Error-First Callbacks

Introduction

The error-first callback pattern (also called Node.js style) passes errors as the first argument to the callback. If there’s no error, the first argument is null.

Why Do We Need This?

This convention standardizes error handling across async APIs, making it predictable and consistent.

The Pattern

function readFile(path, callback) {
  // ... read file
  if (error) {
    callback(new Error('File not found'), null);  // Error first
  } else {
    callback(null, fileData);                       // null = no error
  }
}

// Usage
readFile('/path/to/file', (err, data) => {
  if (err) {
    console.error('Error:', err.message);
    return;
  }
  console.log(data);
});

Multiple Results

function getUser(id, callback) {
  setTimeout(() => {
    if (!id) {
      callback(new Error('Invalid ID'), null);
      return;
    }
    callback(null, { id, name: 'Alice', email: 'alice@email.com' });
  }, 100);
}

getUser(1, (err, user) => {
  if (err) return console.error(err);
  console.log(user.name); // 'Alice'
});

Summary

First argument is always error (or null if success)
Second argument is the result data
Standard in Node.js core APIs
Always check for errors first in the callback

Error-First Callbacks

Error-First Callbacks

Introduction

Why Do We Need This?

The Pattern

Multiple Results

Summary

Further Reading