Merge pull request #183

main sync

Author: bogdanbacosca

1-js/05-data-types/01-primitives-methods/1-string-new-property/task.md

@@ -15,4 +15,4 @@ str.test = 5;
 alert(str.test);
 ```
 
-How do you think, will it work? What will be shown?
+What do you think, will it work? What will be shown?

1-js/05-data-types/01-primitives-methods/article.md

@@ -39,7 +39,7 @@ Objects are "heavier" than primitives. They require additional resources to supp
 
 Here's the paradox faced by the creator of JavaScript:
 
-- There are many things one would want to do with a primitive like a string or a number. It would be great to access them using methods.
+- There are many things one would want to do with a primitive, like a string or a number. It would be great to access them using methods.
 - Primitives must be as fast and lightweight as possible.
 
 The solution looks a little bit awkward, but here it is:
@@ -104,9 +104,10 @@ if (zero) { // zero is true, because it's an object
 }
 ```
 
-On the other hand, using the same functions `String/Number/Boolean` without `new` is a totally sane and useful thing. They convert a value to the corresponding type: to a string, a number, or a boolean (primitive).
+On the other hand, using the same functions `String/Number/Boolean` without `new` is totally fine and useful thing. They convert a value to the corresponding type: to a string, a number, or a boolean (primitive).
 
 For example, this is entirely valid:
+
 ```js
 let num = Number("123"); // convert a string to number
 ```

1-js/05-data-types/02-number/2-why-rounded-down/solution.md

@@ -28,6 +28,6 @@ Note that `63.5` has no precision loss at all. That's because the decimal part `
 
 
 ```js run
-alert( Math.round(6.35 * 10) / 10); // 6.35 -> 63.5 -> 64(rounded) -> 6.4
+alert( Math.round(6.35 * 10) / 10 ); // 6.35 -> 63.5 -> 64(rounded) -> 6.4
 ```
 

1-js/05-data-types/02-number/article.md

@@ -2,9 +2,9 @@
 
 In modern JavaScript, there are two types of numbers:
 
-1. Regular numbers in JavaScript are stored in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754-2008_revision), also known as "double precision floating point numbers". These are numbers that we're using most of the time, and we'll talk about them in this chapter.
+1. Regular numbers in JavaScript are stored in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754), also known as "double precision floating point numbers". These are numbers that we're using most of the time, and we'll talk about them in this chapter.
 
-2. BigInt numbers, to represent integers of arbitrary length. They are sometimes needed, because a regular number can't safely exceed <code>2<sup>53</sup></code> or be less than <code>-2<sup>53</sup></code>. As bigints are used in few special areas, we devote them a special chapter <info:bigint>.
+2. BigInt numbers represent integers of arbitrary length. They are sometimes needed because a regular integer number can't safely exceed <code>(2<sup>53</sup>-1)</code> or be less than <code>-(2<sup>53</sup>-1)</code>, as we mentioned earlier in the chapter <info:types>. As bigints are used in few special areas, we devote them a special chapter <info:bigint>.
 
 So here we'll talk about regular numbers. Let's expand our knowledge of them.
 
@@ -22,7 +22,7 @@ We also can use underscore `_` as the separator:
 let billion = 1_000_000_000;
 ```
 
-Here the underscore `_` plays the role of the "syntactic sugar", it makes the number more readable. The JavaScript engine simply ignores `_` between digits, so it's exactly the same one billion as above.
+Here the underscore `_` plays the role of the "[syntactic sugar](https://en.wikipedia.org/wiki/Syntactic_sugar)", it makes the number more readable. The JavaScript engine simply ignores `_` between digits, so it's exactly the same one billion as above.
 
 In real life though, we try to avoid writing long sequences of zeroes. We're too lazy for that. We'll try to write something like `"1bn"` for a billion or `"7.3bn"` for 7 billion 300 million. The same is true for most large numbers.
 
@@ -47,10 +47,10 @@ Now let's write something very small. Say, 1 microsecond (one millionth of a sec
 let mсs = 0.000001;
 ```
 
-Just like before, using `"e"` can help. If we'd like to avoid writing the zeroes explicitly, we could say the same as:
+Just like before, using `"e"` can help. If we'd like to avoid writing the zeroes explicitly, we could write the same as:
 
 ```js
-let mcs = 1e-6; // six zeroes to the left from 1
+let mcs = 1e-6; // five zeroes to the left from 1
 ```
 
 If we count the zeroes in `0.000001`, there are 6 of them. So naturally it's `1e-6`.
@@ -63,6 +63,9 @@ In other words, a negative number after `"e"` means a division by 1 with the giv
 
 // -6 divides by 1 with 6 zeroes
 1.23e-6 === 1.23 / 1000000; // 0.00000123
+
+// an example with a bigger number
+1234e-2 === 1234 / 100; // 12.34, decimal point moves 2 times
 ```
 
 ### Hex, binary and octal numbers
@@ -157,7 +160,7 @@ There are two ways to do so:
 
 1. Multiply-and-divide.
 
-    For example, to round the number to the 2nd digit after the decimal, we can multiply the number by `100` (or a bigger power of 10), call the rounding function and then divide it back.
+    For example, to round the number to the 2nd digit after the decimal, we can multiply the number by `100`, call the rounding function and then divide it back.
     ```js run
     let num = 1.23456;
 
@@ -178,28 +181,28 @@ There are two ways to do so:
     alert( num.toFixed(1) ); // "12.4"
     ```
 
-    Please note that result of `toFixed` is a string. If the decimal part is shorter than required, zeroes are appended to the end:
+    Please note that the result of `toFixed` is a string. If the decimal part is shorter than required, zeroes are appended to the end:
 
     ```js run
     let num = 12.34;
     alert( num.toFixed(5) ); // "12.34000", added zeroes to make exactly 5 digits
     ```
 
-    We can convert it to a number using the unary plus or a `Number()` call: `+num.toFixed(5)`.
+    We can convert it to a number using the unary plus or a `Number()` call, e.g write `+num.toFixed(5)`.
 
 ## Imprecise calculations
 
-Internally, a number is represented in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754-2008_revision), so there are exactly 64 bits to store a number: 52 of them are used to store the digits, 11 of them store the position of the decimal point (they are zero for integer numbers), and 1 bit is for the sign.
+Internally, a number is represented in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754), so there are exactly 64 bits to store a number: 52 of them are used to store the digits, 11 of them store the position of the decimal point, and 1 bit is for the sign.
 
-If a number is too big, it would overflow the 64-bit storage, potentially giving an infinity:
+If a number is really huge, it may overflow the 64-bit storage and become a special numeric value `Infinity`:
 
 ```js run
 alert( 1e500 ); // Infinity
 ```
 
 What may be a little less obvious, but happens quite often, is the loss of precision.
 
-Consider this (falsy!) test:
+Consider this (falsy!) equality test:
 
 ```js run
 alert( 0.1 + 0.2 == 0.3 ); // *!*false*/!*
@@ -213,13 +216,13 @@ Strange! What is it then if not `0.3`?
 alert( 0.1 + 0.2 ); // 0.30000000000000004
 ```
 
-Ouch! There are more consequences than an incorrect comparison here. Imagine you're making an e-shopping site and the visitor puts `$0.10` and `$0.20` goods into their cart. The order total will be `$0.30000000000000004`. That would surprise anyone.
+Ouch! Imagine you're making an e-shopping site and the visitor puts `$0.10` and `$0.20` goods into their cart. The order total will be `$0.30000000000000004`. That would surprise anyone.
 
 But why does this happen?
 
 A number is stored in memory in its binary form, a sequence of bits - ones and zeroes. But fractions like `0.1`, `0.2` that look simple in the decimal numeric system are actually unending fractions in their binary form.
 
-In other words, what is `0.1`? It is one divided by ten `1/10`, one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: `1/3`. It becomes an endless fraction `0.33333(3)`.
+What is `0.1`? It is one divided by ten `1/10`, one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: `1/3`. It becomes an endless fraction `0.33333(3)`.
 
 So, division by powers `10` is guaranteed to work well in the decimal system, but division by `3` is not. For the same reason, in the binary numeral system, the division by powers of `2` is guaranteed to work, but `1/10` becomes an endless binary fraction.
 
@@ -246,7 +249,7 @@ Can we work around the problem? Sure, the most reliable method is to round the r
 
 ```js run
 let sum = 0.1 + 0.2;
-alert( sum.toFixed(2) ); // 0.30
+alert( sum.toFixed(2) ); // "0.30"
 ```
 
 Please note that `toFixed` always returns a string. It ensures that it has 2 digits after the decimal point. That's actually convenient if we have an e-shopping and need to show `$0.30`. For other cases, we can use the unary plus to coerce it into a number:
@@ -305,7 +308,7 @@ They belong to the type `number`, but are not "normal" numbers, so there are spe
     alert( isNaN("str") ); // true
     ```
 
-    But do we need this function? Can't we just use the comparison `=== NaN`? Sorry, but the answer is no. The value `NaN` is unique in that it does not equal anything, including itself:
+    But do we need this function? Can't we just use the comparison `=== NaN`? Unfortunately not. The value `NaN` is unique in that it does not equal anything, including itself:
 
     ```js run
     alert( NaN === NaN ); // false
@@ -331,16 +334,44 @@ alert( isFinite(num) );
 
 Please note that an empty or a space-only string is treated as `0` in all numeric functions including `isFinite`.
 
-```smart header="Compare with `Object.is`"
+````smart header="`Number.isNaN` and `Number.isFinite`"
+[Number.isNaN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN) and [Number.isFinite](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isFinite) methods are the more "strict" versions of `isNaN` and `isFinite` functions. They do not autoconvert their argument into a number, but check if it belongs to the `number` type instead.
+
+- `Number.isNaN(value)` returns `true` if the argument belongs to the `number` type and it is `NaN`. In any other case it returns `false`.
+
+    ```js run
+    alert( Number.isNaN(NaN) ); // true
+    alert( Number.isNaN("str" / 2) ); // true
+
+    // Note the difference:
+    alert( Number.isNaN("str") ); // false, because "str" belongs to the string type, not the number type
+    alert( isNaN("str") ); // true, because isNaN converts string "str" into a number and gets NaN as a result of this conversion
+    ```
+
+- `Number.isFinite(value)` returns `true` if the argument belongs to the `number` type and it is not `NaN/Infinity/-Infinity`. In any other case it returns `false`.
 
-There is a special built-in method [`Object.is`](mdn:js/Object/is) that compares values like `===`, but is more reliable for two edge cases:
+    ```js run
+    alert( Number.isFinite(123) ); // true
+    alert( Number.isFinite(Infinity) ); // false
+    alert( Number.isFinite(2 / 0) ); // false
+
+    // Note the difference:
+    alert( Number.isFinite("123") ); // false, because "123" belongs to the string type, not the number type
+    alert( isFinite("123") ); // true, because isFinite converts string "123" into a number 123
+    ```
+
+In a way, `Number.isNaN` and `Number.isFinite` are simpler and more straightforward than `isNaN` and `isFinite` functions. In practice though, `isNaN` and `isFinite` are mostly used, as they're shorter to write.
+````
+
+```smart header="Comparison with `Object.is`"
+There is a special built-in method `Object.is` that compares values like `===`, but is more reliable for two edge cases:
 
 1. It works with `NaN`: `Object.is(NaN, NaN) === true`, that's a good thing.
-2. Values `0` and `-0` are different: `Object.is(0, -0) === false`, technically that's true, because internally the number has a sign bit that may be different even if all other bits are zeroes.
+2. Values `0` and `-0` are different: `Object.is(0, -0) === false`, technically that's correct, because internally the number has a sign bit that may be different even if all other bits are zeroes.
 
 In all other cases, `Object.is(a, b)` is the same as `a === b`.
 
-This way of comparison is often used in JavaScript specification. When an internal algorithm needs to compare two values for being exactly the same, it uses `Object.is` (internally called [SameValue](https://tc39.github.io/ecma262/#sec-samevalue)).
+We mention `Object.is` here, because it's often used in JavaScript specification. When an internal algorithm needs to compare two values for being exactly the same, it uses `Object.is` (internally called [SameValue](https://tc39.github.io/ecma262/#sec-samevalue)).
 ```
 
 
@@ -400,8 +431,8 @@ A few examples:
     alert( Math.random() ); // ... (any random numbers)
     ```
 
-`Math.max(a, b, c...)` / `Math.min(a, b, c...)`
-: Returns the greatest/smallest from the arbitrary number of arguments.
+`Math.max(a, b, c...)` and `Math.min(a, b, c...)`
+: Returns the greatest and smallest from the arbitrary number of arguments.
 
     ```js run
     alert( Math.max(3, 5, -10, 0, 1) ); // 5
@@ -430,6 +461,13 @@ For different numeral systems:
 - `parseInt(str, base)` parses the string `str` into an integer in numeral system with given `base`, `2 ≤ base ≤ 36`.
 - `num.toString(base)` converts a number to a string in the numeral system with the given `base`.
 
+For regular number tests:
+
+- `isNaN(value)` converts its argument to a number and then tests it for being `NaN`
+- `Number.isNaN(value)` checks whether its argument belongs to the `number` type, and if so, tests it for being `NaN`
+- `isFinite(value)` converts its argument to a number and then tests it for not being `NaN/Infinity/-Infinity`
+- `Number.isFinite(value)` checks whether its argument belongs to the `number` type, and if so, tests it for not being `NaN/Infinity/-Infinity`
+
 For converting values like `12pt` and `100px` to a number:
 
 - Use `parseInt/parseFloat` for the "soft" conversion, which reads a number from a string and then returns the value they could read before the error.

1-js/05-data-types/03-string/1-ucfirst/solution.md

@@ -8,12 +8,7 @@ let newStr = str[0].toUpperCase() + str.slice(1);
 
 There's a small problem though. If `str` is empty, then `str[0]` is `undefined`, and as `undefined` doesn't have the `toUpperCase()` method, we'll get an error.
 
-There are two variants here:
-
-1. Use `str.charAt(0)`, as it always returns a string (maybe empty).
-2. Add a test for an empty string.
-
-Here's the 2nd variant:
+The easiest way out is to add a test for an empty string, like this:
 
 ```js run demo
 function ucFirst(str) {
@@ -24,4 +19,3 @@ function ucFirst(str) {
 
 alert( ucFirst("john") ); // John
 ```
-