Practice JavaScript right here — no installs, no signups.
JavaScript runs in every browser on earth, which makes it the fastest language to start with — you write a line, press Run, and see the result immediately. This online JavaScript compiler executes your code in a sandboxed environment and prints everything you send to console.log() straight into the output panel below the editor.
Use it for practising the fundamentals: variables and scope, template literals, array methods like map/filter/reduce, objects and destructuring, promises and async/await, and classes. Because there is no build step, you can iterate on a snippet dozens of times a minute — which is exactly how the syntax sticks.
Copy any snippet into the editor above and press Run. Each one is short on purpose — retype it from memory afterwards.
const scores = [42, 87, 13, 96, 65];
const passed = scores
.filter((s) => s >= 50)
.map((s) => `${s} marks`);
console.log(passed);
console.log("Average:", scores.reduce((a, b) => a + b, 0) / scores.length);filter narrows the list, map reshapes each item, reduce collapses the list to one value. All three return new arrays or values — the original stays untouched.
const wait = (ms) => new Promise((r) => setTimeout(r, ms));
async function main() {
console.log("start");
await wait(500);
console.log("half a second later");
}
main();await pauses only the async function, not the whole program. Anything after main() runs before the awaited line resolves.
Uncaught ReferenceError: x is not defined
Why: You used a variable before declaring it, or misspelled its name.
Fix: Declare it with let or const above first use, and check the spelling and capitalisation — JavaScript is case sensitive.
Cannot read properties of undefined (reading 'name')
Why: The object you indexed into does not exist at that point.
Fix: Guard with optional chaining: user?.name, or log the object first to confirm its shape.
Unexpected token }
Why: A brace, bracket or parenthesis is unbalanced.
Fix: Count the opening and closing pairs — the editor highlights the matching brace when the cursor sits next to one.
| Concept | Syntax |
|---|---|
| Declare a constant | const total = 10; |
| Arrow function | const add = (a, b) => a + b; |
| Template literal | `Hello ${name}` |
| Loop an array | for (const item of list) { } |
| Class | class Dog { constructor(n) { this.n = n } } |
| Try/catch | try { risky() } catch (e) { console.log(e) } |
Yes. Arrow functions, destructuring, spread, optional chaining, classes, modules-free async/await and template literals all work.
The runner is a headless sandbox, so document and window APIs are limited. Use console.log for output; for DOM work use the HTML compiler instead.
Running snippets builds speed; the 17-chapter course builds understanding. Work through the JavaScript chapters, take the quiz, then claim your certificate.
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