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JavaScript: Difference between revisions

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JavaScript is a single-threaded programming language. In the browser, JS is event-driven, meaning that after initialization, the JS engine is idle until an event occurs such as a button being pressed in the DOM.
This page is a mostly about browser-based JavaScript or ECMAScript usage and interaction with the HTML DOM (window).
This page is a mostly about browser-based JavaScript or ECMAScript usage and interaction with the HTML DOM (window).
For server and desktop application JavaScript usage, please see the [[NodeJS]] page.
For server and desktop application JavaScript usage, please see the [[NodeJS]] page.


==Usage==
==Language Syntax==
 
General Ecmascript syntax. Parts of this section applies to browser-side JS and server-side JS (Node.js, Deno), though details may vary between runtimes.
===Inputs===
 
====Showing an input image====
<syntaxhighlight lang="javascript">
const myInput = document.getElementById("myInput");
const myImage = document.getElementById("myImage");
myInput.addEventListener('change', function() {
    if (myInput.files.length !== 1) {
        return;
    }
    let image = myInput.files[0];
    const reader = new FileReader();
    reader.onload = function(e) {
        myImage.src = e.target.result;
    };
    reader.readAsDataURL(image);
});
</syntaxhighlight>
 
===Canvas===
 
===Picture===
 
===Video===
Your HTML:
<syntaxhighlight lang="html">
<video id="myVideoElt" controls>
</video>
</syntaxhighlight>
Your JS:
<syntaxhighlight lang="javascript">
const myVideo = document.getElementById("myVideoElt");
 
// Create a new source
const newSource = document.createElement("source");
const myVideoUrl = "https://interactive-examples.mdn.mozilla.net/media/examples/flower.webm";
newSource.setAttribute("src", myVideoUrl);
myVideo.appendChild(newSource);
myVideo.play();
</syntaxhighlight>


===Regular Expressions (Regex)===
===Regular Expressions (Regex)===
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions Reference]
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions MDN Guide]
<syntaxhighlight lang="javascript">
<syntaxhighlight lang="javascript">
var myRegex = /(\d+),(\d+)/;
const myRegex = /(\d+),(\d+)/;
var myStr = "124,52";
const myStr = "124,52";
var match = myStr.match(myRegex);
const match = myStr.match(myRegex);
// Captures
// Captures
console.log(match[1], match[2]);
console.log(match[1], match[2]);
console.table(match);
console.table(match);
</syntaxhighlight>
</syntaxhighlight>


===Classes===
===Classes===
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===Async-await===
===Async-await===
Added in ES2017<br>
Added in ES2017
 
See [https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await Async_await]<br>
See [https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await Async_await]<br>
Async functions return implicit promises. They allow you to use keyword <code>await</code> to get results from other promises or functions.
Async functions return implicit promises. They allow you to use keyword <code>await</code> to get results from other promises or functions.
===Enums===
See [https://stackoverflow.com/questions/287903/what-is-the-preferred-syntax-for-defining-enums-in-javascript Stack overflow]
Ecmascript doesn't have a built-in way to define ENUMS. One way is to use <code>Object.freeze</code>:
<syntaxhighlight lang="js">
const WORKERSTATUS = Object.freeze({
  ACTIVE: "ACTIVE",
  DISABLED: "DISABLED",
});
</syntaxhighlight>
; Notes
* This does not enforce any kind of type. If you need that, use [https://www.typescriptlang.org/docs/handbook/enums.html typescript enums].
===<code>var</code> vs <code>let</code>, <code>const</code>===
* <code>var</code> is the old way of declaring variables. It has function-scope rather than block-scope. Note that <code>var</code> can also be redefined which can hide errors.
* <code>let</code> and <code>const</code> are post-ES6 ways which have block-scope.
These days you should never use <code>var</code>. 
The Google [https://google.github.io/styleguide/jsguide.html JS Style Guide] and [https://google.github.io/styleguide/tsguide.html TS Style Guide] suggest using <code>const</code> by default.
==Browser Usage==
For basic dom manipulation, see [https://htmldom.dev/ https://htmldom.dev/].
===Inputs===
====Showing an input image====
<syntaxhighlight lang="javascript">
const myInput = document.getElementById("myInput");
const myImage = document.getElementById("myImage");
myInput.addEventListener('change', function() {
    if (myInput.files.length !== 1) {
        return;
    }
    let image = myInput.files[0];
    const reader = new FileReader();
    reader.onload = function(e) {
        myImage.src = e.target.result;
    };
    reader.readAsDataURL(image);
});
</syntaxhighlight>
===Canvas===
===Picture===
===Video===
Your HTML:
<syntaxhighlight lang="html">
<video id="myVideoElt" controls>
</video>
</syntaxhighlight>
Your JS:
<syntaxhighlight lang="javascript">
const myVideo = document.getElementById("myVideoElt");
// Create a new source
const newSource = document.createElement("source");
const myVideoUrl = "https://interactive-examples.mdn.mozilla.net/media/examples/flower.webm";
newSource.setAttribute("src", myVideoUrl);
myVideo.appendChild(newSource);
myVideo.play();
</syntaxhighlight>


==Compilation==
==Compilation==
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===Map===
===Map===
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map Reference]
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map MDN Map]
[https://www.ecma-international.org/ecma-262/10.0/index.html#sec-map-objects ES2019 (ES10) Map Specification] 
This is your typical hashmap.
 
<syntaxhighlight lang="js">
<syntaxhighlight lang="js">
let myMap = new Map();
let myMap = new Map();
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my_set.clear();
my_set.clear();


// Returns if key has in the set
// Returns true if key was in the set
my_set.delete(key);
my_set.delete(key);
</syntaxhighlight>
</syntaxhighlight>
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==Modules==
==Modules==
These days, we can use modules for everything. 
Note that in most instances, you should compile these using Webpack, browserify or similar. 
For Node.js, you will need to transpile using Babel. 
However some modern browsers now support importing modules directly using script tags. 
* [[Caniuse: Modules]]
===Getting Started===
===Getting Started===
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules MDN Guide to Modules]<br>
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules MDN Guide to Modules]
 
Example Module
<syntaxhighlight lang="javascript">
<syntaxhighlight lang="javascript">
// Import three.js as a module. webpack will resolve this.
import * as THREE from 'three';
// Import MyClass as a module. webpack will resolve this.
import {MyClass} from "./MyClass.js";


// Pretend we're writing another class
export class MyOtherClass {
  constructor() {}
}
</syntaxhighlight>
</syntaxhighlight>
;Notes
* The Google style guide suggests always using named exports rather than using <code>export default</code>.
==Web Workers==
By default, JavaScript is a single-threaded language. For things that take a long time, people usually use asynchronous JS through promises and callbacks. However, if you have a heavy calculation that needs to be performed, you can use spawn web workers which will run in background threads. NodeJS also has a similar concept called [https://nodejs.org/api/worker_threads.html Worker Threads].
===Getting Started===
;Your main JS file:
;Your worker JS file:
Resources
* [https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers MDN: Using web workers]
* [https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API Web Workers API]
==Documentation==
Use [https://jsdoc.app/ JSDoc] for documentation. 
Example:
<syntaxhighlight lang="js">
</syntaxhighlight>
==Testing==
Frameworks for testing JS:
* [https://jestjs.io/ https://jestjs.io/]


==Useful Packages==
==Useful Packages==
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npm install mathjs
npm install mathjs
</syntaxhighlight>
</syntaxhighlight>
[[Category:Programming languages]]
==Misc==
* [[Javascript CDNs]]
Retrieved from "https://wiki.davidl.me/view/JavaScript"