Online JavaScript Compiler

This free JavaScript compiler runs your code in your own browser, in a separate worker. Nothing is sent to a server, so the result appears at once and your code stays private. You see everything you print with console.log(), plus any error.

Use it to practise JavaScript, test an array method, or check a small function before you put it in your project.

1. Write JavaScript in the editor. Use console.log() to print values.
2. Press Run. The output appears below the editor at once.
3. If the code runs for more than 2 seconds, it is stopped, so a loop that never ends cannot freeze the page.

Open any of these from the Examples tab in the side bar.

console.log("Hello, World!");

const marks = [72, 45, 88, 39, 91];

const passed = marks.filter((m) => m >= 40);
const doubled = marks.map((m) => m * 2);
const total = marks.reduce((sum, m) => sum + m, 0);

console.log("Passed:", passed);
console.log("Doubled:", doubled);
console.log("Total:", total);

const student = { name: "Meera", skills: ["HTML", "CSS", "JS"] };

const json = JSON.stringify(student);
console.log(json);

const back = JSON.parse(json);
console.log(back.skills.length, "skills");

JavaScript is the language of the web. Every browser runs it. With Node.js it also runs on servers, so one language can build a full web app — the front end and the back end.

This page runs your code in the browser’s JavaScript engine. Browser features such as Math, Date, JSON, Map, Set and Promise work. Node.js features such as require(), fs and readline do not, because a browser does not have them.

• ReferenceError: x is not definedThe variable name is spelled wrong, or it was not declared with let, const or var.
• TypeError: Cannot read properties of undefinedYou used a property on a value that is undefined. Check that the object exists first.
• SyntaxError: Unexpected tokenA bracket, brace or comma is missing or extra near the line shown.

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