Callbacks in Javascript

Introduction

Callbacks let you run a function after another function finishes. This helps when you want to wait for something to happen before moving on.

When you want to do something after a button is clicked.

When you need to wait for data to load from the internet before showing it.

When you want to run a task after a timer ends.

When you want to handle a response after a file is read.

When you want to run code after a user finishes typing.

Syntax

Javascript

function doSomething(callback) {
  // do some work
  callback();
}

doSomething(function() {
  // code to run after work is done
});

The callback is a function passed as an argument.

It runs inside the main function when ready.

Examples

This example greets a person, then runs the callback to say the greeting is done.

Javascript

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

greet('Alice', function() {
  console.log('Greeting done');
});

Here, the callback runs after 1 second delay using setTimeout.

Javascript

setTimeout(() => {
  console.log('Waited 1 second');
}, 1000);

Sample Program

This program simulates fetching data with a 2-second delay. After data is ready, the callback runs and prints the data.

Javascript

function fetchData(callback) {
  console.log('Start fetching data...');
  setTimeout(() => {
    console.log('Data fetched');
    callback('Here is your data');
  }, 2000);
}

fetchData(function(data) {
  console.log('Callback received:', data);
});

Important Notes

Callbacks help manage tasks that take time, like loading data.

Without callbacks, code might run too early before tasks finish.

Too many callbacks can make code hard to read, called "callback hell".

Summary

Callbacks are functions passed to other functions to run later.

They help wait for tasks to finish before continuing.

Use callbacks for events, timers, and asynchronous work.

Practice

1. What is a callback function in JavaScript?

easy

A. A function that runs immediately without waiting

B. A function passed as an argument to another function to be called later

C. A function that only runs once when the program starts

D. A function that cannot be passed to other functions

Solution

1. Step 1: Understand the role of callbacks

   A callback is a function given to another function to run after some work is done.

2. Step 2: Match the definition

   A function passed as an argument to another function to be called later correctly describes this behavior as a function passed as an argument to be called later.

3. Final Answer:

   A function passed as an argument to another function to be called later -> Option B

4. Quick Check:

   Callback = function passed to run later [OK]

Hint: Callbacks are functions given to run after another function finishes [OK]

Common Mistakes:

• Thinking callbacks run immediately
• Confusing callbacks with event listeners only
• Believing callbacks cannot be passed as arguments

2. Which of the following is the correct syntax to use a callback in JavaScript?

easy

A. function greet(callback) { callback }

B. function greet() { callback(); }

C. function greet(callback) { callback; }

D. function greet(callback) { callback(); }

Solution

1. Step 1: Check how the callback is called

   The callback function must be called with parentheses: callback()

2. Step 2: Identify correct syntax

   function greet(callback) { callback(); } correctly defines a function that takes a callback and calls it with callback()

3. Final Answer:

   function greet(callback) { callback(); } -> Option D

4. Quick Check:

   Call callback with parentheses to execute [OK]

Hint: Call callbacks with parentheses to run them [OK]

Common Mistakes:

• Forgetting parentheses when calling callback
• Not passing callback as a parameter
• Using callback without calling it

3. What will be the output of this code?

function sayHello(name, callback) {
  callback('Hello ' + name);
}
sayHello('Alice', message => console.log(message));

medium

A. Hello Alice

B. message

C. Hello name

D. Error: callback is not a function

Solution

1. Step 1: Understand the function call

   sayHello is called with 'Alice' and a callback that logs the message.

2. Step 2: Trace the callback execution

   The callback receives 'Hello Alice' and logs it to the console.

3. Final Answer:

   Hello Alice -> Option A

4. Quick Check:

   Callback logs 'Hello Alice' [OK]

Hint: Callbacks run with passed data, so output matches callback argument [OK]

Common Mistakes:

• Expecting variable names printed instead of values
• Confusing callback parameter with string 'message'
• Thinking callback is not called

4. Find the error in this code:

function processData(data, callback) {
  callback(data);
}
processData('Test');

medium

A. Callback should not take parameters

B. processData should not have parameters

C. Missing callback function when calling processData

D. No error, code runs fine

Solution

1. Step 1: Check function call arguments

   processData expects two arguments: data and callback function.

2. Step 2: Identify missing argument

   Only one argument ('Test') is passed, so callback is undefined causing error.

3. Final Answer:

   Missing callback function when calling processData -> Option C

4. Quick Check:

   Callback argument missing causes error [OK]

Hint: Always pass callback function when required [OK]

Common Mistakes:

• Not passing callback argument
• Assuming callback is optional without default
• Ignoring function parameter requirements

5. You want to run a function after a timer finishes. Which code correctly uses a callback with setTimeout to print 'Done' after 1 second?

hard

A. setTimeout(() => console.log('Done'), 1000);

B. setTimeout(console.log('Done'), 1000);

C. setTimeout(console.log, 1000);

D. setTimeout(console.log('Done'));

Solution

1. Step 1: Understand setTimeout usage

   setTimeout expects a function as first argument and delay in ms as second.

2. Step 2: Check each option

   setTimeout(() => console.log('Done'), 1000); passes a function that logs 'Done' after 1000ms delay correctly.

3. Final Answer:

   setTimeout(() => console.log('Done'), 1000); -> Option A

4. Quick Check:

   Callback function with delay = correct usage [OK]

Hint: Pass a function, not a function call, to setTimeout [OK]

Common Mistakes:

• Calling function immediately instead of passing it
• Missing delay argument
• Passing wrong argument types