Error-first callback convention in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a callback that handles an error first.

Node.js

function callback([1], data) {
  if ([1]) {
    console.error('Error:', [1]);
  } else {
    console.log('Data:', data);
  }
}

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

Aerror

Bresult

Cresponse

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using the data argument as the first parameter instead of the error.

Naming the first argument something unrelated like 'result'.

The first argument in an error-first callback is the error object, commonly named error.

2fill in blank

medium

Complete the code to call the callback with an error.

Node.js

function fetchData(callback) {
  const error = new Error('Failed to fetch');
  callback([1], null);
}

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

Aundefined

Bnull

Cerror

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Passing null as the first argument when there is an error.

Passing false or undefined instead of the error object.

When there is an error, the first argument to the callback should be the error object.

3fill in blank

hard

Fix the error in the callback usage to follow the error-first convention.

Node.js

function processData(callback) {
  const data = { value: 42 };
  callback([1]);
}

processData(function(err, data) {
  if (err) {
    console.error(err);
  } else {
    console.log(data.value);
  }
});

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

Anull, data

Bdata

Cundefined

Ddata, null

Attempts:

3 left

💡 Hint

Common Mistakes

Passing data as the first argument instead of the second.

Omitting the second argument when there is no error.

The callback should be called with null as the first argument to indicate no error, and the data as the second argument.

4fill in blank

hard

Fill both blanks to create an error-first callback that logs error or success message.

Node.js

function handleResult([1], [2]) {
  if ([1]) {
    console.log('Error occurred:', [1]);
  } else {
    console.log('Success:', [2]);
  }
}

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

Aerr

Berror

Cresult

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of error and data parameters.

Using inconsistent variable names.

The first parameter is the error (named error), and the second is the data (named data).

5fill in blank

hard

Fill all three blanks to define a function that calls a callback with error or data properly.

Node.js

function getUser(id, [1]) {
  if (id <= 0) {
    [1](new Error('Invalid ID'), null);
  } else {
    const user = { id: id, name: 'Alice' };
    [1](null, [2]);
  }
}

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

Acallback

Buser

Cdata

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the callback parameter.

Passing wrong variables to the callback.

The function uses callback as the callback name. It calls callback with an error or with user data.

Practice

1. What is the main purpose of the error-first callback convention in Node.js?

easy

A. To avoid using callbacks and use promises instead

B. To pass the result as the first argument and error as the second

C. To handle errors only after all callbacks have finished

D. To always pass the error as the first argument to the callback function

Solution

1. Step 1: Understand the callback argument order

   The error-first callback convention means the first argument is always the error if any occurred.

2. Step 2: Recognize the purpose of this order

   This helps developers check for errors before processing results, making code safer and clearer.

3. Final Answer:

   To always pass the error as the first argument to the callback function -> Option D

4. Quick Check:

   Error is first argument [OK]

Hint: Error always comes first in callbacks [OK]

Common Mistakes:

• Thinking result comes before error
• Confusing error-first with promise usage
• Ignoring error handling in callbacks

2. Which of the following is the correct syntax for an error-first callback function in Node.js?

easy

A. function callback(error, result) { ... }

B. function callback(result, error) { ... }

C. function callback() { ... }

D. function callback(result) { ... }

Solution

1. Step 1: Identify the correct parameter order

   The error-first callback convention requires the first parameter to be error, second to be result.

2. Step 2: Match the syntax

   Only the function with parameters (error, result) follows this convention correctly.

3. Final Answer:

   function callback(error, result) { ... } -> Option A

4. Quick Check:

   Callback params: error first, result second [OK]

Hint: Error is first parameter in callback functions [OK]

Common Mistakes:

• Swapping error and result parameters
• Omitting error parameter
• Using only one parameter for result

3. Consider the following code snippet:

function readFile(callback) {
  setTimeout(() => {
    callback(null, 'file content');
  }, 100);
}

readFile((err, data) => {
  if (err) {
    console.log('Error:', err);
  } else {
    console.log('Data:', data);
  }
});

What will be printed to the console?

medium

A. Error: null

B. Data: file content

C. Error: file content

D. Nothing will be printed

Solution

1. Step 1: Analyze the callback invocation

   The callback is called with null as error and 'file content' as data after 100ms.

2. Step 2: Check the callback logic

   Since err is null (no error), the else branch runs and logs 'Data: file content'.

3. Final Answer:

   Data: file content -> Option B

4. Quick Check:

   Null error means success, so data logs [OK]

Hint: Null error means no error, print data [OK]

Common Mistakes:

• Printing error when error is null
• Confusing error and data values
• Expecting no output due to async

4. Identify the error in this code using error-first callback convention:

function getData(callback) {
  const error = null;
  const result = 'Success';
  callback(result, error);
}

getData((err, data) => {
  if (err) {
    console.log('Error:', err);
  } else {
    console.log('Data:', data);
  }
});

medium

A. No error handling is needed here

B. The callback function is missing

C. The callback arguments are reversed; error should be first

D. The error variable should be a string, not null

Solution

1. Step 1: Check callback argument order

   The callback is called with (result, error) but error-first convention requires (error, result).

2. Step 2: Understand impact of reversed arguments

   This reversal causes the callback to treat 'Success' as error and null as data, breaking logic.

3. Final Answer:

   The callback arguments are reversed; error should be first -> Option C

4. Quick Check:

   Callback args must be (error, result) [OK]

Hint: Error must be first argument in callback calls [OK]

Common Mistakes:

• Passing result before error
• Ignoring argument order in callbacks
• Assuming error can be second argument

5. You have a function that reads user data asynchronously and uses an error-first callback:

function fetchUser(id, callback) {
  if (id <= 0) {
    callback(new Error('Invalid ID'), null);
  } else {
    setTimeout(() => {
      callback(null, { id, name: 'Alice' });
    }, 50);
  }
}

How should you call fetchUser to correctly handle errors and print the user's name or the error message?

hard

A. fetchUser(1, (err, user) => { if (err) console.log(err.message); else console.log(user.name); });

B. fetchUser(1, (err, user) => { if (user) console.log(user.name); else console.log(err.message); });

C. fetchUser(1, (user, err) => { if (err) console.log(err.message); else console.log(user.name); });

D. fetchUser(1, (err, user) => { console.log(user.name); });

Solution

1. Step 1: Check callback parameter order and error handling

   The callback parameters must be (err, user). We check if err exists first to handle errors.

2. Step 2: Verify correct conditional logic

   If err exists, print err.message; otherwise, print user.name. fetchUser(1, (err, user) => { if (err) console.log(err.message); else console.log(user.name); }); follows this correctly.

3. Final Answer:

   fetchUser(1, (err, user) => { if (err) console.log(err.message); else console.log(user.name); }); -> Option A

4. Quick Check:

   Check error first, then use user [OK]

Hint: Always check error before using result in callback [OK]

Common Mistakes:

• Swapping error and user parameters
• Not checking error before accessing user
• Ignoring error handling completely