How to create a custom Promise in JavaScript

Short answer

Use the new Promise((resolve, reject) => { ... }) constructor to wrap callback-based or event-based code into a standardized Promise.

Steps

1. Wrap a callback API: javascript function readFilePromise(path) { return new Promise((resolve, reject) => { fs.readFile(path, "utf8", (err, data) => { if (err) reject(err); else resolve(data); }); }); }
2. Wrap a timeout: javascript function delay(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } await delay(500);
3. Convert an event to a Promise: javascript function waitForEvent(el, name) { return new Promise(resolve => { el.addEventListener(name, resolve, { once: true }); }); }

Tips

• Always call resolve or reject exactly once.
• Avoid mixing Promise constructors with existing Promise chains.

Common issues

• Creating a Promise but never resolving or rejecting it causes an indefinite hang.
• Synchronous throws inside a Promise constructor are not caught; wrap in try/catch and call reject.