Lesson 23 — Promisify a callback

Wrap old callback APIs

Older code reports results through a callback — a function you pass in, called as cb(error, value) when the work finishes (error-first style). Promisifying wraps such a function so it returns a Promise instead: reject when there is an error, resolve with the value otherwise. Node even ships util.promisify for this.

function readLater(cb) { setTimeout(() => cb(null, "data"), 10); }
// promisified: readLaterP().then(console.log)

Why it matters: it lets you use await with legacy libraries instead of nesting callbacks.