JavaScript Online Compiler

Run JavaScript code online

Write, run, and debug JavaScript snippets in your browser. Try interview algorithms, inspect console output, and experiment without installing Node.js.

Practice interview code in your browser

Use the editor above to try algorithms, change the sample input, and run a focused snippet. The terminal displays program output and errors so you can iterate quickly.

Example program

Start with this interview-style example, then modify it to test edge cases and alternative approaches.

function twoSum(nums, target) {
  const seen = new Map();
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
  return [];
}

console.log(twoSum([2, 7, 11, 15], 9));

Runtime and compatibility

This runner uses the browser JavaScript engine, not Node.js. Browser APIs are available only where supported by the worker; Node built-ins such as fs, process spawning, npm packages, and node:test are not available; use a single-file snippet without import statements.

Frequently asked questions