Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript
JavaScript | MDN
JavaScript (JS) is a lightweight interpreted (or just-in-time compiled) programming language with first-class functions. While it is most well-known as the scripting language for Web pages, many non-browser environments also use it, such as Node.js, Apache CouchDB and Adobe Acrobat.
MDN Web Docs
developer.mozilla.org › en-US › docs › Learn_web_development › Core › Scripting
Dynamic scripting with JavaScript - Learn web development | MDN
An excellent resource for aspiring web developers — Learn JavaScript in an interactive environment, with short lessons and interactive tests, guided by automated assessment. The first 40 lessons are free, and the complete course is available for a small one-time payment.
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Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Expressions_and_operators
Expressions and operators - JavaScript | MDN
This chapter describes JavaScript's expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, ternary and more.
MDN Web Docs
developer.mozilla.org › en-US › docs › Glossary › JavaScript
JavaScript - Glossary | MDN
JavaScript (or "JS") is a programming language used most often for dynamic client-side scripts on webpages, but it is also often used on the server-side, using a runtime such as Node.js, Deno, and Bun.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference
JavaScript reference - JavaScript | MDN
The JavaScript reference serves as a repository of facts about the JavaScript language. The entire language is described here in detail. As you write JavaScript code, you'll refer to these pages often (thus the title "JavaScript reference").
Mozilla
developer.mozilla.org › en-US
MDN Web Docs
Documenting CSS, HTML, and JavaScript, since 2005. ... MDN turns 20!
Mozilla
developer.mozilla.org › en-US › curriculum › core › javascript-fundamentals
6. JavaScript fundamentals | MDN Curriculum
JavaScript is a huge topic, with so many different features, styles, and techniques to learn, and so many APIs and tools built on top of it. This module focuses mostly on the essentials of the core language, plus some key surrounding topics — learning these topics will give you a solid basis to work from. ... Understand what variables are and why they are so important in programming generally, not just JavaScript.
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Introduction
Introduction - JavaScript | MDN
This chapter introduces JavaScript and discusses some of its fundamental concepts.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › in
in - JavaScript | MDN
class Person { #age; constructor(age) { this.#age = age; } static [Symbol.hasInstance](o) { // Testing `this` to prevent false-positives when // calling `instanceof SubclassOfPerson` return this === Person && #age in o; } ageDifference(other) { return this.#age - other.#age; } } const p1 = new Person(20); const p2 = new Person(30); if (p1 instanceof Person && p2 instanceof Person) { console.log(p1.ageDifference(p2)); // -10 } For more examples, see Private elements and the class guide. ... This page was last modified on Jul 8, 2025 by MDN contributors.
MDN
mdn2.netlify.app › en-us › docs › web › javascript
JavaScript | MDN
JavaScript (JS) is a lightweight, interpreted, or just-in-time compiled programming language with first-class functions. While it is most well-known as the scripting language for Web pages, many non-browser environments also use it, such as Node.js, Apache CouchDB and Adobe Acrobat.
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Grammar_and_types
Grammar and types - JavaScript | MDN
This chapter discusses JavaScript's basic grammar, variable declarations, data types and literals.
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide
JavaScript Guide - JavaScript | MDN
The JavaScript Guide shows you how to use JavaScript and gives an overview of the language. If you need exhaustive information about a language feature, have a look at the JavaScript reference.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Statements › for
for - JavaScript | MDN
You can create two counters that are updated simultaneously in a for loop using the comma operator. Multiple let and var declarations can also be joined with commas. ... const arr = [1, 2, 3, 4, 5, 6]; for (let l = 0, r = arr.length - 1; l < r; l++, r--) { console.log(arr[l], arr[r]); } // 1 6 // 2 5 // 3 4 ... This page was last modified on Jul 8, 2025 by MDN contributors.
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Data_structures
JavaScript data types and data structures - JavaScript | MDN
Programming languages all have built-in data structures, but these often differ from one language to another. This article attempts to list the built-in data structures available in JavaScript and what properties they have. These can be used to build other data structures.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Operators › this
this - JavaScript | MDN
// In web browsers, the window object is also the global object: console.log(this === window); // true this.b = "MDN"; console.log(window.b); // "MDN" console.log(b); // "MDN"
MDN Web Docs
developer.mozilla.org › en-US › docs › Learn_web_development › Core › Scripting › What_is_JavaScript
What is JavaScript? - Learn web development | MDN
JavaScript is a scripting or programming language that allows you to implement complex features on web pages — every time a web page does more than just sit there and display static information for you to look at — displaying timely content updates, interactive maps, animated 2D/3D graphics, ...
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Statements › with
with - JavaScript | MDN
const namespace = new Proxy( {}, { has(target, key) { // Avoid trapping global properties like `console` if (key in globalThis) { return false; } // Trap all property lookups return true; }, get(target, key) { return key; }, }, ); with (namespace) { console.log(a, b, c); // "a" "b" "c" } ... This page was last modified on Oct 30, 2025 by MDN contributors.
Mozilla
developer.mozilla.org › en-US › docs › Web › JavaScript › Guide › Working_with_objects
Working with objects - JavaScript | MDN
Objects in JavaScript, just as in many other programming languages, can be compared to objects in real life. In JavaScript, an object is a standalone entity, with properties and type. Compare it with a cup, for example. A cup is an object with properties. A cup has a color, a design, weight, a material it is made of, etc.
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Statements › for...in
for...in - JavaScript | MDN
const obj = { a: 1, b: 2, c: 3 }; for (const prop in obj) { console.log(`obj.${prop} = ${obj[prop]}`); Object.defineProperty(obj, "c", { enumerable: false }); } for...of · for · Enumerability and ownership of properties · Object.getOwnPropertyNames() Object.hasOwn() Array.prototype.forEach() Was this page helpful to you? Yes · No Learn how to contribute · This page was last modified on Jul 29, 2025 by MDN contributors.