Callbacks in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the callback function.

Javascript

function greet(name, [1]) {
  [1](name);
}

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

Drag options to blanks, or click blank then click option'

Acallback

Bgreet

Cname

Dconsole

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong parameter name to call the callback.

Trying to call a variable that is not a function.

The callback function is passed as the second argument and called inside the greet function using the name 'callback'.

2fill in blank

medium

Complete the code to pass a callback that logs a message.

Javascript

function processData(data, [1]) {
  console.log('Processing: ' + data);
  [1]();
}

processData('info', function() {
  console.log('Done processing');
});

Drag options to blanks, or click blank then click option'

Adata

Bcallback

CprocessData

Dconsole

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling the callback function.

Using the wrong parameter name for the callback.

The callback function is passed as the second argument and called inside processData using the name 'callback'.

3fill in blank

hard

Fix the error in calling the callback function.

Javascript

function fetchData(callback) {
  setTimeout(function() {
    const data = 'Sample data';
    [1](data);
  }, 1000);
}

fetchData(function(result) {
  console.log('Received:', result);
});

Drag options to blanks, or click blank then click option'

Acallback

BfetchData

CsetTimeout

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a variable that is not the callback function.

Forgetting to pass the data to the callback.

The callback function should be called by its name 'callback' with the data as argument.

4fill in blank

hard

Fill both blanks to create a callback that doubles numbers.

Javascript

function doubleNumbers(numbers, [1]) {
  const result = numbers.map(function(num) {
    return num * 2;
  });
  [2](result);
}

doubleNumbers([1, 2, 3], function(doubled) {
  console.log(doubled);
});

Drag options to blanks, or click blank then click option'

Acallback

Bnumbers

Cconsole.log

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable name to call the callback.

Not calling the callback at all.

The callback function is passed as 'callback' and called with the doubled array as argument.

5fill in blank

hard

Fill all three blanks to create a callback that filters even numbers and logs them.

Javascript

function filterEven(numbers, [1]) {
  const evens = numbers.filter(function(num) {
    return num [2] 2 === 0;
  });
  [3](evens);
}

filterEven([1, 2, 3, 4], function(result) {
  console.log(result);
});

Drag options to blanks, or click blank then click option'

Acallback

B%

Cconsole.log

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '%' for checking even numbers.

Calling the wrong function instead of the callback.

The callback is named 'callback' and called with the filtered evens. The modulo operator '%' checks for even numbers.

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