`).
+Please note that such `isHidden` returns `true` for elements that are on-screen, but have zero sizes.
````
## clientTop/Left
diff --git a/2-ui/1-document/10-size-and-scroll-window/article.md b/2-ui/1-document/10-size-and-scroll-window/article.md
index 95a5cd48b..08a2f6576 100644
--- a/2-ui/1-document/10-size-and-scroll-window/article.md
+++ b/2-ui/1-document/10-size-and-scroll-window/article.md
@@ -73,6 +73,12 @@ alert('Current scroll from the left: ' + window.pageXOffset);
These properties are read-only.
+```smart header="Also available as `window` properties `scrollX` and `scrollY`"
+For historical reasons, both properties exist, but they are the same:
+- `window.pageXOffset` is an alias of `window.scrollX`.
+- `window.pageYOffset` is an alias of `window.scrollY`.
+```
+
## Scrolling: scrollTo, scrollBy, scrollIntoView [#window-scroll]
```warn
diff --git a/2-ui/1-document/11-coordinates/article.md b/2-ui/1-document/11-coordinates/article.md
index 4775ff0eb..fc605c414 100644
--- a/2-ui/1-document/11-coordinates/article.md
+++ b/2-ui/1-document/11-coordinates/article.md
@@ -36,7 +36,7 @@ Additionally, there are derived properties:
```online
For instance click this button to see its window coordinates:
-
+
diff --git a/2-ui/3-event-details/6-pointer-events/slider.view/style.css b/2-ui/3-event-details/6-pointer-events/slider.view/style.css
index 9b3d3b82d..a84cd5e7e 100644
--- a/2-ui/3-event-details/6-pointer-events/slider.view/style.css
+++ b/2-ui/3-event-details/6-pointer-events/slider.view/style.css
@@ -8,6 +8,7 @@
}
.thumb {
+ touch-action: none;
width: 10px;
height: 25px;
border-radius: 3px;
diff --git a/2-ui/3-event-details/7-keyboard-events/article.md b/2-ui/3-event-details/7-keyboard-events/article.md
index 54bde42b4..12fe63201 100644
--- a/2-ui/3-event-details/7-keyboard-events/article.md
+++ b/2-ui/3-event-details/7-keyboard-events/article.md
@@ -107,7 +107,7 @@ So, `event.code` may match a wrong character for unexpected layout. Same letters
To reliably track layout-dependent characters, `event.key` may be a better way.
-On the other hand, `event.code` has the benefit of staying always the same, bound to the physical key location, even if the visitor changes languages. So hotkeys that rely on it work well even in case of a language switch.
+On the other hand, `event.code` has the benefit of staying always the same, bound to the physical key location. So hotkeys that rely on it work well even in case of a language switch.
Do we want to handle layout-dependant keys? Then `event.key` is the way to go.
@@ -139,22 +139,25 @@ For instance, the `` below expects a phone number, so it does not accept
```html autorun height=60 run
```
-Please note that special keys, such as `key:Backspace`, `key:Left`, `key:Right`, `key:Ctrl+V`, do not work in the input. That's a side-effect of the strict filter `checkPhoneKey`.
+The `onkeydown` handler here uses `checkPhoneKey` to check for the key pressed. If it's valid (from `0..9` or one of `+-()`), then it returns `true`, otherwise `false`.
-Let's relax it a little bit:
+As we know, the `false` value returned from the event handler, assigned using a DOM property or an attribute, such as above, prevents the default action, so nothing appears in the `` for keys that don't pass the test. (The `true` value returned doesn't affect anything, only returning `false` matters)
+Please note that special keys, such as `key:Backspace`, `key:Left`, `key:Right`, do not work in the input. That's a side effect of the strict filter `checkPhoneKey`. These keys make it return `false`.
+
+Let's relax the filter a little bit by allowing arrow keys `key:Left`, `key:Right` and `key:Delete`, `key:Backspace`:
```html autorun height=60 run
@@ -162,7 +165,9 @@ function checkPhoneKey(key) {
Now arrows and deletion works well.
-...But we still can enter anything by using a mouse and right-click + Paste. So the filter is not 100% reliable. We can just let it be like that, because most of time it works. Or an alternative approach would be to track the `input` event -- it triggers after any modification. There we can check the new value and highlight/modify it when it's invalid.
+Even though we have the key filter, one still can enter anything using a mouse and right-click + Paste. Mobile devices provide other means to enter values. So the filter is not 100% reliable.
+
+The alternative approach would be to track the `oninput` event -- it triggers *after* any modification. There we can check the new `input.value` and modify it/highlight the `` when it's invalid. Or we can use both event handlers together.
## Legacy
diff --git a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html
index 401062830..a0d5a4f40 100644
--- a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html
+++ b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html
@@ -28,7 +28,7 @@
-
+
diff --git a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js
index 5eba24c7a..d97f7a7b5 100644
--- a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js
+++ b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js
@@ -5,6 +5,8 @@ let lastTime = Date.now();
function handle(e) {
if (form.elements[e.type + 'Ignore'].checked) return;
+ area.scrollTop = 1e6;
+
let text = e.type +
' key=' + e.key +
' code=' + e.code +
diff --git a/2-ui/4-forms-controls/1-form-elements/article.md b/2-ui/4-forms-controls/1-form-elements/article.md
index 689301e4d..7bc87a0f0 100644
--- a/2-ui/4-forms-controls/1-form-elements/article.md
+++ b/2-ui/4-forms-controls/1-form-elements/article.md
@@ -155,7 +155,7 @@ Let's talk about form controls.
### input and textarea
-We can access their value as `input.value` (string) or `input.checked` (boolean) for checkboxes.
+We can access their value as `input.value` (string) or `input.checked` (boolean) for checkboxes and radio buttons.
Like this:
@@ -244,7 +244,7 @@ This syntax is optional. We can use `document.createElement('option')` and set a
- `defaultSelected` -- if `true`, then `selected` HTML-attribute is created,
- `selected` -- if `true`, then the option is selected.
-The difference between `defaultSelected` and `selected` is that `defaultSelected` sets the HTML-attribute (that we can get using `option.getAttribute('selected')`, while `selected` sets whether the option is selected or not.
+The difference between `defaultSelected` and `selected` is that `defaultSelected` sets the HTML-attribute (that we can get using `option.getAttribute('selected')`), while `selected` sets whether the option is selected or not.
In practice, one should usually set _both_ values to `true` or `false`. (Or, simply omit them; both default to `false`.)
diff --git a/2-ui/4-forms-controls/2-focus-blur/4-edit-td-click/task.md b/2-ui/4-forms-controls/2-focus-blur/4-edit-td-click/task.md
index 2cccea020..378bd1f54 100644
--- a/2-ui/4-forms-controls/2-focus-blur/4-edit-td-click/task.md
+++ b/2-ui/4-forms-controls/2-focus-blur/4-edit-td-click/task.md
@@ -6,7 +6,7 @@ importance: 5
Make table cells editable on click.
-- On click -- the cell should became "editable" (textarea appears inside), we can change HTML. There should be no resize, all geometry should remain the same.
+- On click -- the cell should become "editable" (textarea appears inside), we can change HTML. There should be no resize, all geometry should remain the same.
- Buttons OK and CANCEL appear below the cell to finish/cancel the editing.
- Only one cell may be editable at a moment. While a `` is in "edit mode", clicks on other cells are ignored.
- The table may have many cells. Use event delegation.
diff --git a/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md b/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md
index fc48c21ff..644d814d9 100644
--- a/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md
+++ b/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md
@@ -9,4 +9,5 @@ Focus on the mouse. Then use arrow keys to move it:
[demo src="solution"]
P.S. Don't put event handlers anywhere except the `#mouse` element.
+
P.P.S. Don't modify HTML/CSS, the approach should be generic and work with any element.
diff --git a/2-ui/4-forms-controls/2-focus-blur/article.md b/2-ui/4-forms-controls/2-focus-blur/article.md
index d4348d25b..c253dc11d 100644
--- a/2-ui/4-forms-controls/2-focus-blur/article.md
+++ b/2-ui/4-forms-controls/2-focus-blur/article.md
@@ -90,6 +90,8 @@ If we enter something into the input and then try to use `key:Tab` or click away
Please note that we can't "prevent losing focus" by calling `event.preventDefault()` in `onblur`, because `onblur` works *after* the element lost the focus.
+In practice though, one should think well, before implementing something like this, because we generally *should show errors* to the user, but *should not prevent their progress* in filling our form. They may want to fill other fields first.
+
```warn header="JavaScript-initiated focus loss"
A focus loss can occur for many reasons.
@@ -104,7 +106,7 @@ The best recipe is to be careful when using these events. If we want to track us
```
## Allow focusing on any element: tabindex
-By default many elements do not support focusing.
+By default, many elements do not support focusing.
The list varies a bit between browsers, but one thing is always correct: `focus/blur` support is guaranteed for elements that a visitor can interact with: `
(253-1) (đó là 9007199254740991) hoặc nhỏ hơn -(253-1) đối với số âm. Đó là một hạn chế kỹ thuật do sự biểu diễn nội bộ của chúng gây ra.
Đối với hầu hết các mục đích đó là đủ, nhưng đôi khi chúng ta cần những con số thực sự lớn, ví dụ cho mật mã hoặc thời điểm chính xác đến micro giây.
+=======
+In JavaScript, the "number" type cannot safely represent integer values larger than (253-1) (that's `9007199254740991`), or less than -(253-1) for negatives.
+
+To be really precise, the "number" type can store larger integers (up to 1.7976931348623157 * 10308), but outside of the safe integer range ±(253-1) there'll be a precision error, because not all digits fit into the fixed 64-bit storage. So an "approximate" value may be stored.
+
+For example, these two numbers (right above the safe range) are the same:
+
+```js
+console.log(9007199254740991 + 1); // 9007199254740992
+console.log(9007199254740991 + 2); // 9007199254740992
+```
+
+So to say, all odd integers greater than (253-1) can't be stored at all in the "number" type.
+
+For most purposes ±(253-1) range is quite enough, but sometimes we need the entire range of really big integers, e.g. for cryptography or microsecond-precision timestamps.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
Kiểu `BigInt` gần đây đã được thêm vào ngôn ngữ để biểu diễn các số nguyên có độ dài tùy ý.
@@ -81,6 +108,7 @@ const bigInt = 1234567890123456789012345678901234567890n;
Vì các số `BigInt` hiếm khi cần thiết, chúng tôi không trình bày chúng ở đây mà dành cho chúng một chương riêng ±(253-1).
- `bigint` dành cho các số nguyên có độ dài tùy ý.
- `string` dành cho các chuỗi. Một chuỗi có thể có không hoặc nhiều ký tự, không có kiểu ký tự đơn riêng biệt.
@@ -265,11 +321,29 @@ Có 8 kiểu dữ liệu cơ bản trong JavaScript.
- `undefined` dành cho các giá trị chưa được gán -- một kiểu độc lập có một giá trị duy nhất `undefined`.
- `object` dành cho các cấu trúc dữ liệu phức tạp hơn.
- `symbol` dành cho các định danh duy nhất.
+=======
+- Seven primitive data types:
+ - `number` for numbers of any kind: integer or floating-point, integers are limited by ±(253-1).
+ - `bigint` for integer numbers of arbitrary length.
+ - `string` for strings. A string may have zero or more characters, there's no separate single-character type.
+ - `boolean` for `true`/`false`.
+ - `null` for unknown values -- a standalone type that has a single value `null`.
+ - `undefined` for unassigned values -- a standalone type that has a single value `undefined`.
+ - `symbol` for unique identifiers.
+- And one non-primitive data type:
+ - `object` for more complex data structures.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
Toán tử `typeof` cho phép chúng ta xem kiểu nào được lưu trữ trong một biến.
+<<<<<<< HEAD
- Hai dạng: `typeof x` hoặc `typeof(x)`.
- Trả về một chuỗi với tên của kiểu dữ liêu, ví dụ `"string"`.
- Với `null` nó trả về `"object"` -- đây là một lỗi còn tồn tại trong ngôn ngữ, nó không thực sự là một đối tượng.
+=======
+- Usually used as `typeof x`, but `typeof(x)` is also possible.
+- Returns a string with the name of the type, like `"string"`.
+- For `null` returns `"object"` -- this is an error in the language, it's not actually an object.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
Trong các chương tiếp theo, chúng ta sẽ tập trung vào các giá trị nguyên thủy và khi chúng ta đã quen thuộc với chúng, chúng ta sẽ chuyển sang các đối tượng.
diff --git a/1-js/02-first-steps/07-type-conversions/article.md b/1-js/02-first-steps/07-type-conversions/article.md
index cf97b3307..329556141 100644
--- a/1-js/02-first-steps/07-type-conversions/article.md
+++ b/1-js/02-first-steps/07-type-conversions/article.md
@@ -7,7 +7,7 @@ For example, `alert` automatically converts any value to a string to show it. Ma
There are also cases when we need to explicitly convert a value to the expected type.
```smart header="Not talking about objects yet"
-In this chapter, we won't cover objects. For now we'll just be talking about primitives.
+In this chapter, we won't cover objects. For now, we'll just be talking about primitives.
Later, after we learn about objects, in the chapter true and false | `1` and `0` |
-| `string` | Whitespaces from the start and end are removed. If the remaining string is empty, the result is `0`. Otherwise, the number is "read" from the string. An error gives `NaN`. |
+| `string` | Whitespaces (includes spaces, tabs `\t`, newlines `\n` etc.) from the start and end are removed. If the remaining string is empty, the result is `0`. Otherwise, the number is "read" from the string. An error gives `NaN`. |
Examples:
@@ -130,7 +130,7 @@ The conversion follows the rules:
|`undefined`|`NaN`|
|`null`|`0`|
|true / false | `1 / 0` |
-| `string` | The string is read "as is", whitespaces from both sides are ignored. An empty string becomes `0`. An error gives `NaN`. |
+| `string` | The string is read "as is", whitespaces (includes spaces, tabs `\t`, newlines `\n` etc.) from both sides are ignored. An empty string becomes `0`. An error gives `NaN`. |
**`Boolean Conversion`** -- Occurs in logical operations. Can be performed with `Boolean(value)`.
diff --git a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md
index dfd061cb6..7370b66af 100644
--- a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md
+++ b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md
@@ -22,4 +22,4 @@ undefined + 1 = NaN // (6)
4. The subtraction always converts to numbers, so it makes `" -9 "` a number `-9` (ignoring spaces around it).
5. `null` becomes `0` after the numeric conversion.
6. `undefined` becomes `NaN` after the numeric conversion.
-7. Space characters, are trimmed off string start and end when a string is converted to a number. Here the whole string consists of space characters, such as `\t`, `\n` and a "regular" space between them. So, similarly to an empty string, it becomes `0`.
+7. Space characters are trimmed off string start and end when a string is converted to a number. Here the whole string consists of space characters, such as `\t`, `\n` and a "regular" space between them. So, similarly to an empty string, it becomes `0`.
diff --git a/1-js/02-first-steps/08-operators/article.md b/1-js/02-first-steps/08-operators/article.md
index 360ebd22e..26734ec9d 100644
--- a/1-js/02-first-steps/08-operators/article.md
+++ b/1-js/02-first-steps/08-operators/article.md
@@ -51,8 +51,14 @@ Kết quả của `a % b` là [phần dư](https://en.wikipedia.org/wiki/Remaind
Ví dụ:
```js run
+<<<<<<< HEAD
alert( 5 % 2 ); // 1, phần dư của 5 chia 2
alert( 8 % 3 ); // 2, phần dư của 8 chia 3
+=======
+alert( 5 % 2 ); // 1, the remainder of 5 divided by 2
+alert( 8 % 3 ); // 2, the remainder of 8 divided by 3
+alert( 8 % 4 ); // 0, the remainder of 8 divided by 4
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
```
### Exponentiation **
@@ -64,12 +70,12 @@ In school maths, we write that as ab.
For instance:
```js run
-alert( 2 ** 2 ); // 2² = 4
-alert( 2 ** 3 ); // 2³ = 8
+alert( 2 ** 2 ); // 2² = 4
+alert( 2 ** 3 ); // 2³ = 8
alert( 2 ** 4 ); // 2⁴ = 16
```
-Just like in maths, the exponentiation operator is defined for non-integer numbers as well.
+Just like in maths, the exponentiation operator is defined for non-integer numbers as well.
For example, a square root is an exponentiation by ½:
@@ -81,7 +87,7 @@ alert( 8 ** (1/3) ); // 2 (power of 1/3 is the same as a cubic root)
## String concatenation with binary +
-Let's meet features of JavaScript operators that are beyond school arithmetics.
+Let's meet the features of JavaScript operators that are beyond school arithmetics.
Usually, the plus operator `+` sums numbers.
@@ -205,6 +211,7 @@ Có nhiều toán tử trong JavaScript. Mỗi toán tử có một số ưu ti
| Độ ưu tiên | Tên | Kí hiệu |
|------------|-----|---------|
| ... | ... | ... |
+<<<<<<< HEAD
| 17 | cộng đơn nguyên | `+` |
| 17 | trừ đơn nguyên | `-` |
| 16 | lũy thừa | `**` |
@@ -217,10 +224,28 @@ Có nhiều toán tử trong JavaScript. Mỗi toán tử có một số ưu ti
| ... | ... | ... |
Như chúng ta thấy, phép "cộng đơn nguyên" có độ ưu tiên là `17` cao hơn `13` của phép "cộng" (cộng nhị phân). Đó là lý do, trong biểu thức `"+apples + +oranges"`, cộng đơn nguyên chạy trước cộng nhị phân.
+=======
+| 14 | unary plus | `+` |
+| 14 | unary negation | `-` |
+| 13 | exponentiation | `**` |
+| 12 | multiplication | `*` |
+| 12 | division | `/` |
+| 11 | addition | `+` |
+| 11 | subtraction | `-` |
+| ... | ... | ... |
+| 2 | assignment | `=` |
+| ... | ... | ... |
+
+As we can see, the "unary plus" has a priority of `14` which is higher than the `11` of "addition" (binary plus). That's why, in the expression `"+apples + +oranges"`, unary pluses work before the addition.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
## Phép gán
+<<<<<<< HEAD
Hãy lưu ý rằng một phép gán `=` cũng là một toán tử. Nó được liệt kê trong bảng ưu tiên với mức ưu tiên rất thấp là `3`.
+=======
+Let's note that an assignment `=` is also an operator. It is listed in the precedence table with the very low priority of `2`.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
Đó là lý do tại sao, khi chúng ta gán một biến, như `x = 2 * 2 + 1`, các phép tính được thực hiện trước và sau đó dấu ` = `được chạy, lưu trữ kết quả trong` x`.
@@ -315,9 +340,13 @@ Các toán tử như vậy có cùng mức độ ưu tiên như một phép gán
```js run
let n = 2;
-n *= 3 + 5;
+n *= 3 + 5; // right part evaluated first, same as n *= 8
+<<<<<<< HEAD
alert( n ); // 16 (phần bên phải thực hiện trước, giống như n *= 8)
+=======
+alert( n ); // 16
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
```
## Tăng/giảm
@@ -451,7 +480,11 @@ Danh sách các toán tử:
- RIGHT SHIFT ( `>>` )
- ZERO-FILL RIGHT SHIFT ( `>>>` )
+<<<<<<< HEAD
Các toán tử này rất hiếm khi được sử dụng, khi chúng ta cần xử lý các số ở mức rất thấp (bitwise). Chúng ta sẽ không cần đến những toán tử này trong thời gian tới, vì việc phát triển web ít sử dụng chúng, nhưng trong một số lĩnh vực đặc biệt, chẳng hạn như mật mã, chúng rất hữu ích. Bạn có thể đọc chương [Toán tử Bitwise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Bitwise) trên MDN khi có nhu cầu.
+=======
+These operators are used very rarely, when we need to fiddle with numbers on the very lowest (bitwise) level. We won't need these operators any time soon, as web development has little use of them, but in some special areas, such as cryptography, they are useful. You can read the [Bitwise Operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#bitwise_operators) chapter on MDN when a need arises.
+>>>>>>> 035c5267ba80fa7b55878f7213cbde449b4092d9
## Dấu phẩy
diff --git a/1-js/02-first-steps/09-comparison/article.md b/1-js/02-first-steps/09-comparison/article.md
index ead7922ff..a69317fee 100644
--- a/1-js/02-first-steps/09-comparison/article.md
+++ b/1-js/02-first-steps/09-comparison/article.md
@@ -7,7 +7,7 @@ In JavaScript they are written like this:
- Greater/less than: a > b, a < b.
- Greater/less than or equals: a >= b, a <= b.
- Equals: `a == b`, please note the double equality sign `==` means the equality test, while a single one `a = b` means an assignment.
-- Not equals. In maths the notation is ≠, but in JavaScript it's written as a != b.
+- Not equals: In maths the notation is ≠, but in JavaScript it's written as a != b.
In this article we'll learn more about different types of comparisons, how JavaScript makes them, including important peculiarities.
diff --git a/1-js/02-first-steps/10-ifelse/article.md b/1-js/02-first-steps/10-ifelse/article.md
index 51514062f..82e8800b9 100644
--- a/1-js/02-first-steps/10-ifelse/article.md
+++ b/1-js/02-first-steps/10-ifelse/article.md
@@ -68,7 +68,7 @@ if (cond) {
## The "else" clause
-The `if` statement may contain an optional "else" block. It executes when the condition is falsy.
+The `if` statement may contain an optional `else` block. It executes when the condition is falsy.
For example:
```js run
@@ -181,9 +181,9 @@ alert( message );
It may be difficult at first to grasp what's going on. But after a closer look, we can see that it's just an ordinary sequence of tests:
1. The first question mark checks whether `age < 3`.
-2. If true -- it returns `'Hi, baby!'`. Otherwise, it continues to the expression after the colon '":"', checking `age < 18`.
-3. If that's true -- it returns `'Hello!'`. Otherwise, it continues to the expression after the next colon '":"', checking `age < 100`.
-4. If that's true -- it returns `'Greetings!'`. Otherwise, it continues to the expression after the last colon '":"', returning `'What an unusual age!'`.
+2. If true -- it returns `'Hi, baby!'`. Otherwise, it continues to the expression after the colon ":", checking `age < 18`.
+3. If that's true -- it returns `'Hello!'`. Otherwise, it continues to the expression after the next colon ":", checking `age < 100`.
+4. If that's true -- it returns `'Greetings!'`. Otherwise, it continues to the expression after the last colon ":", returning `'What an unusual age!'`.
Here's how this looks using `if..else`:
diff --git a/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md b/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md
index 5c2455ef4..368b59409 100644
--- a/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md
+++ b/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md
@@ -1,6 +1,6 @@
The answer: `null`, because it's the first falsy value from the list.
```js run
-alert( 1 && null && 2 );
+alert(1 && null && 2);
```
diff --git a/1-js/02-first-steps/11-logical-operators/article.md b/1-js/02-first-steps/11-logical-operators/article.md
index 97f5d738a..78c4fd2f1 100644
--- a/1-js/02-first-steps/11-logical-operators/article.md
+++ b/1-js/02-first-steps/11-logical-operators/article.md
@@ -123,7 +123,7 @@ This leads to some interesting usage compared to a "pure, classical, boolean-onl
It means that `||` processes its arguments until the first truthy value is reached, and then the value is returned immediately, without even touching the other argument.
- That importance of this feature becomes obvious if an operand isn't just a value, but an expression with a side effect, such as a variable assignment or a function call.
+ The importance of this feature becomes obvious if an operand isn't just a value, but an expression with a side effect, such as a variable assignment or a function call.
In the example below, only the second message is printed:
diff --git a/1-js/02-first-steps/12-nullish-coalescing-operator/article.md b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md
index b84dff892..0b2f092ab 100644
--- a/1-js/02-first-steps/12-nullish-coalescing-operator/article.md
+++ b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md
@@ -4,7 +4,7 @@
The nullish coalescing operator is written as two question marks `??`.
-As it treats `null` and `undefined` similarly, we'll use a special term here, in this article. We'll say that an expression is "defined" when it's neither `null` nor `undefined`.
+As it treats `null` and `undefined` similarly, we'll use a special term here, in this article. For brevity, we'll say that a value is "defined" when it's neither `null` nor `undefined`.
The result of `a ?? b` is:
- if `a` is defined, then `a`,
@@ -22,14 +22,14 @@ result = (a !== null && a !== undefined) ? a : b;
Now it should be absolutely clear what `??` does. Let's see where it helps.
-The common use case for `??` is to provide a default value for a potentially undefined variable.
+The common use case for `??` is to provide a default value.
-For example, here we show `user` if defined, otherwise `Anonymous`:
+For example, here we show `user` if its value isn't `null/undefined`, otherwise `Anonymous`:
```js run
let user;
-alert(user ?? "Anonymous"); // Anonymous (user not defined)
+alert(user ?? "Anonymous"); // Anonymous (user is undefined)
```
Here's the example with `user` assigned to a name:
@@ -37,14 +37,14 @@ Here's the example with `user` assigned to a name:
```js run
let user = "John";
-alert(user ?? "Anonymous"); // John (user defined)
+alert(user ?? "Anonymous"); // John (user is not null/undefined)
```
We can also use a sequence of `??` to select the first value from a list that isn't `null/undefined`.
-Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be not defined, if the user decided not to enter a value.
+Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be not defined, if the user decided not to fill in the corresponding values.
-We'd like to display the user name using one of these variables, or show "Anonymous" if all of them aren't defined.
+We'd like to display the user name using one of these variables, or show "Anonymous" if all of them are `null/undefined`.
Let's use the `??` operator for that:
@@ -76,7 +76,7 @@ alert(firstName || lastName || nickName || "Anonymous"); // Supercoder
*/!*
```
-Historically, the OR `||` operator was there first. It exists since the beginning of JavaScript, so developers were using it for such purposes for a long time.
+Historically, the OR `||` operator was there first. It's been there since the beginning of JavaScript, so developers were using it for such purposes for a long time.
On the other hand, the nullish coalescing operator `??` was added to JavaScript only recently, and the reason for that was that people weren't quite happy with `||`.
@@ -106,11 +106,11 @@ In practice, the zero height is often a valid value, that shouldn't be replaced
## Precedence
-The precedence of the `??` operator is about the same as `||`, just a bit lower. It equals `5` in the [MDN table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table), while `||` is `6`.
+The precedence of the `??` operator is the same as `||`. They both equal `3` in the [MDN table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table).
That means that, just like `||`, the nullish coalescing operator `??` is evaluated before `=` and `?`, but after most other operations, such as `+`, `*`.
-So if we'd like to choose a value with `??` in an expression with other operators, consider adding parentheses:
+So we may need to add parentheses in expressions like this:
```js run
let height = null;
@@ -128,7 +128,7 @@ Otherwise, if we omit parentheses, then as `*` has the higher precedence than `?
// without parentheses
let area = height ?? 100 * width ?? 50;
-// ...works the same as this (probably not what we want):
+// ...works this way (not what we want):
let area = height ?? (100 * width) ?? 50;
```
diff --git a/1-js/02-first-steps/13-while-for/article.md b/1-js/02-first-steps/13-while-for/article.md
index 2579b647d..d1b749888 100644
--- a/1-js/02-first-steps/13-while-for/article.md
+++ b/1-js/02-first-steps/13-while-for/article.md
@@ -6,6 +6,19 @@ For example, outputting goods from a list one after another or just running the
*Loops* are a way to repeat the same code multiple times.
+```smart header="The for..of and for..in loops"
+A small announcement for advanced readers.
+
+This article covers only basic loops: `while`, `do..while` and `for(..;..;..)`.
+
+If you came to this article searching for other types of loops, here are the pointers:
+
+- See [for..in](info:object#forin) to loop over object properties.
+- See [for..of](info:array#loops) and [iterables](info:iterable) for looping over arrays and iterable objects.
+
+Otherwise, please read on.
+```
+
## The "while" loop
The `while` loop has the following syntax:
@@ -106,7 +119,7 @@ Let's examine the `for` statement part-by-part:
| part | | |
|-------|----------|----------------------------------------------------------------------------|
-| begin | `i = 0` | Executes once upon entering the loop. |
+| begin | `let i = 0` | Executes once upon entering the loop. |
| condition | `i < 3`| Checked before every loop iteration. If false, the loop stops. |
| body | `alert(i)`| Runs again and again while the condition is truthy. |
| step| `i++` | Executes after the body on each iteration. |
@@ -162,10 +175,8 @@ for (i = 0; i < 3; i++) { // use an existing variable
alert(i); // 3, visible, because declared outside of the loop
```
-
````
-
### Skipping parts
Any part of `for` can be skipped.
@@ -268,7 +279,7 @@ for (let i = 0; i < 10; i++) {
From a technical point of view, this is identical to the example above. Surely, we can just wrap the code in an `if` block instead of using `continue`.
-But as a side-effect, this created one more level of nesting (the `alert` call inside the curly braces). If the code inside of `if` is longer than a few lines, that may decrease the overall readability.
+But as a side effect, this created one more level of nesting (the `alert` call inside the curly braces). If the code inside of `if` is longer than a few lines, that may decrease the overall readability.
````
````warn header="No `break/continue` to the right side of '?'"
@@ -286,7 +297,6 @@ if (i > 5) {
...and rewrite it using a question mark:
-
```js no-beautify
(i > 5) ? alert(i) : *!*continue*/!*; // continue isn't allowed here
```
@@ -321,6 +331,7 @@ We need a way to stop the process if the user cancels the input.
The ordinary `break` after `input` would only break the inner loop. That's not sufficient -- labels, come to the rescue!
A *label* is an identifier with a colon before a loop:
+
```js
labelName: for (...) {
...
@@ -342,6 +353,7 @@ The `break 253 or be less than -253. As bigints are used in few special areas, we devote them a special chapter (253-1) or be less than -(253-1), as we mentioned earlier in the chapter func`string`. The function `func` is called automatically, receives the string and embedded expressions and can process them. This is called "tagged templates". This feature makes it easier to implement custom templating, but is rarely used in practice. You can read more about it in the [manual](mdn:/JavaScript/Reference/Template_literals#Tagged_templates).
+Backticks also allow us to specify a "template function" before the first backtick. The syntax is: func`string`. The function `func` is called automatically, receives the string and embedded expressions and can process them. This feature is called "tagged templates", it's rarely seen, but you can read about it in the MDN: [Template literals](mdn:/JavaScript/Reference/Template_literals#Tagged_templates).
## Special characters
@@ -59,10 +59,10 @@ It is still possible to create multiline strings with single and double quotes b
```js run
let guestList = "Guests:\n * John\n * Pete\n * Mary";
-alert(guestList); // a multiline list of guests
+alert(guestList); // a multiline list of guests, same as above
```
-For example, these two lines are equal, just written differently:
+As a simpler example, these two lines are equal, just written differently:
```js run
let str1 = "Hello\nWorld"; // two lines using a "newline symbol"
@@ -74,33 +74,26 @@ World`;
alert(str1 == str2); // true
```
-There are other, less common "special" characters.
-
-Here's the full list:
+There are other, less common special characters:
| Character | Description |
|-----------|-------------|
|`\n`|New line|
-|`\r`|Carriage return: not used alone. Windows text files use a combination of two characters `\r\n` to represent a line break. |
-|`\'`, `\"`|Quotes|
+|`\r`|In Windows text files a combination of two characters `\r\n` represents a new break, while on non-Windows OS it's just `\n`. That's for historical reasons, most Windows software also understands `\n`. |
+|`\'`, `\"`, \\`|Quotes|
|`\\`|Backslash|
|`\t`|Tab|
-|`\b`, `\f`, `\v`| Backspace, Form Feed, Vertical Tab -- kept for compatibility, not used nowadays. |
-|`\xXX`|Unicode character with the given hexadecimal Unicode `XX`, e.g. `'\x7A'` is the same as `'z'`.|
-|`\uXXXX`|A Unicode symbol with the hex code `XXXX` in UTF-16 encoding, for instance `\u00A9` -- is a Unicode for the copyright symbol `©`. It must be exactly 4 hex digits. |
-|`\u{X…XXXXXX}` (1 to 6 hex characters)|A Unicode symbol with the given UTF-32 encoding. Some rare characters are encoded with two Unicode symbols, taking 4 bytes. This way we can insert long codes. |
+|`\b`, `\f`, `\v`| Backspace, Form Feed, Vertical Tab -- mentioned for completeness, coming from old times, not used nowadays (you can forget them right now). |
+
+As you can see, all special characters start with a backslash character `\`. It is also called an "escape character".
-Examples with Unicode:
+Because it's so special, if we need to show an actual backslash `\` within the string, we need to double it:
```js run
-alert( "\u00A9" ); // ©
-alert( "\u{20331}" ); // 佫, a rare Chinese hieroglyph (long Unicode)
-alert( "\u{1F60D}" ); // 😍, a smiling face symbol (another long Unicode)
+alert( `The backslash: \\` ); // The backslash: \
```
-All special characters start with a backslash character `\`. It is also called an "escape character".
-
-We might also use it if we wanted to insert a quote into the string.
+So-called "escaped" quotes `\'`, `\"`, \\` are used to insert a quote into the same-quoted string.
For instance:
@@ -113,18 +106,10 @@ As you can see, we have to prepend the inner quote by the backslash `\'`, becaus
Of course, only the quotes that are the same as the enclosing ones need to be escaped. So, as a more elegant solution, we could switch to double quotes or backticks instead:
```js run
-alert( `I'm the Walrus!` ); // I'm the Walrus!
+alert( "I'm the Walrus!" ); // I'm the Walrus!
```
-Note that the backslash `\` serves for the correct reading of the string by JavaScript, then disappears. The in-memory string has no `\`. You can clearly see that in `alert` from the examples above.
-
-But what if we need to show an actual backslash `\` within the string?
-
-That's possible, but we need to double it like `\\`:
-
-```js run
-alert( `The backslash: \\` ); // The backslash: \
-```
+Besides these special characters, there's also a special notation for Unicode codes `\u…`, it's rarely used and is covered in the optional chapter about [Unicode](info:unicode).
## String length
@@ -139,33 +124,36 @@ Note that `\n` is a single "special" character, so the length is indeed `3`.
```warn header="`length` is a property"
People with a background in some other languages sometimes mistype by calling `str.length()` instead of just `str.length`. That doesn't work.
-Please note that `str.length` is a numeric property, not a function. There is no need to add parenthesis after it.
+Please note that `str.length` is a numeric property, not a function. There is no need to add parenthesis after it. Not `.length()`, but `.length`.
```
## Accessing characters
-To get a character at position `pos`, use square brackets `[pos]` or call the method [str.charAt(pos)](mdn:js/String/charAt). The first character starts from the zero position:
+To get a character at position `pos`, use square brackets `[pos]` or call the method [str.at(pos)](mdn:js/String/at). The first character starts from the zero position:
```js run
let str = `Hello`;
// the first character
alert( str[0] ); // H
-alert( str.charAt(0) ); // H
+alert( str.at(0) ); // H
// the last character
alert( str[str.length - 1] ); // o
+alert( str.at(-1) );
```
-The square brackets are a modern way of getting a character, while `charAt` exists mostly for historical reasons.
+As you can see, the `.at(pos)` method has a benefit of allowing negative position. If `pos` is negative, then it's counted from the end of the string.
-The only difference between them is that if no character is found, `[]` returns `undefined`, and `charAt` returns an empty string:
+So `.at(-1)` means the last character, and `.at(-2)` is the one before it, etc.
+
+The square brackets always return `undefined` for negative indexes, for instance:
```js run
let str = `Hello`;
-alert( str[1000] ); // undefined
-alert( str.charAt(1000) ); // '' (an empty string)
+alert( str[-2] ); // undefined
+alert( str.at(-2) ); // l
```
We can also iterate over characters using `for..of`:
@@ -214,7 +202,7 @@ alert( 'Interface'.toLowerCase() ); // interface
Or, if we want a single character lowercased:
-```js
+```js run
alert( 'Interface'[0].toLowerCase() ); // 'i'
```
@@ -310,45 +298,6 @@ if (str.indexOf("Widget") != -1) {
}
```
-#### The bitwise NOT trick
-
-One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation.
-
-In practice, that means a simple thing: for 32-bit integers `~n` equals `-(n+1)`.
-
-For instance:
-
-```js run
-alert( ~2 ); // -3, the same as -(2+1)
-alert( ~1 ); // -2, the same as -(1+1)
-alert( ~0 ); // -1, the same as -(0+1)
-*!*
-alert( ~-1 ); // 0, the same as -(-1+1)
-*/!*
-```
-
-As we can see, `~n` is zero only if `n == -1` (that's for any 32-bit signed integer `n`).
-
-So, the test `if ( ~str.indexOf("...") )` is truthy only if the result of `indexOf` is not `-1`. In other words, when there is a match.
-
-People use it to shorten `indexOf` checks:
-
-```js run
-let str = "Widget";
-
-if (~str.indexOf("Widget")) {
- alert( 'Found it!' ); // works
-}
-```
-
-It is usually not recommended to use language features in a non-obvious way, but this particular trick is widely used in old code, so we should understand it.
-
-Just remember: `if (~str.indexOf(...))` reads as "if found".
-
-To be precise though, as big numbers are truncated to 32 bits by `~` operator, there exist other numbers that give `0`, the smallest is `~4294967295=0`. That makes such check correct only if a string is not that long.
-
-Right now we can see this trick only in the old code, as modern JavaScript provides `.includes` method (see below).
-
### includes, startsWith, endsWith
The more modern method [str.includes(substr, pos)](mdn:js/String/includes) returns `true/false` depending on whether `str` contains `substr` within.
@@ -371,8 +320,8 @@ alert( "Widget".includes("id", 3) ); // false, from position 3 there is no "id"
The methods [str.startsWith](mdn:js/String/startsWith) and [str.endsWith](mdn:js/String/endsWith) do exactly what they say:
```js run
-alert( "Widget".startsWith("Wid") ); // true, "Widget" starts with "Wid"
-alert( "Widget".endsWith("get") ); // true, "Widget" ends with "get"
+alert( "*!*Wid*/!*get".startsWith("Wid") ); // true, "Widget" starts with "Wid"
+alert( "Wid*!*get*/!*".endsWith("get") ); // true, "Widget" ends with "get"
```
## Getting a substring
@@ -407,9 +356,9 @@ There are 3 methods in JavaScript to get a substring: `substring`, `substr` and
```
`str.substring(start [, end])`
-: Returns the part of the string *between* `start` and `end`.
+: Returns the part of the string *between* `start` and `end` (not including `end`).
- This is almost the same as `slice`, but it allows `start` to be greater than `end`.
+ This is almost the same as `slice`, but it allows `start` to be greater than `end` (in this case it simply swaps `start` and `end` values).
For instance:
@@ -445,18 +394,22 @@ There are 3 methods in JavaScript to get a substring: `substring`, `substr` and
alert( str.substr(-4, 2) ); // 'gi', from the 4th position get 2 characters
```
+ This method resides in the [Annex B](https://tc39.es/ecma262/#sec-string.prototype.substr) of the language specification. It means that only browser-hosted Javascript engines should support it, and it's not recommended to use it. In practice, it's supported everywhere.
+
Let's recap these methods to avoid any confusion:
| method | selects... | negatives |
|--------|-----------|-----------|
| `slice(start, end)` | from `start` to `end` (not including `end`) | allows negatives |
-| `substring(start, end)` | between `start` and `end` | negative values mean `0` |
+| `substring(start, end)` | between `start` and `end` (not including `end`)| negative values mean `0` |
| `substr(start, length)` | from `start` get `length` characters | allows negative `start` |
```smart header="Which one to choose?"
All of them can do the job. Formally, `substr` has a minor drawback: it is described not in the core JavaScript specification, but in Annex B, which covers browser-only features that exist mainly for historical reasons. So, non-browser environments may fail to support it. But in practice it works everywhere.
-Of the other two variants, `slice` is a little bit more flexible, it allows negative arguments and shorter to write. So, it's enough to remember solely `slice` of these three methods.
+Of the other two variants, `slice` is a little bit more flexible, it allows negative arguments and shorter to write.
+
+So, for practical use it's enough to remember only `slice`.
```
## Comparing strings
@@ -479,17 +432,18 @@ Although, there are some oddities.
This may lead to strange results if we sort these country names. Usually people would expect `Zealand` to come after `Österreich` in the list.
-To understand what happens, let's review the internal representation of strings in JavaScript.
+To understand what happens, we should be aware that strings in Javascript are encoded using [UTF-16](https://en.wikipedia.org/wiki/UTF-16). That is: each character has a corresponding numeric code.
-All strings are encoded using [UTF-16](https://en.wikipedia.org/wiki/UTF-16). That is: each character has a corresponding numeric code. There are special methods that allow to get the character for the code and back.
+There are special methods that allow to get the character for the code and back:
`str.codePointAt(pos)`
-: Returns the code for the character at position `pos`:
+: Returns a decimal number representing the code for the character at position `pos`:
```js run
// different case letters have different codes
- alert( "z".codePointAt(0) ); // 122
alert( "Z".codePointAt(0) ); // 90
+ alert( "z".codePointAt(0) ); // 122
+ alert( "z".codePointAt(0).toString(16) ); // 7a (if we need a hexadecimal value)
```
`String.fromCodePoint(code)`
@@ -497,13 +451,7 @@ All strings are encoded using [UTF-16](https://en.wikipedia.org/wiki/UTF-16). Th
```js run
alert( String.fromCodePoint(90) ); // Z
- ```
-
- We can also add Unicode characters by their codes using `\u` followed by the hex code:
-
- ```js run
- // 90 is 5a in hexadecimal system
- alert( '\u005a' ); // Z
+ alert( String.fromCodePoint(0x5a) ); // Z (we can also use a hex value as an argument)
```
Now let's see the characters with codes `65..220` (the latin alphabet and a little bit extra) by making a string of them:
@@ -515,6 +463,7 @@ for (let i = 65; i <= 220; i++) {
str += String.fromCodePoint(i);
}
alert( str );
+// Output:
// ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~
// ¡¢£¤¥¦§¨©ª«¬®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜ
```
@@ -526,7 +475,7 @@ Now it becomes obvious why `a > Z`.
The characters are compared by their numeric code. The greater code means that the character is greater. The code for `a` (97) is greater than the code for `Z` (90).
- All lowercase letters go after uppercase letters because their codes are greater.
-- Some letters like `Ö` stand apart from the main alphabet. Here, it's code is greater than anything from `a` to `z`.
+- Some letters like `Ö` stand apart from the main alphabet. Here, its code is greater than anything from `a` to `z`.
### Correct comparisons [#correct-comparisons]
@@ -534,7 +483,7 @@ The "right" algorithm to do string comparisons is more complex than it may seem,
So, the browser needs to know the language to compare.
-Luckily, all modern browsers (IE10- requires the additional library [Intl.js](https://github.com/andyearnshaw/Intl.js/)) support the internationalization standard [ECMA-402](http://www.ecma-international.org/ecma-402/1.0/ECMA-402.pdf).
+Luckily, modern browsers support the internationalization standard [ECMA-402](https://www.ecma-international.org/publications-and-standards/standards/ecma-402/).
It provides a special method to compare strings in different languages, following their rules.
@@ -552,119 +501,11 @@ alert( 'Österreich'.localeCompare('Zealand') ); // -1
This method actually has two additional arguments specified in [the documentation](mdn:js/String/localeCompare), which allows it to specify the language (by default taken from the environment, letter order depends on the language) and setup additional rules like case sensitivity or should `"a"` and `"á"` be treated as the same etc.
-## Internals, Unicode
-
-```warn header="Advanced knowledge"
-The section goes deeper into string internals. This knowledge will be useful for you if you plan to deal with emoji, rare mathematical or hieroglyphic characters or other rare symbols.
-
-You can skip the section if you don't plan to support them.
-```
-
-### Surrogate pairs
-
-All frequently used characters have 2-byte codes. Letters in most european languages, numbers, and even most hieroglyphs, have a 2-byte representation.
-
-But 2 bytes only allow 65536 combinations and that's not enough for every possible symbol. So rare symbols are encoded with a pair of 2-byte characters called "a surrogate pair".
-
-The length of such symbols is `2`:
-
-```js run
-alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X
-alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY
-alert( '𩷶'.length ); // 2, a rare Chinese hieroglyph
-```
-
-Note that surrogate pairs did not exist at the time when JavaScript was created, and thus are not correctly processed by the language!
-
-We actually have a single symbol in each of the strings above, but the `length` shows a length of `2`.
-
-`String.fromCodePoint` and `str.codePointAt` are few rare methods that deal with surrogate pairs right. They recently appeared in the language. Before them, there were only [String.fromCharCode](mdn:js/String/fromCharCode) and [str.charCodeAt](mdn:js/String/charCodeAt). These methods are actually the same as `fromCodePoint/codePointAt`, but don't work with surrogate pairs.
-
-Getting a symbol can be tricky, because surrogate pairs are treated as two characters:
-
-```js run
-alert( '𝒳'[0] ); // strange symbols...
-alert( '𝒳'[1] ); // ...pieces of the surrogate pair
-```
-
-Note that pieces of the surrogate pair have no meaning without each other. So the alerts in the example above actually display garbage.
-
-Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval of `0xd800..0xdbff`, then it is the first part of the surrogate pair. The next character (second part) must have the code in interval `0xdc00..0xdfff`. These intervals are reserved exclusively for surrogate pairs by the standard.
-
-In the case above:
-
-```js run
-// charCodeAt is not surrogate-pair aware, so it gives codes for parts
-
-alert( '𝒳'.charCodeAt(0).toString(16) ); // d835, between 0xd800 and 0xdbff
-alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3, between 0xdc00 and 0xdfff
-```
-
-You will find more ways to deal with surrogate pairs later in the chapter + This functionality is supported in most modern browsers. After the actions are taken, the
alert will trigger immediately.
+
+## FinalizationRegistry
+
+Now it is time to talk about finalizers. Before we move on, let's clarify the terminology:
+
+**Cleanup callback (finalizer)** - is a function that is executed, when an object, registered in the `FinalizationRegistry`, is deleted from memory by the garbage collector.
+
+Its purpose - is to provide the ability to perform additional operations, related to the object, after it has been finally deleted from memory.
+
+**Registry** (or `FinalizationRegistry`) - is a special object in JavaScript that manages the registration and unregistration of objects and their cleanup callbacks.
+
+This mechanism allows registering an object to track and associate a cleanup callback with it.
+Essentially it is a structure that stores information about registered objects and their cleanup callbacks, and then automatically invokes those callbacks when the objects are deleted from memory.
+
+To create an instance of the `FinalizationRegistry`, it needs to call its constructor, which takes a single argument - the cleanup callback (finalizer).
+
+Syntax:
+
+```js
+function cleanupCallback(heldValue) {
+ // cleanup callback code
+}
+
+const registry = new FinalizationRegistry(cleanupCallback);
+```
+
+Here:
+
+- `cleanupCallback` - a cleanup callback that will be automatically called when a registered object is deleted from memory.
+- `heldValue` - the value that is passed as an argument to the cleanup callback. If `heldValue` is an object, the registry keeps a strong reference to it.
+- `registry` - an instance of `FinalizationRegistry`.
+
+`FinalizationRegistry` methods:
+
+- `register(target, heldValue [, unregisterToken])` - used to register objects in the registry.
+
+ `target` - the object being registered for tracking. If the `target` is garbage collected, the cleanup callback will be called with `heldValue` as its argument.
+
+ Optional `unregisterToken` – an unregistration token. It can be passed to unregister an object before the garbage collector deletes it. Typically, the `target` object is used as `unregisterToken`, which is the standard practice.
+- `unregister(unregisterToken)` - the `unregister` method is used to unregister an object from the registry. It takes one argument - `unregisterToken` (the unregister token that was obtained when registering the object).
+
+Now let's move on to a simple example. Let's use the already-known `user` object and create an instance of `FinalizationRegistry`:
+
+```js
+let user = { name: "John" };
+
+const registry = new FinalizationRegistry((heldValue) => {
+ console.log(`${heldValue} has been collected by the garbage collector.`);
+});
+```
+
+Then, we will register the object, that requires a cleanup callback by calling the `register` method:
+
+```js
+registry.register(user, user.name);
+```
+
+The registry does not keep a strong reference to the object being registered, as this would defeat its purpose. If the registry kept a strong reference, then the object would never be garbage collected.
+
+If the object is deleted by the garbage collector, our cleanup callback may be called at some point in the future, with the `heldValue` passed to it:
+
+```js
+// When the user object is deleted by the garbage collector, the following message will be printed in the console:
+"John has been collected by the garbage collector."
+```
+
+There are also situations where, even in implementations that use a cleanup callback, there is a chance that it will not be called.
+
+For example:
+- When the program fully terminates its operation (for example, when closing a tab in a browser).
+- When the `FinalizationRegistry` instance itself is no longer reachable to JavaScript code.
+ If the object that creates the `FinalizationRegistry` instance goes out of scope or is deleted, the cleanup callbacks registered in that registry might also not be invoked.
+
+## Caching with FinalizationRegistry
+
+Returning to our *weak* cache example, we can notice the following:
+- Even though the values wrapped in the `WeakRef` have been collected by the garbage collector, there is still an issue of "memory leakage" in the form of the remaining keys, whose values have been collected by the garbage collector.
+
+Here is an improved caching example using `FinalizationRegistry`:
+
+```js
+function fetchImg() {
+ // abstract function for downloading images...
+}
+
+function weakRefCache(fetchImg) {
+ const imgCache = new Map();
+
+ *!*
+ const registry = new FinalizationRegistry((imgName) => { // (1)
+ const cachedImg = imgCache.get(imgName);
+ if (cachedImg && !cachedImg.deref()) imgCache.delete(imgName);
+ });
+ */!*
+
+ return (imgName) => {
+ const cachedImg = imgCache.get(imgName);
+
+ if (cachedImg?.deref()) {
+ return cachedImg?.deref();
+ }
+
+ const newImg = fetchImg(imgName);
+ imgCache.set(imgName, new WeakRef(newImg));
+ *!*
+ registry.register(newImg, imgName); // (2)
+ */!*
+
+ return newImg;
+ };
+}
+
+const getCachedImg = weakRefCache(fetchImg);
+```
+
+1. To manage the cleanup of "dead" cache entries, when the associated `WeakRef` objects are collected by the garbage collector, we create a `FinalizationRegistry` cleanup registry.
+
+ The important point here is, that in the cleanup callback, it should be checked, if the entry was deleted by the garbage collector and not re-added, in order not to delete a "live" entry.
+2. Once the new value (image) is downloaded and put into the cache, we register it in the finalizer registry to track the `WeakRef` object.
+
+This implementation contains only actual or "live" key/value pairs.
+In this case, each `WeakRef` object is registered in the `FinalizationRegistry`.
+And after the objects are cleaned up by the garbage collector, the cleanup callback will delete all `undefined` values.
+
+Here is a visual representation of the updated code:
+
+
+
+A key aspect of the updated implementation is that finalizers allow parallel processes to be created between the "main" program and cleanup callbacks.
+In the context of JavaScript, the "main" program - is our JavaScript-code, that runs and executes in our application or web page.
+
+Hence, from the moment an object is marked for deletion by the garbage collector, and to the actual execution of the cleanup callback, there may be a certain time gap.
+It is important to understand that during this time gap, the main program can make any changes to the object or even bring it back to memory.
+
+That's why, in the cleanup callback, we must check to see if an entry has been added back to the cache by the main program to avoid deleting "live" entries.
+Similarly, when searching for a key in the cache, there is a chance that the value has been deleted by the garbage collector, but the cleanup callback has not been executed yet.
+
+Such situations require special attention if you are working with `FinalizationRegistry`.
+
+## Using WeakRef and FinalizationRegistry in practice
+
+Moving from theory to practice, imagine a real-life scenario, where a user synchronizes their photos on a mobile device with some cloud service
+(such as [iCloud](https://en.wikipedia.org/wiki/ICloud) or [Google Photos](https://en.wikipedia.org/wiki/Google_Photos)),
+and wants to view them from other devices. In addition to the basic functionality of viewing photos, such services offer a lot of additional features, for example:
+
+- Photo editing and video effects.
+- Creating "memories" and albums.
+- Video montage from a series of photos.
+- ...and much more.
+
+Here, as an example, we will use a fairly primitive implementation of such a service.
+The main point - is to show a possible scenario of using `WeakRef` and `FinalizationRegistry` together in real life.
+
+Here is what it looks like:
+
+
+
++On the left side, there is a cloud library of photos (they are displayed as thumbnails). +We can select the images we need and create a collage, by clicking the "Create collage" button on the right side of the page. +Then, the resulting collage can be downloaded as an image. +
+ +To increase page loading speed, it would be reasonable to download and display photo thumbnails in *compressed* quality. +But, to create a collage from selected photos, download and use them in *full-size* quality. + +Below, we can see, that the intrinsic size of the thumbnails is 240x240 pixels. +The size was chosen on purpose to increase loading speed. +Moreover, we do not need full-size photos in preview mode. + + + +
+Let's assume, that we need to create a collage of 4 photos: we select them, and then click the "Create collage" button. +At this stage, the already known to us
weakRefCache function checks whether the required image is in the cache.
+If not, it downloads it from the cloud and puts it in the cache for further use.
+This happens for each selected image:
++ + + + + +Paying attention to the output in the console, you can see, which of the photos were downloaded from the cloud - this is indicated by FETCHED_IMAGE. +Since this is the first attempt to create a collage, this means, that at this stage the "weak cache" was still empty, and all the photos were downloaded from the cloud and put in it. + +But, along with the process of downloading images, there is also a process of memory cleanup by the garbage collector. +This means, that the object stored in the cache, which we refer to, using a weak reference, is deleted by the garbage collector. +And our finalizer executes successfully, thereby deleting the key, by which the image was stored in the cache. +CLEANED_IMAGE notifies us about it: + + + +
+Next, we realize that we do not like the resulting collage, and decide to change one of the images and create a new one. +To do this, just deselect the unnecessary image, select another one, and click the "Create collage" button again: +
+ + + +
+But this time not all images were downloaded from the network, and one of them was taken from the weak cache: the CACHED_IMAGE message tells us about it. +This means that at the time of collage creation, the garbage collector had not yet deleted our image, and we boldly took it from the cache, +thereby reducing the number of network requests and speeding up the overall time of the collage creation process: +
+ + + +
+Let's "play around" a little more, by replacing one of the images again and creating a new collage: +
+ + + +
+This time the result is even more impressive. Of the 4 images selected, 3 of them were taken from the weak cache, and only one had to be downloaded from the network. +The reduction in network load was about 75%. Impressive, isn't it? +
+ + + + + +Of course, it is important to remember, that such behavior is not guaranteed, and depends on the specific implementation and operation of the garbage collector. + +Based on this, a completely logical question immediately arises: why do not we use an ordinary cache, where we can manage its entities ourselves, instead of relying on the garbage collector? +That's right, in the vast majority of cases there is no need to use `WeakRef` and `FinalizationRegistry`. + +Here, we simply demonstrated an alternative implementation of similar functionality, using a non-trivial approach with interesting language features. +Still, we cannot rely on this example, if we need a constant and predictable result. + +You can [open this example in the sandbox](sandbox:weakref-finalizationregistry). + +## Summary + +`WeakRef` - designed to create weak references to objects, allowing them to be deleted from memory by the garbage collector if there are no longer strong references to them. +This is beneficial for addressing excessive memory usage and optimizing the utilization of system resources in applications. + +`FinalizationRegistry` - is a tool for registering callbacks, that are executed when objects that are no longer strongly referenced, are destroyed. +This allows releasing resources associated with the object or performing other necessary operations before deleting the object from memory. \ No newline at end of file diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/google-chrome-developer-tools.png b/1-js/99-js-misc/07-weakref-finalizationregistry/google-chrome-developer-tools.png new file mode 100644 index 000000000..021637342 Binary files /dev/null and b/1-js/99-js-misc/07-weakref-finalizationregistry/google-chrome-developer-tools.png differ diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.css b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.css new file mode 100644 index 000000000..f6df812d0 --- /dev/null +++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.css @@ -0,0 +1,49 @@ +.app { + display: flex; + flex-direction: column; + gap: 16px; +} + +.start-messages { + width: fit-content; +} + +.window { + width: 100%; + border: 2px solid #464154; + overflow: hidden; +} + +.window__header { + position: sticky; + padding: 8px; + display: flex; + justify-content: space-between; + align-items: center; + background-color: #736e7e; +} + +.window__title { + margin: 0; + font-size: 24px; + font-weight: 700; + color: white; + letter-spacing: 1px; +} + +.window__button { + padding: 4px; + background: #4f495c; + outline: none; + border: 2px solid #464154; + color: white; + font-size: 16px; + cursor: pointer; +} + +.window__body { + height: 250px; + padding: 16px; + overflow: scroll; + background-color: #736e7e33; +} \ No newline at end of file diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.html b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.html new file mode 100644 index 000000000..7f93af4c7 --- /dev/null +++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.html @@ -0,0 +1,28 @@ + + + + + + +
+
+
+
+
+
+
+
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.js b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.js
new file mode 100644
index 000000000..ea55b4478
--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-dom.view/index.js
@@ -0,0 +1,24 @@
+const startMessagesBtn = document.querySelector('.start-messages'); // (1)
+const closeWindowBtn = document.querySelector('.window__button'); // (2)
+const windowElementRef = new WeakRef(document.querySelector(".window__body")); // (3)
+
+startMessagesBtn.addEventListener('click', () => { // (4)
+ startMessages(windowElementRef);
+ startMessagesBtn.disabled = true;
+});
+
+closeWindowBtn.addEventListener('click', () => document.querySelector(".window__body").remove()); // (5)
+
+
+const startMessages = (element) => {
+ const timerId = setInterval(() => { // (6)
+ if (element.deref()) { // (7)
+ const payload = document.createElement("p");
+ payload.textContent = `Message: System status OK: ${new Date().toLocaleTimeString()}`;
+ element.deref().append(payload);
+ } else { // (8)
+ alert("The element has been deleted."); // (9)
+ clearInterval(timerId);
+ }
+ }, 1000);
+};
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-01.svg b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-01.svg
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--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-01.svg
@@ -0,0 +1,32 @@
+
+
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-02.svg b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-02.svg
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--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-02.svg
@@ -0,0 +1,33 @@
+
+
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-03.svg b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-03.svg
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--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-03.svg
@@ -0,0 +1,75 @@
+
+
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-04.svg b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-04.svg
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--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-04.svg
@@ -0,0 +1,77 @@
+
+
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-05.svg b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry-05.svg
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@@ -0,0 +1,103 @@
+
+
\ No newline at end of file
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diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.css b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.css
new file mode 100644
index 000000000..e6c9e3960
--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.css
@@ -0,0 +1,285 @@
+:root {
+ --mineralGreen: 60, 98, 85;
+ --viridianGreen: 97, 135, 110;
+ --swampGreen: 166, 187, 141;
+ --fallGreen: 234, 231, 177;
+ --brinkPink: #FA7070;
+ --silverChalice: 178, 178, 178;
+ --white: 255, 255, 255;
+ --black: 0, 0, 0;
+
+ --topBarHeight: 64px;
+ --itemPadding: 32px;
+ --containerGap: 8px;
+}
+
+@keyframes zoom-in {
+ 0% {
+ transform: scale(1, 1);
+ }
+
+ 100% {
+ transform: scale(1.30, 1.30);
+ }
+}
+
+body, html {
+ margin: 0;
+ padding: 0;
+}
+
+.app {
+ min-height: 100vh;
+ background-color: rgba(var(--viridianGreen), 0.5);
+}
+
+.header {
+ height: var(--topBarHeight);
+ padding: 0 24px;
+ display: flex;
+ justify-content: space-between;
+ align-items: center;
+ background-color: rgba(var(--mineralGreen), 1);
+}
+
+.header-text {
+ color: white;
+}
+
+.container {
+ display: flex;
+ gap: 24px;
+ padding: var(--itemPadding);
+}
+
+.item {
+ width: 50%;
+}
+
+.item--scrollable {
+ overflow-y: scroll;
+ height: calc(100vh - var(--topBarHeight) - (var(--itemPadding) * 2));
+}
+
+.thumbnails-container {
+ display: flex;
+ flex-wrap: wrap;
+ gap: 8px;
+ justify-content: center;
+ align-items: center;
+}
+
+.thumbnail-item {
+ width: calc(25% - var(--containerGap));
+ cursor: pointer;
+ position: relative;
+}
+
+.thumbnail-item:hover {
+ z-index: 1;
+ animation: zoom-in 0.1s forwards;
+}
+
+.thumbnail-item--selected {
+ outline: 3px solid rgba(var(--fallGreen), 1);
+ outline-offset: -3px;
+}
+
+.badge {
+ width: 16px;
+ height: 16px;
+ display: flex;
+ justify-content: center;
+ align-items: center;
+ padding: 4px;
+ position: absolute;
+ right: 8px;
+ bottom: 8px;
+ border-radius: 50%;
+ border: 2px solid rgba(var(--fallGreen), 1);
+ background-color: rgba(var(--swampGreen), 1);
+}
+
+.check {
+ display: inline-block;
+ transform: rotate(45deg);
+ border-bottom: 2px solid white;
+ border-right: 2px solid white;
+ width: 6px;
+ height: 12px;
+}
+
+.img {
+ width: 100%;
+ height: 100%;
+ object-fit: cover;
+}
+
+.actions {
+ display: flex;
+ flex-wrap: wrap;
+ justify-content: center;
+ align-content: center;
+ padding: 0 0 16px 0;
+ gap: 8px;
+}
+
+.select {
+ padding: 16px;
+ cursor: pointer;
+ font-weight: 700;
+ color: rgba(var(--black), 1);
+ border: 2px solid rgba(var(--swampGreen), 0.5);
+ background-color: rgba(var(--swampGreen), 1);
+}
+
+.select:disabled {
+ cursor: not-allowed;
+ background-color: rgba(var(--silverChalice), 1);
+ color: rgba(var(--black), 0.5);
+ border: 2px solid rgba(var(--black), 0.25);
+}
+
+.btn {
+ outline: none;
+ padding: 16px;
+ cursor: pointer;
+ font-weight: 700;
+ color: rgba(var(--black), 1);
+ border: 2px solid rgba(var(--black), 0.5);
+}
+
+.btn--primary {
+ background-color: rgba(var(--mineralGreen), 1);
+}
+
+.btn--primary:hover:not([disabled]) {
+ background-color: rgba(var(--mineralGreen), 0.85);
+}
+
+.btn--secondary {
+ background-color: rgba(var(--viridianGreen), 0.5);
+}
+
+.btn--secondary:hover:not([disabled]) {
+ background-color: rgba(var(--swampGreen), 0.25);
+}
+
+.btn--success {
+ background-color: rgba(var(--fallGreen), 1);
+}
+
+.btn--success:hover:not([disabled]) {
+ background-color: rgba(var(--fallGreen), 0.85);
+}
+
+.btn:disabled {
+ cursor: not-allowed;
+ background-color: rgba(var(--silverChalice), 1);
+ color: rgba(var(--black), 0.5);
+ border: 2px solid rgba(var(--black), 0.25);
+}
+
+.previewContainer {
+ margin-bottom: 16px;
+ display: flex;
+ width: 100%;
+ height: 40vh;
+ overflow: scroll;
+ border: 3px solid rgba(var(--black), 1);
+}
+
+.previewContainer--disabled {
+ background-color: rgba(var(--black), 0.1);
+ cursor: not-allowed;
+}
+
+.canvas {
+ margin: auto;
+ display: none;
+}
+
+.canvas--ready {
+ display: block;
+}
+
+.spinnerContainer {
+ display: flex;
+ gap: 8px;
+ flex-direction: column;
+ align-content: center;
+ align-items: center;
+ margin: auto;
+}
+
+.spinnerContainer--hidden {
+ display: none;
+}
+
+.spinnerText {
+ margin: 0;
+ color: rgba(var(--mineralGreen), 1);
+}
+
+.spinner {
+ display: inline-block;
+ width: 50px;
+ height: 50px;
+ margin: auto;
+ border: 3px solid rgba(var(--mineralGreen), 0.3);
+ border-radius: 50%;
+ border-top-color: rgba(var(--mineralGreen), 0.9);
+ animation: spin 1s ease-in-out infinite;
+}
+
+@keyframes spin {
+ to {
+ transform: rotate(360deg);
+ }
+}
+
+.loggerContainer {
+ display: flex;
+ flex-direction: column;
+ gap: 8px;
+ padding: 0 8px 8px 8px;
+ width: 100%;
+ min-height: 30vh;
+ max-height: 30vh;
+ overflow: scroll;
+ border-left: 3px solid rgba(var(--black), 0.25);
+}
+
+.logger-title {
+ display: flex;
+ align-items: center;
+ padding: 8px;
+ position: sticky;
+ height: 40px;
+ min-height: 40px;
+ top: 0;
+ left: 0;
+ background-color: rgba(var(--viridianGreen), 1);
+ font-size: 24px;
+ font-weight: 700;
+ margin: 0;
+}
+
+.logger-item {
+ font-size: 14px;
+ padding: 8px;
+ border: 2px solid #5a5a5a;
+ color: white;
+}
+
+.logger--primary {
+ background-color: #13315a;
+}
+
+.logger--success {
+ background-color: #385a4e;
+}
+
+.logger--error {
+ background-color: #5a1a24;
+}
\ No newline at end of file
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.html b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.html
new file mode 100644
index 000000000..7ce52f927
--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.html
@@ -0,0 +1,49 @@
+
+
+
+
+
+
+
+
+
+
+ Messages:
+ +
+ No messages.
+
+
+
+
+
+
+
+
+
+
diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.js b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.js
new file mode 100644
index 000000000..983b34d9a
--- /dev/null
+++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/index.js
@@ -0,0 +1,228 @@
+import {
+ createImageFile,
+ loadImage,
+ weakRefCache,
+ LAYOUTS,
+ images,
+ THUMBNAIL_PARAMS,
+ stateObj,
+} from "./utils.js";
+
+export const state = new Proxy(stateObj, {
+ set(target, property, value) {
+ const previousValue = target[property];
+
+ target[property] = value;
+
+ if (previousValue !== value) {
+ handleStateChange(target);
+ }
+
+ return true;
+ },
+});
+
+// Elements.
+const thumbnailsContainerEl = document.querySelector(".thumbnails-container");
+const selectEl = document.querySelector(".select");
+const previewContainerEl = document.querySelector(".previewContainer");
+const canvasEl = document.querySelector(".canvas");
+const createCollageBtn = document.querySelector(".btn-create-collage");
+const startOverBtn = document.querySelector(".btn-start-over");
+const downloadBtn = document.querySelector(".btn-download");
+const spinnerContainerEl = document.querySelector(".spinnerContainer");
+const spinnerTextEl = document.querySelector(".spinnerText");
+const loggerContainerEl = document.querySelector(".loggerContainer");
+
+// Renders.
+// Render thumbnails previews.
+images.forEach((img) => {
+ const thumbnail = document.createElement("div");
+ thumbnail.classList.add("thumbnail-item");
+
+ thumbnail.innerHTML = `
+ + Photo Library Collage +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ Logger:
+Logger:
'; +}; + +const downloadCollage = () => { + const date = new Date(); + const fileName = `Collage-${date.getDay()}-${date.getMonth()}-${date.getFullYear()}.png`; + const img = canvasEl.toDataURL("image/png"); + const link = document.createElement("a"); + link.download = fileName; + link.href = img; + link.click(); + link.remove(); +}; + +const changeLayout = ({ target }) => { + state.currentLayout = JSON.parse(target.value); +}; + +// Listeners. +selectEl.addEventListener("change", changeLayout); +createCollageBtn.addEventListener("click", createCollage); +startOverBtn.addEventListener("click", startOver); +downloadBtn.addEventListener("click", downloadCollage); diff --git a/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/utils.js b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/utils.js new file mode 100644 index 000000000..f0140c116 --- /dev/null +++ b/1-js/99-js-misc/07-weakref-finalizationregistry/weakref-finalizationregistry.view/utils.js @@ -0,0 +1,321 @@ +const loggerContainerEl = document.querySelector(".loggerContainer"); + +export const images = [ + { + img: "https://images.unsplash.com/photo-1471357674240-e1a485acb3e1", + }, + { + img: "https://images.unsplash.com/photo-1589118949245-7d38baf380d6", + }, + { + img: "https://images.unsplash.com/photo-1527631746610-bca00a040d60", + }, + { + img: "https://images.unsplash.com/photo-1500835556837-99ac94a94552", + }, + { + img: "https://images.unsplash.com/photo-1503220317375-aaad61436b1b", + }, + { + img: "https://images.unsplash.com/photo-1501785888041-af3ef285b470", + }, + { + img: "https://images.unsplash.com/photo-1528543606781-2f6e6857f318", + }, + { + img: "https://images.unsplash.com/photo-1523906834658-6e24ef2386f9", + }, + { + img: "https://images.unsplash.com/photo-1539635278303-d4002c07eae3", + }, + { + img: "https://images.unsplash.com/photo-1533105079780-92b9be482077", + }, + { + img: "https://images.unsplash.com/photo-1516483638261-f4dbaf036963", + }, + { + img: "https://images.unsplash.com/photo-1502791451862-7bd8c1df43a7", + }, + { + img: "https://plus.unsplash.com/premium_photo-1663047367140-91adf819d007", + }, + { + img: "https://images.unsplash.com/photo-1506197603052-3cc9c3a201bd", + }, + { + img: "https://images.unsplash.com/photo-1517760444937-f6397edcbbcd", + }, + { + img: "https://images.unsplash.com/photo-1518684079-3c830dcef090", + }, + { + img: "https://images.unsplash.com/photo-1505832018823-50331d70d237", + }, + { + img: "https://images.unsplash.com/photo-1524850011238-e3d235c7d4c9", + }, + { + img: "https://plus.unsplash.com/premium_photo-1661277758451-b5053309eea1", + }, + { + img: "https://images.unsplash.com/photo-1541410965313-d53b3c16ef17", + }, + { + img: "https://images.unsplash.com/photo-1528702748617-c64d49f918af", + }, + { + img: "https://images.unsplash.com/photo-1502003148287-a82ef80a6abc", + }, + { + img: "https://plus.unsplash.com/premium_photo-1661281272544-5204ea3a481a", + }, + { + img: "https://images.unsplash.com/photo-1503457574462-bd27054394c1", + }, + { + img: "https://images.unsplash.com/photo-1499363536502-87642509e31b", + }, + { + img: "https://images.unsplash.com/photo-1551918120-9739cb430c6d", + }, + { + img: "https://plus.unsplash.com/premium_photo-1661382219642-43e54f7e81d7", + }, + { + img: "https://images.unsplash.com/photo-1497262693247-aa258f96c4f5", + }, + { + img: "https://images.unsplash.com/photo-1525254134158-4fd5fdd45793", + }, + { + img: "https://plus.unsplash.com/premium_photo-1661274025419-4c54107d5c48", + }, + { + img: "https://images.unsplash.com/photo-1553697388-94e804e2f0f6", + }, + { + img: "https://images.unsplash.com/photo-1574260031597-bcd9eb192b4f", + }, + { + img: "https://images.unsplash.com/photo-1536323760109-ca8c07450053", + }, + { + img: "https://images.unsplash.com/photo-1527824404775-dce343118ebc", + }, + { + img: "https://images.unsplash.com/photo-1612278675615-7b093b07772d", + }, + { + img: "https://images.unsplash.com/photo-1522010675502-c7b3888985f6", + }, + { + img: "https://images.unsplash.com/photo-1501555088652-021faa106b9b", + }, + { + img: "https://plus.unsplash.com/premium_photo-1669223469435-27e091439169", + }, + { + img: "https://images.unsplash.com/photo-1506012787146-f92b2d7d6d96", + }, + { + img: "https://images.unsplash.com/photo-1511739001486-6bfe10ce785f", + }, + { + img: "https://images.unsplash.com/photo-1553342385-111fd6bc6ab3", + }, + { + img: "https://images.unsplash.com/photo-1516546453174-5e1098a4b4af", + }, + { + img: "https://images.unsplash.com/photo-1527142879-95b61a0b8226", + }, + { + img: "https://images.unsplash.com/photo-1520466809213-7b9a56adcd45", + }, + { + img: "https://images.unsplash.com/photo-1516939884455-1445c8652f83", + }, + { + img: "https://images.unsplash.com/photo-1545389336-cf090694435e", + }, + { + img: "https://plus.unsplash.com/premium_photo-1669223469455-b7b734c838f4", + }, + { + img: "https://images.unsplash.com/photo-1454391304352-2bf4678b1a7a", + }, + { + img: "https://images.unsplash.com/photo-1433838552652-f9a46b332c40", + }, + { + img: "https://images.unsplash.com/photo-1506125840744-167167210587", + }, + { + img: "https://images.unsplash.com/photo-1522199873717-bc67b1a5e32b", + }, + { + img: "https://images.unsplash.com/photo-1495904786722-d2b5a19a8535", + }, + { + img: "https://images.unsplash.com/photo-1614094082869-cd4e4b2905c7", + }, + { + img: "https://images.unsplash.com/photo-1474755032398-4b0ed3b2ae5c", + }, + { + img: "https://images.unsplash.com/photo-1501554728187-ce583db33af7", + }, + { + img: "https://images.unsplash.com/photo-1515859005217-8a1f08870f59", + }, + { + img: "https://images.unsplash.com/photo-1531141445733-14c2eb7d4c1f", + }, + { + img: "https://images.unsplash.com/photo-1500259783852-0ca9ce8a64dc", + }, + { + img: "https://images.unsplash.com/photo-1510662145379-13537db782dc", + }, + { + img: "https://images.unsplash.com/photo-1573790387438-4da905039392", + }, + { + img: "https://images.unsplash.com/photo-1512757776214-26d36777b513", + }, + { + img: "https://images.unsplash.com/photo-1518855706573-84de4022b69b", + }, + { + img: "https://images.unsplash.com/photo-1500049242364-5f500807cdd7", + }, + { + img: "https://images.unsplash.com/photo-1528759335187-3b683174c86a", + }, +]; +export const THUMBNAIL_PARAMS = "w=240&h=240&fit=crop&auto=format"; + +// Console styles. +export const CONSOLE_BASE_STYLES = [ + "font-size: 12px", + "padding: 4px", + "border: 2px solid #5a5a5a", + "color: white", +].join(";"); +export const CONSOLE_PRIMARY = [ + CONSOLE_BASE_STYLES, + "background-color: #13315a", +].join(";"); +export const CONSOLE_SUCCESS = [ + CONSOLE_BASE_STYLES, + "background-color: #385a4e", +].join(";"); +export const CONSOLE_ERROR = [ + CONSOLE_BASE_STYLES, + "background-color: #5a1a24", +].join(";"); + +// Layouts. +export const LAYOUT_4_COLUMNS = { + name: "Layout 4 columns", + columns: 4, + itemWidth: 240, + itemHeight: 240, +}; +export const LAYOUT_8_COLUMNS = { + name: "Layout 8 columns", + columns: 8, + itemWidth: 240, + itemHeight: 240, +}; +export const LAYOUTS = [LAYOUT_4_COLUMNS, LAYOUT_8_COLUMNS]; + +export const createImageFile = async (src) => + new Promise((resolve, reject) => { + const img = new Image(); + img.src = src; + img.onload = () => resolve(img); + img.onerror = () => reject(new Error("Failed to construct image.")); + }); + +export const loadImage = async (url) => { + try { + const response = await fetch(url); + if (!response.ok) { + throw new Error(String(response.status)); + } + + return await response.blob(); + } catch (e) { + console.log(`%cFETCHED_FAILED: ${e}`, CONSOLE_ERROR); + } +}; + +export const weakRefCache = (fetchImg) => { + const imgCache = new Map(); + const registry = new FinalizationRegistry(({ imgName, size, type }) => { + const cachedImg = imgCache.get(imgName); + if (cachedImg && !cachedImg.deref()) { + imgCache.delete(imgName); + console.log( + `%cCLEANED_IMAGE: Url: ${imgName}, Size: ${size}, Type: ${type}`, + CONSOLE_ERROR, + ); + + const logEl = document.createElement("div"); + logEl.classList.add("logger-item", "logger--error"); + logEl.innerHTML = `CLEANED_IMAGE: Url: ${imgName}, Size: ${size}, Type: ${type}`; + loggerContainerEl.appendChild(logEl); + loggerContainerEl.scrollTop = loggerContainerEl.scrollHeight; + } + }); + + return async (imgName) => { + const cachedImg = imgCache.get(imgName); + + if (cachedImg?.deref() !== undefined) { + console.log( + `%cCACHED_IMAGE: Url: ${imgName}, Size: ${cachedImg.size}, Type: ${cachedImg.type}`, + CONSOLE_SUCCESS, + ); + + const logEl = document.createElement("div"); + logEl.classList.add("logger-item", "logger--success"); + logEl.innerHTML = `CACHED_IMAGE: Url: ${imgName}, Size: ${cachedImg.size}, Type: ${cachedImg.type}`; + loggerContainerEl.appendChild(logEl); + loggerContainerEl.scrollTop = loggerContainerEl.scrollHeight; + + return cachedImg?.deref(); + } + + const newImg = await fetchImg(imgName); + console.log( + `%cFETCHED_IMAGE: Url: ${imgName}, Size: ${newImg.size}, Type: ${newImg.type}`, + CONSOLE_PRIMARY, + ); + + const logEl = document.createElement("div"); + logEl.classList.add("logger-item", "logger--primary"); + logEl.innerHTML = `FETCHED_IMAGE: Url: ${imgName}, Size: ${newImg.size}, Type: ${newImg.type}`; + loggerContainerEl.appendChild(logEl); + loggerContainerEl.scrollTop = loggerContainerEl.scrollHeight; + + imgCache.set(imgName, new WeakRef(newImg)); + registry.register(newImg, { + imgName, + size: newImg.size, + type: newImg.type, + }); + + return newImg; + }; +}; + +export const stateObj = { + loading: false, + drawing: true, + collageRendered: false, + currentLayout: LAYOUTS[0], + selectedImages: new Set(), +}; diff --git a/2-ui/1-document/01-browser-environment/article.md b/2-ui/1-document/01-browser-environment/article.md index 56b568833..eedc28fb3 100644 --- a/2-ui/1-document/01-browser-environment/article.md +++ b/2-ui/1-document/01-browser-environment/article.md @@ -1,10 +1,10 @@ # Browser environment, specs -The JavaScript language was initially created for web browsers. Since then it has evolved and become a language with many uses and platforms. +The JavaScript language was initially created for web browsers. Since then, it has evolved into a language with many uses and platforms. -A platform may be a browser, or a web-server or another *host*, even a "smart" coffee machine, if it can run JavaScript. Each of them provides platform-specific functionality. The JavaScript specification calls that a *host environment*. +A platform may be a browser, or a web-server or another *host*, or even a "smart" coffee machine if it can run JavaScript. Each of these provides platform-specific functionality. The JavaScript specification calls that a *host environment*. -A host environment provides own objects and functions additional to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on. +A host environment provides its own objects and functions in addition to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on. Here's a bird's-eye view of what we have when JavaScript runs in a web browser: @@ -15,9 +15,9 @@ There's a "root" object called `window`. It has two roles: 1. First, it is a global object for JavaScript code, as described in the chapter- Chapter 1 -
- Chapter 1 +
- Chapter 2
Hello
`. + - [Comment](https://dom.spec.whatwg.org/#interface-comment) -- the class for comments. They are not shown, but each comment becomes a member of DOM. + +- [Element](https://dom.spec.whatwg.org/#interface-element) -- is the base class for DOM elements. + + It provides element-level navigation like `nextElementSibling`, `children` and searching methods like `getElementsByTagName`, `querySelector`. + + A browser supports not only HTML, but also XML and SVG. So the `Element` class serves as a base for more specific classes: `SVGElement`, `XMLElement` (we don't need them here) and `HTMLElement`. + +- Finally, [HTMLElement](https://html.spec.whatwg.org/multipage/dom.html#htmlelement) is the basic class for all HTML elements. We'll work with it most of the time. + + It is inherited by concrete HTML elements: - [HTMLInputElement](https://html.spec.whatwg.org/multipage/forms.html#htmlinputelement) -- the class for `` elements, - [HTMLBodyElement](https://html.spec.whatwg.org/multipage/semantics.html#htmlbodyelement) -- the class for `` elements, - [HTMLAnchorElement](https://html.spec.whatwg.org/multipage/semantics.html#htmlanchorelement) -- the class for `` elements, - - ...and so on, each tag has its own class that may provide specific properties and methods. + - ...and so on. + +There are many other tags with their own classes that may have specific properties and methods, while some elements, such as ``, `