Promises

Promises fix callback hell. A Promise is an object that represents the eventual completion (or failure) of an async operation. It can be in one of three states: pending, fulfilled, or rejected.

Creating a Promise

const fetchData = new Promise((resolve, reject) => {
  setTimeout(() => {
    const data = { id: 1, name: "Alice" };
    resolve(data);  // Success
    // reject(new Error("Failed"));  // Failure
  }, 1000);
});

Consuming Promises

fetchData
  .then(data => {
    console.log("Data:", data);
    return data.name;
  })
  .then(name => {
    console.log("Name:", name);
  })
  .catch(error => {
    console.error("Error:", error);
  })
  .finally(() => {
    console.log("Done!");
  });

Converting Callbacks to Promises

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

// Promise-based file reading
async function readFile() {
  try {
    const data = await fs.readFile("data.txt", "utf-8");
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

// Promisify callback-based functions
const { promisify } = require("util");
const readFile = promisify(fs.readFile);

Promise.all and Promise.race

// Run multiple promises in parallel
const results = await Promise.all([
  fetch("/api/users"),
  fetch("/api/posts"),
  fetch("/api/comments")
]);

// Use first to complete
const fastest = await Promise.race([
  fetch("/api/server1"),
  fetch("/api/server2"),
]);

// Wait for all, handle individual results
const settled = await Promise.allSettled([
  fetch("/api/option1"),
  fetch("/api/option2"),
]);

Best Practice: Always return Promises from .then() handlers so you can chain them. And always handle errors with .catch() — unhandled Promise rejections crash your Node.js process.