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‎1-js/01-getting-started/1-intro/article.md‎

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‎1-js/02-first-steps/01-hello-world/1-hello-alert/index.html‎

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<!DOCTYPE html>
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<html>
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<body>
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<script>
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alert( "I'm JavaScript!" );
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</script>
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</body>
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</html>

‎1-js/02-first-steps/01-hello-world/1-hello-alert/solution.md‎

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[html src="index.html"]

‎1-js/02-first-steps/01-hello-world/article.md‎

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আমরা পুরো URL টিও দিতে পারি। উদাহরণস্বরূপ:
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```html
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<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js"></script>
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<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.js"></script>
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```
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একাধিক ট্যাগ ব্যবহার করে, একাধিক স্ক্রিপ্টগুলো সংযুক্ত করতে পারিঃ

‎1-js/02-first-steps/02-structure/article.md‎

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আমরা স্টেটমেন্টের শেষে সেমিকোলন দিতে পরামর্শ দেই, এমনকি যদি স্টেটমেন্টগুলো আলাদা লাইনেও হয়ে থাকে। এই রুলটি কমিউনিটিতে ব্যাপকভাবে গ্রহণ করা হয়েছে। আরও একবার এভাবে বলা যায় -- অধিকাংশ সময় সেমিকোলন ঊহ্য রাখা **সম্ভব**। কিন্তু এটি ব্যবহার করা নিরাপদ -- বিশেষ করে শিক্ষানবিশ/অনভিজ্ঞদের জন্য।
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## কমেন্ট/মন্তব্য
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## কমেন্ট/মন্তব্য [#code-comments]
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সময়ের সাথে সাথে প্রোগ্রামগুলো অধিক থেকে অধিকতর জটিল হতে থাকে। *কমেন্ট/মন্তব্য* লিখার মাধ্যমে কোড কি কাজ করে এবং কেন করে তা প্রয়োজনীয় হয়ে দাঁড়ায়।
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alert('ওয়ার্ল্ড');
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```
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```smart header="হট-কী ব্যবহার করুন"
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```smart header="Use hotkeys!"
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অধিকাংশ এডিটরে, কোডের কোন অংশ কমেন্ট করতে, এক লাইনের কমেন্টের জন্য `key:Ctrl+/` হট-কী এবং একাধিক লাইনের কমেন্টের জন্য `key:Ctrl+Shift+/` হট-কী ব্যবহার করা হয় (কোডের অংশটি সিলেক্ট করে হট-কী প্রেস করা হয়)। ম্যাকের জন্য `key:Ctrl` এর পরিবর্তে `key:Cmd` ব্যবহার করে চেষ্টা করে দেখুন।
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```
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‎1-js/02-first-steps/03-strict-mode/article.md‎

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...
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```
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<<<<<<< HEAD
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শিঘ্রই আমরা ফাংশন (কমান্ড গ্রুপ করার একটি উপায়) শিখব। একটু এগিয়ে গিয়ে নোট করে নেয়া যাক, `"use strict"` কোন ফাংশনের বডির শুরুতেও দেয়া যায়। এতে করে শুধু ঐ ফাংশনে স্ট্রিক্ট মোড সক্রিয় হয়। তবে সাধারণত পুরো স্ক্রিপ্টের জন্যই স্ট্রিক্ট মোড ব্যবহার করা হয়।
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=======
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Quite soon we're going to learn functions (a way to group commands), so let's note in advance that `"use strict"` can be put at the beginning of a function. Doing that enables strict mode in that function only. But usually people use it for the whole script.
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>>>>>>> d6e88647b42992f204f57401160ebae92b358c0d
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````warn header="\"use strict\" সবার শুরুতে থাকতে হবে"
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নিশ্চিত করুন যেন `"use strict"` আপনার স্ক্রিপ্টগুলোর একেবারে সবার উপরে থাকে। নাহলে স্ট্রিক্ট মোড নাও সক্রিয় হতে পারে।
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## ব্রাউজার কনসোল
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<<<<<<< HEAD
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ভবিষ্যতে আপনি যখন ব্রাউজার কনসোলে কোন ফিচার পরীক্ষা করতে যাবেন, মনে রাখবেন ডিফল্টভাবে সেটা স্ট্রিক্ট মোডে থাকে না।
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=======
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When you use a [developer console](info:devtools) to run code, please note that it doesn't `use strict` by default.
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কখনো কখনো যখন `use strict`-এ পার্থক্য হয়, আপনি ভুল ফলাফল পাবেন।
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<<<<<<< HEAD
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একাধিক লাইন দেয়ার জন্য `key:Shift+Enter` চেপে সবার উপরে `use strict` দিয়ে দেখতে পারেন, এভাবে:
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=======
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So, how to actually `use strict` in the console?
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First, you can try to press `key:Shift+Enter` to input multiple lines, and put `use strict` on top, like this:
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```js
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'use strict'; <নতুন লাইনের জন্য Shift+Enter>
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বেশিরভাগ ব্রাউজার যেমন ক্রোম, ফায়ারফক্সে এটা কাজ করে।
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<<<<<<< HEAD
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যদি কাজ না করে তাহলে `use strict` নিশ্চিত করার সবচেয়ে নির্ভরযোগ্য উপায় হল কোডটাকে কনসোলে এভাবে ইনপুট দেয়া:
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=======
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If it doesn't, e.g. in an old browser, there's an ugly, but reliable way to ensure `use strict`. Put it inside this kind of wrapper:
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>>>>>>> d6e88647b42992f204f57401160ebae92b358c0d
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```js
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(function() {
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'use strict';
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<<<<<<< HEAD
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// ...আপনার কোড...
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})()
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```
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২. একটি স্ক্রিপ্ট কিংবা ফাংশনের সবার উপরে `"use strict"` দিয়ে স্ট্রিক্ট মোড সক্রিয় করা যায়। ভাষার কিছু ফিচার, যেমন "classes" এবং "modules" স্বয়ংক্রিয়ভাবে স্ট্রিক্ট মোড সক্রিয় করে।
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৩. সব মর্ডান ব্রাউজারই স্ট্রিক্ট মোড সমর্থন করে।
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৪. আমরা পরামর্শ দিয়েছি সব সময় `"use strict"` দিয়ে স্ক্রিপ্ট শুরু করতে। এই টিউটোরিয়ালের সকল উদাহরণ ধরে নেয় যে স্ট্রিক্ট মোড চালু আছে, যদিনা অন্যথা (খুবই বিরল) উল্লেখ করে দেয়া থাকে।
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=======
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// ...your code here...
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})()
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```
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## Should we "use strict"?
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The question may sound obvious, but it's not so.
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One could recommend to start scripts with `"use strict"`... But you know what's cool?
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Modern JavaScript supports "classes" and "modules" - advanced language structures (we'll surely get to them), that enable `use strict` automatically. So we don't need to add the `"use strict"` directive, if we use them.
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**So, for now `"use strict";` is a welcome guest at the top of your scripts. Later, when your code is all in classes and modules, you may omit it.**
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As of now, we've got to know about `use strict` in general.
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In the next chapters, as we learn language features, we'll see the differences between the strict and old modes. Luckily, there aren't many and they actually make our lives better.
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All examples in this tutorial assume strict mode unless (very rarely) specified otherwise.
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>>>>>>> d6e88647b42992f204f57401160ebae92b358c0d

‎1-js/02-first-steps/04-variables/article.md‎

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Technically, all these variants do the same thing. So, it's a matter of personal taste and aesthetics.
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````smart header="`var` instead of `let`"
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In older scripts, you may also find another keyword: `var` instead of `let`:
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alert(message); // Hello world!
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````warn header="Declaring twice triggers an error"
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A variable should be declared only once.
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A repeated declaration of the same variable is an error:
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```js run
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let message = "This";
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// repeated 'let' leads to an error
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let message = "That"; // SyntaxError: 'message' has already been declared
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```
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So, we should declare a variable once and then refer to it without `let`.
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````
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```smart header="Functional languages"
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It's interesting to note that there exist [functional](https://en.wikipedia.org/wiki/Functional_programming) programming languages, like [Scala](http://www.scala-lang.org/) or [Erlang](http://www.erlang.org/) that forbid changing variable values.
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Technically, there is no error here, such names are allowed, but there is an international tradition to use English in variable names. Even if we're writing a small script, it may have a long life ahead. People from other countries may need to read it some time.
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Technically, there is no error here. Such names are allowed, but there is an international convention to use English in variable names. Even if we're writing a small script, it may have a long life ahead. People from other countries may need to read it some time.
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````warn header="Reserved names"

‎1-js/02-first-steps/05-types/article.md‎

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# Data types
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A variable in JavaScript can contain any data. A variable can at one moment be a string and at another be a number:
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A value in JavaScript is always of a certain type. For example, a string or a number.
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There are eight basic data types in JavaScript. Here, we'll cover them in general and in the next chapters we'll talk about each of them in detail.
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We can put any type in a variable. For example, a variable can at one moment be a string and then store a number:
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Programming languages that allow such things are called "dynamically typed", meaning that there are data types, but variables are not bound to any of them.
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There are eight basic data types in JavaScript. Here, we'll cover them in general and in the next chapters we'll talk about each of them in detail.
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Programming languages that allow such things, such as JavaScript, are called "dynamically typed", meaning that there exist data types, but variables are not bound to any of them.
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## BigInt
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In JavaScript, the "number" type cannot represent integer values larger than <code>2<sup>53</sup></code> (or less than <code>-2<sup>53</sup></code> for negatives), that's a technical limitation caused by their internal representation. That's about 16 decimal digits, so for most purposes the limitation isn't a problem, but sometimes we need really big numbers, e.g. for cryptography or microsecond-precision timestamps.
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In JavaScript, the "number" type cannot represent integer values larger than <code>(2<sup>53</sup>-1)</code> (that's `9007199254740991`), or less than <code>-(2<sup>53</sup>-1)</code> for negatives. It's a technical limitation caused by their internal representation.
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For most purposes that's quite enough, but sometimes we need really big numbers, e.g. for cryptography or microsecond-precision timestamps.
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`BigInt` type was recently added to the language to represent integers of arbitrary length.
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A `BigInt` is created by appending `n` to the end of an integer literal:
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A `BigInt` value is created by appending `n` to the end of an integer:
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As `BigInt` numbers are rarely needed, we devoted them a separate chapter <info:bigint>.
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As `BigInt` numbers are rarely needed, we don't cover them here, but devoted them a separate chapter <info:bigint>. Read it when you need such big numbers.
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Right now `BigInt` is supported in Firefox and Chrome, but not in Safari/IE/Edge.
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```smart header="Compatibility issues"
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Right now, `BigInt` is supported in Firefox/Chrome/Edge/Safari, but not in IE.
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You can check [*MDN* BigInt compatibility table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt#Browser_compatibility) to know which versions of a browser are supported.
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## String
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A string in JavaScript must be surrounded by quotes.
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In some languages, there is a special "character" type for a single character. For example, in the C language and in Java it is called "char".
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In JavaScript, there is no such type. There's only one type: `string`. A string may consist of only one character or many of them.
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In JavaScript, there is no such type. There's only one type: `string`. A string may consist of zero characters (be empty), one character or many of them.
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It's just a special value which represents "nothing", "empty" or "value unknown".
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The code above states that `age` is unknown or empty for some reason.
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The code above states that `age` is unknown.
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## The "undefined" value
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Technically, it is possible to assign `undefined` to any variable:
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Technically, it is possible to explicitly assign `undefined` to a variable:
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...But we don't recommend doing that. Normally, we use `null` to assign an "empty" or "unknown" value to a variable, and we use `undefined` for checks like seeing if a variable has been assigned.
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...But we don't recommend doing that. Normally, one uses `null` to assign an "empty" or "unknown" value to a variable, while `undefined` is reserved as a default initial value for unassigned things.
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## Objects and Symbols
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The `object` type is special.
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All other types are called "primitive" because their values can contain only a single thing (be it a string or a number or whatever). In contrast, objects are used to store collections of data and more complex entities. We'll deal with them later in the chapter <info:object> after we learn more about primitives.
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All other types are called "primitive" because their values can contain only a single thing (be it a string or a number or whatever). In contrast, objects are used to store collections of data and more complex entities.
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Being that important, objects deserve a special treatment. We'll deal with them later in the chapter <info:object>, after we learn more about primitives.
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The `symbol` type is used to create unique identifiers for objects. We mention it here for completeness, but we'll study it after objects.
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The `symbol` type is used to create unique identifiers for objects. We have to mention it here for the sake of completeness, but also postpone the details till we know objects.
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## The typeof operator [#type-typeof]
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The last three lines may need additional explanation:
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1. `Math` is a built-in object that provides mathematical operations. We will learn it in the chapter <info:number>. Here, it serves just as an example of an object.
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2. The result of `typeof null` is `"object"`. That's wrong. It is an officially recognized error in `typeof`, kept for compatibility. Of course, `null` is not an object. It is a special value with a separate type of its own. So, again, this is an error in the language.
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3. The result of `typeof alert` is `"function"`, because `alert` is a function. We'll study functions in the next chapters where we'll also see that there's no special "function" type in JavaScript. Functions belong to the object type. But `typeof` treats them differently, returning `"function"`. That's not quite correct, but very convenient in practice.
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2. The result of `typeof null` is `"object"`. That's an officially recognized error in `typeof` behavior, coming from the early days of JavaScript and kept for compatibility. Definitely, `null` is not an object. It is a special value with a separate type of its own.
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3. The result of `typeof alert` is `"function"`, because `alert` is a function. We'll study functions in the next chapters where we'll also see that there's no special "function" type in JavaScript. Functions belong to the object type. But `typeof` treats them differently, returning `"function"`. That also comes from the early days of JavaScript. Technically, such behavior isn't correct, but can be convenient in practice.
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## Summary
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There are 8 basic data types in JavaScript.
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- `number` for numbers of any kind: integer or floating-point, integers are limited by ±2<sup>53</sup>.
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- `number` for numbers of any kind: integer or floating-point, integers are limited by <code>±(2<sup>53</sup>-1)</code>.
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- `bigint` is for integer numbers of arbitrary length.
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- `string` for strings. A string may have one or more characters, there's no separate single-character type.
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- `string` for strings. A string may have zero or more characters, there's no separate single-character type.
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- `boolean` for `true`/`false`.
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- `null` for unknown values -- a standalone type that has a single value `null`.
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- `undefined` for unassigned values -- a standalone type that has a single value `undefined`.

‎1-js/02-first-steps/09-alert-prompt-confirm/1-simple-page/solution.md‎ renamed to ‎1-js/02-first-steps/06-alert-prompt-confirm/1-simple-page/solution.md‎

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