Callback pitfalls in Javascript - Mini Project: Build & Apply

Callback Pitfalls

📖 Scenario: You are building a simple program that simulates fetching user data from a server. The server takes some time to respond, so you use callbacks to handle the data once it arrives.However, callbacks can sometimes cause confusion if not used carefully. This project will help you understand how callbacks work and how to avoid common mistakes.

🎯 Goal: Create a program that fetches user names using callbacks and prints them in order. You will learn how to set up callbacks correctly and avoid pitfalls like calling callbacks too early or too late.

📋 What You'll Learn

Create a function that simulates fetching user data with a callback

Use a callback function to handle the fetched data

Avoid calling the callback before the data is ready

Print the user names after fetching them

💡 Why This Matters

🌍 Real World

Callbacks are used in real web apps to handle data loading, user actions, and server responses without freezing the page.

💼 Career

Understanding callbacks is essential for JavaScript developers working on interactive websites and asynchronous programming.

1

Create the user data array

Create an array called users with these exact strings: 'Alice', 'Bob', 'Charlie'.

Javascript

// Create the users array with 'Alice', 'Bob', and 'Charlie'
// Your code here

Need a hint?

Use square brackets [] to create an array and separate the names with commas.

2

Create the fetchUser function with a callback

Create a function called fetchUser that takes two parameters: index and callback. Inside the function, use setTimeout to simulate a delay of 100 milliseconds, then call callback with the user name at position index from the users array.

Javascript

const users = ['Alice', 'Bob', 'Charlie'];

// Create fetchUser function with index and callback parameters
// Your code here

Need a hint?

Use setTimeout to delay calling the callback. The callback should receive the user name from the users array at the given index.

3

Fetch all users using callbacks

Use fetchUser to fetch each user by index 0, 1, and 2. For each call, pass a callback function that takes a parameter name and adds it to an array called fetchedUsers. Create the fetchedUsers array before the calls.

Javascript

const users = ['Alice', 'Bob', 'Charlie'];

function fetchUser(index, callback) {
    setTimeout(() => {
        callback(users[index]);
    }, 100);
}

// Create an empty array called fetchedUsers
// Use fetchUser for index 0, 1, and 2
// For each, pass a callback that adds the name to fetchedUsers
// Your code here

Need a hint?

Remember to create the fetchedUsers array before calling fetchUser. Each callback should add the received name to this array.

4

Print the fetched users after delay

Use setTimeout with a delay of 500 milliseconds to print the fetchedUsers array using console.log. This ensures all callbacks have finished before printing.

Javascript

const users = ['Alice', 'Bob', 'Charlie'];

function fetchUser(index, callback) {
    setTimeout(() => {
        callback(users[index]);
    }, 100);
}

const fetchedUsers = [];
fetchUser(0, (name) => {
    fetchedUsers.push(name);
});
fetchUser(1, (name) => {
    fetchedUsers.push(name);
});
fetchUser(2, (name) => {
    fetchedUsers.push(name);
});

// Use setTimeout with 500ms delay to print fetchedUsers
// Your code here

Need a hint?

Use setTimeout to delay printing so all user names are fetched and added to the array first.