Node.js — Callbacks

What is a Callback?

A function passed as an argument to be executed later.

function greet(name, callback) {
    console.log('Hello, ' + name);
    callback();
}

greet('John', () => {
    console.log('Callback executed!');
});

Error-First Callbacks

function readData(callback) {
    fs.readFile('data.json', (err, data) => {
        if (err) return callback(err);
        callback(null, JSON.parse(data));
    });
}

readData((err, data) => {
    if (err) console.error(err);
    else console.log(data);
});

Callback Hell

getData((a) => {
    processA(a, (b) => {
        processB(b, (c) => {
            processC(c, (d) => {
                console.log(d);
            });
        });
    });
});

Convert to Promise

function readData() {
    return new Promise((resolve, reject) => {
        fs.readFile('data.json', (err, data) => {
            if (err) reject(err);
            else resolve(JSON.parse(data));
        });
    });
}

Mini Practice

1. Write a callback function
2. Use error-first callbacks
3. Flatten callback hell
4. Convert callbacks to promises

Up Next

Continue with Promises — promise-based patterns.