JavaScript Async
JavaScript Callbacks: Asynchronous Programming Fundamentals
Master JavaScript callbacks for asynchronous programming. Learn callback patterns, error handling, callback hell solutions, and best practices for writing clean async code.
By JavaScript Document Team•
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Callbacks are functions passed as arguments to other functions and executed after some operation completes. They're fundamental to asynchronous programming in JavaScript, enabling non-blocking code execution.
What are Callbacks?
A callback is a function that is passed to another function as a parameter and is executed after some operation has been completed.
Basic Callback Example
// Simple callback function
function greet(name, callback) {
console.log('Hello ' + name);
callback();
}
function sayGoodbye() {
console.log('Goodbye!');
}
// Pass sayGoodbye as a callback
greet('Alice', sayGoodbye);
// Output:
// Hello Alice
// Goodbye!
// Using anonymous function as callback
greet('Bob', function () {
console.log('Nice to meet you!');
});
// Arrow function as callback
greet('Charlie', () => {
console.log('See you later!');
});
Synchronous Callbacks
Callbacks can be synchronous, executing immediately as part of the normal flow.
Array Methods with Callbacks
// map() with callback
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(function (num) {
return num * 2;
});
console.log(doubled); // [2, 4, 6, 8, 10]
// filter() with callback
const evens = numbers.filter((num) => num % 2 === 0);
console.log(evens); // [2, 4]
// forEach() with callback
numbers.forEach((num, index) => {
console.log(`Index ${index}: ${num}`);
});
// reduce() with callback
const sum = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum); // 15
// Custom array method with callback
Array.prototype.customMap = function (callback) {
const result = [];
for (let i = 0; i < this.length; i++) {
result.push(callback(this[i], i, this));
}
return result;
};
const squared = numbers.customMap((x) => x * x);
console.log(squared); // [1, 4, 9, 16, 25]
Event Handlers as Callbacks
// DOM event callbacks
button.addEventListener('click', function (event) {
console.log('Button clicked!');
console.log('Event:', event);
});
// Multiple callbacks for same event
button.addEventListener('click', handleClick);
button.addEventListener('click', logClick);
button.addEventListener('click', updateUI);
function handleClick(e) {
console.log('Handling click');
}
function logClick(e) {
console.log('Logging click');
}
function updateUI(e) {
console.log('Updating UI');
}
// Remove callback
button.removeEventListener('click', handleClick);
Asynchronous Callbacks
Asynchronous callbacks execute after an asynchronous operation completes.
setTimeout and setInterval
// setTimeout callback
console.log('Start');
setTimeout(function () {
console.log('This runs after 2 seconds');
}, 2000);
console.log('End');
// Output:
// Start
// End
// This runs after 2 seconds
// setInterval callback
let count = 0;
const intervalId = setInterval(() => {
count++;
console.log(`Count: ${count}`);
if (count >= 5) {
clearInterval(intervalId);
console.log('Interval cleared');
}
}, 1000);
// Passing parameters to setTimeout callback
function delayedGreeting(name, delay) {
setTimeout(() => {
console.log(`Hello, ${name}!`);
}, delay);
}
delayedGreeting('Alice', 1000);
File Operations (Node.js)
const fs = require('fs');
// Asynchronous file read
fs.readFile('data.txt', 'utf8', function (err, data) {
if (err) {
console.error('Error reading file:', err);
return;
}
console.log('File contents:', data);
});
// Asynchronous file write
const content = 'Hello, World!';
fs.writeFile('output.txt', content, function (err) {
if (err) {
console.error('Error writing file:', err);
return;
}
console.log('File written successfully');
});
// Multiple async operations
fs.readdir('./', function (err, files) {
if (err) {
console.error('Error reading directory:', err);
return;
}
files.forEach((file) => {
fs.stat(file, function (err, stats) {
if (err) {
console.error(`Error getting stats for ${file}:`, err);
return;
}
console.log(`${file}: ${stats.size} bytes`);
});
});
});
AJAX Requests with Callbacks
// XMLHttpRequest with callbacks
function makeRequest(url, callback) {
const xhr = new XMLHttpRequest();
xhr.open('GET', url);
xhr.onload = function () {
if (xhr.status === 200) {
callback(null, xhr.responseText);
} else {
callback(new Error(`Request failed: ${xhr.status}`));
}
};
xhr.onerror = function () {
callback(new Error('Network error'));
};
xhr.send();
}
// Using the callback-based request
makeRequest('https://api.example.com/data', function (err, data) {
if (err) {
console.error('Error:', err);
return;
}
console.log('Data received:', data);
});
// jQuery AJAX with callbacks
$.ajax({
url: 'https://api.example.com/data',
method: 'GET',
success: function (data) {
console.log('Success:', data);
},
error: function (xhr, status, error) {
console.error('Error:', error);
},
complete: function () {
console.log('Request completed');
},
});
Error-First Callbacks
Node.js popularized the error-first callback pattern where the first parameter is reserved for an error.
Error-First Pattern
// Error-first callback pattern
function asyncOperation(callback) {
setTimeout(() => {
const random = Math.random();
if (random < 0.5) {
callback(new Error('Operation failed'));
} else {
callback(null, 'Operation succeeded');
}
}, 1000);
}
// Using error-first callback
asyncOperation(function (err, result) {
if (err) {
console.error('Error:', err.message);
return;
}
console.log('Success:', result);
});
// Database operation example
function fetchUser(userId, callback) {
// Simulate database query
setTimeout(() => {
if (!userId) {
callback(new Error('User ID is required'));
return;
}
if (userId < 0) {
callback(new Error('Invalid user ID'));
return;
}
// Success case
callback(null, {
id: userId,
name: 'John Doe',
email: 'john@example.com',
});
}, 100);
}
// Proper error handling
fetchUser(123, (err, user) => {
if (err) {
console.error('Failed to fetch user:', err.message);
return;
}
console.log('User:', user);
});
Multiple Async Operations
// Sequential operations with callbacks
function step1(callback) {
setTimeout(() => {
console.log('Step 1 complete');
callback(null, 'Result 1');
}, 1000);
}
function step2(previousResult, callback) {
setTimeout(() => {
console.log('Step 2 complete');
callback(null, previousResult + ' -> Result 2');
}, 1000);
}
function step3(previousResult, callback) {
setTimeout(() => {
console.log('Step 3 complete');
callback(null, previousResult + ' -> Result 3');
}, 1000);
}
// Execute in sequence
step1((err, result1) => {
if (err) {
console.error('Step 1 failed:', err);
return;
}
step2(result1, (err, result2) => {
if (err) {
console.error('Step 2 failed:', err);
return;
}
step3(result2, (err, result3) => {
if (err) {
console.error('Step 3 failed:', err);
return;
}
console.log('Final result:', result3);
});
});
});
Callback Hell
Nested callbacks can lead to "callback hell" or "pyramid of doom".
The Problem
// Callback hell example
getData(function (a) {
getMoreData(a, function (b) {
getMoreData(b, function (c) {
getMoreData(c, function (d) {
getMoreData(d, function (e) {
console.log('Final result:', e);
});
});
});
});
});
// Real-world example
getUserData(userId, function (err, user) {
if (err) {
handleError(err);
return;
}
getOrders(user.id, function (err, orders) {
if (err) {
handleError(err);
return;
}
getOrderDetails(orders[0].id, function (err, details) {
if (err) {
handleError(err);
return;
}
calculateTotal(details, function (err, total) {
if (err) {
handleError(err);
return;
}
displayResult(user, orders, details, total);
});
});
});
});
Solutions to Callback Hell
// 1. Named functions
function handleUser(err, user) {
if (err) return handleError(err);
getOrders(user.id, handleOrders);
}
function handleOrders(err, orders) {
if (err) return handleError(err);
getOrderDetails(orders[0].id, handleOrderDetails);
}
function handleOrderDetails(err, details) {
if (err) return handleError(err);
calculateTotal(details, handleTotal);
}
function handleTotal(err, total) {
if (err) return handleError(err);
displayResult(total);
}
getUserData(userId, handleUser);
// 2. Modularization
const userService = {
getUser(id, callback) {
// Implementation
},
getUserWithOrders(id, callback) {
this.getUser(id, (err, user) => {
if (err) return callback(err);
orderService.getOrders(user.id, (err, orders) => {
if (err) return callback(err);
callback(null, { user, orders });
});
});
},
};
// 3. Async library (before Promises)
const async = require('async');
async.waterfall(
[
function (callback) {
getUserData(userId, callback);
},
function (user, callback) {
getOrders(user.id, (err, orders) => {
callback(err, user, orders);
});
},
function (user, orders, callback) {
getOrderDetails(orders[0].id, (err, details) => {
callback(err, user, orders, details);
});
},
],
function (err, user, orders, details) {
if (err) return handleError(err);
displayResult(user, orders, details);
}
);
Advanced Callback Patterns
Callback Factory
// Create specialized callbacks
function createCallback(name) {
return function (err, result) {
if (err) {
console.error(`${name} failed:`, err);
return;
}
console.log(`${name} result:`, result);
};
}
asyncOperation1(createCallback('Operation 1'));
asyncOperation2(createCallback('Operation 2'));
asyncOperation3(createCallback('Operation 3'));
// Callback with context
function createContextCallback(context) {
return function (err, result) {
if (err) {
context.errors.push(err);
return;
}
context.results.push(result);
context.completed++;
if (context.completed === context.total) {
context.onComplete();
}
};
}
const context = {
results: [],
errors: [],
completed: 0,
total: 3,
onComplete() {
console.log('All operations complete');
console.log('Results:', this.results);
console.log('Errors:', this.errors);
},
};
asyncOp1(createContextCallback(context));
asyncOp2(createContextCallback(context));
asyncOp3(createContextCallback(context));
Callback Queue
class CallbackQueue {
constructor() {
this.queue = [];
this.running = false;
}
add(operation) {
this.queue.push(operation);
if (!this.running) {
this.run();
}
}
run() {
if (this.queue.length === 0) {
this.running = false;
return;
}
this.running = true;
const operation = this.queue.shift();
operation((err, result) => {
if (err) {
console.error('Operation failed:', err);
} else {
console.log('Operation result:', result);
}
// Run next operation
this.run();
});
}
}
const queue = new CallbackQueue();
// Add operations to queue
queue.add((callback) => {
setTimeout(() => callback(null, 'First'), 1000);
});
queue.add((callback) => {
setTimeout(() => callback(null, 'Second'), 500);
});
queue.add((callback) => {
setTimeout(() => callback(null, 'Third'), 1500);
});
Parallel Execution with Callbacks
function parallel(tasks, finalCallback) {
const results = [];
let completed = 0;
let hasError = false;
tasks.forEach((task, index) => {
task((err, result) => {
if (hasError) return;
if (err) {
hasError = true;
finalCallback(err);
return;
}
results[index] = result;
completed++;
if (completed === tasks.length) {
finalCallback(null, results);
}
});
});
}
// Usage
parallel(
[
(callback) => setTimeout(() => callback(null, 'Result 1'), 1000),
(callback) => setTimeout(() => callback(null, 'Result 2'), 2000),
(callback) => setTimeout(() => callback(null, 'Result 3'), 1500),
],
(err, results) => {
if (err) {
console.error('Error:', err);
return;
}
console.log('All results:', results);
}
);
// Limit concurrent operations
function parallelLimit(tasks, limit, finalCallback) {
const results = [];
let running = 0;
let completed = 0;
let index = 0;
let hasError = false;
function runNext() {
while (running < limit && index < tasks.length && !hasError) {
const taskIndex = index++;
const task = tasks[taskIndex];
running++;
task((err, result) => {
running--;
if (hasError) return;
if (err) {
hasError = true;
finalCallback(err);
return;
}
results[taskIndex] = result;
completed++;
if (completed === tasks.length) {
finalCallback(null, results);
} else {
runNext();
}
});
}
}
runNext();
}
Callback Best Practices
1. Always Handle Errors
// Bad - no error handling
getData(function (data) {
processData(data);
});
// Good - handle errors
getData(function (err, data) {
if (err) {
console.error('Failed to get data:', err);
// Handle error appropriately
return;
}
processData(data);
});
2. Return Early on Errors
function processFile(filename, callback) {
fs.readFile(filename, 'utf8', (err, data) => {
if (err) {
callback(err);
return; // Exit early
}
try {
const processed = JSON.parse(data);
callback(null, processed);
} catch (parseErr) {
callback(parseErr);
}
});
}
3. Avoid Callback Reuse
// Bad - callback might be called multiple times
function fetchData(callback) {
apiCall1((err, data) => {
if (err) callback(err);
callback(null, data); // Might call twice if error
});
}
// Good - ensure callback is called only once
function fetchData(callback) {
let called = false;
apiCall1((err, data) => {
if (called) return;
called = true;
if (err) {
callback(err);
return;
}
callback(null, data);
});
}
4. Validate Callbacks
function asyncFunction(options, callback) {
// Validate callback
if (typeof callback !== 'function') {
throw new TypeError('Callback must be a function');
}
// Validate options
if (!options || typeof options !== 'object') {
callback(new Error('Options must be an object'));
return;
}
// Perform async operation
setTimeout(() => {
callback(null, 'Success');
}, 1000);
}
5. Document Callback Parameters
/**
* Fetches user data from the database
* @param {number} userId - The user's ID
* @param {function} callback - Callback function
* @param {Error} callback.err - Error object if operation failed
* @param {Object} callback.user - User object if found
* @param {string} callback.user.name - User's name
* @param {string} callback.user.email - User's email
*/
function getUser(userId, callback) {
// Implementation
}
Transitioning from Callbacks
Modern JavaScript provides better alternatives to callbacks:
// Callback version
function fetchDataCallback(callback) {
setTimeout(() => {
callback(null, 'data');
}, 1000);
}
// Promise version
function fetchDataPromise() {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('data');
}, 1000);
});
}
// Async/await version
async function fetchDataAsync() {
return new Promise((resolve) => {
setTimeout(() => {
resolve('data');
}, 1000);
});
}
// Converting callbacks to promises
const util = require('util');
const fs = require('fs');
const readFilePromise = util.promisify(fs.readFile);
// Or manually
function promisify(fn) {
return function (...args) {
return new Promise((resolve, reject) => {
fn(...args, (err, result) => {
if (err) reject(err);
else resolve(result);
});
});
};
}
Conclusion
Callbacks are fundamental to asynchronous JavaScript programming, enabling non-blocking code execution. While they can lead to complex nested structures (callback hell), proper patterns and practices can help manage this complexity. Understanding callbacks is essential even as modern JavaScript moves toward Promises and async/await, as callbacks remain prevalent in many APIs and form the foundation of asynchronous patterns. The key to effective callback usage is consistent error handling, avoiding deep nesting, and knowing when to transition to more modern async patterns.