Callbacks

A callback is a function passed to another function to be called later, usually when an asynchronous task finishes. Callbacks were the original way Node.js handled async work.

A simple callback

function greet(name, callback) {
  console.log("Hi " + name);
  callback();
}

greet("Fausto", () => {
  console.log("Done");
});

Error-first callbacks

Node.js follows a convention: the first argument of a callback is the error (or null if all went well), and the result comes after.

const fs = require("fs");

fs.readFile("note.txt", "utf8", (err, data) => {
  if (err) {
    return console.error("Failed:", err.message);
  }
  console.log("Content:", data);
});

Always check the error first before using the data.

Callback hell

Nesting many callbacks gets hard to read:

doA((err) => {
  doB((err) => {
    doC((err) => {
      // deeply nested
    });
  });
});

This problem is why Promises and async/await were introduced.

Summary

Callbacks run code after an async task. Follow the error-first convention, and switch to Promises when nesting gets deep.