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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==
===Arrays===
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.
<syntaxhighlight lang="javascript">
let arr = [1,2,3];


// Map
arr.map(x => x > 2); // [false, false, true]
// Reduce or Fold
// Note that if you do not provide an initial accumulator,
// then the first element will be your accumulator
// I.e. the first call to your function will be (arr[0], arr[1])
arr.reduce((acc, x) => acc + x, 0); // 6
</syntaxhighlight>
===Canvas===
===Video===
===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===
Line 44: Line 31:
// To add to the prototype
// To add to the prototype
Object.assign(Rectangle.prototype, {
Object.assign(Rectangle.prototype, {
  constructor: Rectangle,
   getSize: function() {
   getSize: function() {
     return this.height * this.width;
     return this.height * this.width;
Line 63: Line 51:
   }
   }
}
}
</syntaxhighlight>
===Promises===
Added in ES2015 (ES6)<br>
See [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise Promises]<br>
In general, a function returns a promise if it needs to perform asyncronous tasks.<br>
<syntaxhighlight lang="javascript">
function myFunc() {
  // Do syncronous things here
  return new Promise((resolve, reject) => {
    // Do asyncronous things here
    // Return data
    resolve(myData);
    // Or if we have an error
    reject(myError);
  });
}
myFunc()
  .then(data => {
    console.log(data);
  }).catch(err => {
    console.error(err);
  });
</syntaxhighlight>
===Async-await===
Added in ES2017
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.
===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>
</syntaxhighlight>


==Compilation==
==Compilation==
===Webpack===
===Babel===
==Websockets==
How to use Websockets
===Getting Started===
<syntaxhighlight lang="javascript">
let ws = new WebSocket(this.SERVER_URL);
ws.onopen = function(event) {
  console.log("Websocket opened");
  ws.send("Hi");
};
ws.onmessage = function(event) {
  console.log("message received");
  console.log(event.data);
};
ws.onclose = function() {
  console.log("WS Closed");
};
</syntaxhighlight>
==Data Structures==
JavaScript traditionally has arrays and objects (hashmap) data structures.<br>
ES2015 (ES6) adds several collections including: Map, Set, WeakMap, WeakSet<br>
[https://www.sitepoint.com/es6-collections-map-set-weakmap-weakset/ ES6 Collections]
===Arrays===
<syntaxhighlight lang="javascript">
let arr = [1,2,3];
// Map
arr.map(x => x > 2); // [false, false, true]
// Reduce or Fold
// Note that if you do not provide an initial accumulator,
// then the first element will be your accumulator
// I.e. the first call to your function will be (arr[0], arr[1])
arr.reduce((acc, x) => acc + x, 0); // 6
</syntaxhighlight>
===Objects===
Objects are maps in JavaScript. They are typically implemented as hashmaps by the JS engine.<br>
Note that you can only use numbers and strings as keys.<br>
Learn more about the implementation at [https://v8.dev/blog/hash-code https://v8.dev/blog/hash-code]<br>
<syntaxhighlight lang="javascript">
let my_map = {};
// or my_map["my_key"]
my_map.my_key = "my_value";
"my_key" in my_map;
// Loop over keys
for (let key in a) {
  console.log("Key:", key);
}
</syntaxhighlight>
===Map===
[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">
let myMap = new Map();
let keyObj = {};
// Set value
myMap.set(keyObj, 'value associated with keyObj');
// Get size
myMap.size;
// Get value
myMap.get(keyObj);
// Check if key is in the map
myMap.has(keyObj);
// Delete
myMap.delete(keyObj);
</syntaxhighlight>
;Notes
* You can mix and match types of keys
* The hash for objects are randomly generated under the hood
* Do not use <code>[]</code> to get or set from the map
* Iterating over a Map will be in order of insertion
;Saving as json
See [https://2ality.com/2015/08/es6-map-json.html es6 map json]<br>
<syntaxhighlight lang="javascript">
const data = JSON.stringify([...myMap]);
myMap = new Map(JSON.parse(data));
</syntaxhighlight>
===Set===
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set MDN Set]
<syntaxhighlight lang="js">
let my_set = new Set();
// Returns the set itself
my_set.add(key);
my_set.has(key);
my_set.clear();
// Returns true if key was in the set
my_set.delete(key);
</syntaxhighlight>
===WeakMap===
Unlike map, weakmap holds weak references to its keys. This allows keys to be garbage collected when no reference to it remains.
Note that once keys are garbage collected, they are also removed from the weakmap since you can no longer access them.
You can not iterate through a weakmap.
===WeakSet===
==WebXR==
<syntaxhighlight lang="javascript">
// Check for VR support
if ("xr" in navigator && "isSessionSupported" in navigator.xr) {
  navigator.xr.isSessionSupported('immersive-vr').then((supported) => {
    console.log("immersive-vr supported:", supported);
  });
}
</syntaxhighlight>


==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===
[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules MDN Guide to Modules]
Example Module
<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>
;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==
Packages which run in the browser and may be useful for developing web applications.
Note that some of these may overlap with the NodeJS page.
===lerp, clamp===
<syntaxhighlight lang="bash">
npm i lerp clamp
</syntaxhighlight>
<syntaxhighlight lang="javascript">
Math.lerp = require("lerp");
Math.clamp = require("clamp");
</syntaxhighlight>
===pako===
[https://github.com/nodeca/pako pako github]<br>
"zlib port to javascript, very fast!"
<syntaxhighlight lang="bash">
npm install pako
</syntaxhighlight>
===url-join===
Basically <code>path.join</code> for the browser.<br>
<syntaxhighlight lang="bash">
npm install url-join
</syntaxhighlight>
===mathjs===
[https://mathjs.org/ https://mathjs.org/]<br>
Some useful math things for JavaScript in the browser and in NodeJS
<syntaxhighlight lang="bash">
npm install mathjs
</syntaxhighlight>
[[Category:Programming languages]]
==Misc==
* [[Javascript CDNs]]
Retrieved from "https://wiki.davidl.me/view/JavaScript"