Error-first callback convention in Node.js - Mini Project: Build & Apply

Error-first callback convention in Node.js

📖 Scenario: You are building a simple Node.js module that reads user data from a pretend database. You want to follow the common Node.js pattern called the error-first callback convention. This means your functions will pass an error as the first argument to the callback if something goes wrong, or null if everything is fine.

🎯 Goal: Create a function called getUserData that takes a userId and a callback. The function should simulate fetching user data. If the userId is not found, call the callback with an error as the first argument. If found, call the callback with null as the error and the user data as the second argument.

📋 What You'll Learn

Create an object called users with exactly these entries: 1: {name: 'Alice', age: 25}, 2: {name: 'Bob', age: 30}, 3: {name: 'Charlie', age: 35}

Create a variable called notFoundError that holds a new Error with the message 'User not found'

Write a function called getUserData that takes userId and callback parameters

Inside getUserData, check if userId exists in users. If not, call callback(notFoundError). If yes, call callback(null, userData) with the user data

Export the getUserData function using module.exports

💡 Why This Matters

🌍 Real World

Many Node.js modules and libraries use the error-first callback pattern to handle asynchronous operations and errors clearly.

💼 Career

Understanding this pattern is essential for working with legacy Node.js code and many popular libraries that have not yet adopted promises or async/await.

1

Create the users data object

Create an object called users with these exact entries: 1: {name: 'Alice', age: 25}, 2: {name: 'Bob', age: 30}, 3: {name: 'Charlie', age: 35}

Node.js

# Create the users object with 3 users
# Your code here

Need a hint?

Use const users = {} and add the exact user entries inside.

2

Create the error variable

Create a variable called notFoundError that holds a new Error with the message 'User not found'

Node.js

const users = {
  1: {name: 'Alice', age: 25},
  2: {name: 'Bob', age: 30},
  3: {name: 'Charlie', age: 35}
};

// Create the notFoundError variable
# Your code here

Need a hint?

Use new Error('User not found') to create the error.

3

Write the getUserData function

Write a function called getUserData that takes userId and callback parameters. Inside, check if userId exists in users. If not, call callback(notFoundError). If yes, call callback(null, userData) with the user data.

Node.js

const users = {
  1: {name: 'Alice', age: 25},
  2: {name: 'Bob', age: 30},
  3: {name: 'Charlie', age: 35}
};

const notFoundError = new Error('User not found');

// Write the getUserData function
# Your code here

Need a hint?

Use if (!users[userId]) to check if user exists. Call callback accordingly.

4

Export the getUserData function

Export the getUserData function using module.exports.

Node.js

const users = {
  1: {name: 'Alice', age: 25},
  2: {name: 'Bob', age: 30},
  3: {name: 'Charlie', age: 35}
};

const notFoundError = new Error('User not found');

function getUserData(userId, callback) {
  if (!users[userId]) {
    callback(notFoundError);
  } else {
    callback(null, users[userId]);
  }
}

// Export getUserData
# Your code here

Need a hint?

Use module.exports = getUserData; to export the function.