Promises

A Promise represents a value that will be available later. It is a cleaner way to handle asynchronous code than nested callbacks.

Using a Promise

A Promise has three states: pending, fulfilled, or rejected. You read the result with then and handle errors with catch.

const fs = require("fs/promises");

fs.readFile("note.txt", "utf8")
  .then((data) => {
    console.log(data);
  })
  .catch((err) => {
    console.error("Error:", err.message);
  });

Creating a Promise

You build one with the Promise constructor. Call resolve on success and reject on failure.

function wait(ms) {
  return new Promise((resolve) => {
    setTimeout(() => resolve("done"), ms);
  });
}

wait(1000).then((result) => console.log(result)); // 'done' after 1s

Chaining

Each then returns a new Promise, so you can chain steps:

wait(500)
  .then(() => "step 1")
  .then((value) => console.log(value)); // 'step 1'

Summary

Promises make async flow easier to read than callbacks. Use then for results, catch for errors, and chain steps in order.