Error-first callback convention in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding error-first callback output

What will be the output of this Node.js code using an error-first callback?

Node.js

function fetchData(callback) {
  setTimeout(() => {
    callback(null, 'Data loaded');
  }, 100);
}

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

A Error: null

B Success: null

C Success: Data loaded

D Error: Data loaded

💡 Hint

Remember, the first argument is the error, the second is the data.

The callback receives null as the error, so the else branch runs, printing 'Success: Data loaded'.

❓ Predict Output

intermediate

Detecting error in error-first callback

What will this code print when the callback receives an error?

Node.js

function readFile(callback) {
  setTimeout(() => {
    callback(new Error('File not found'), null);
  }, 50);
}

readFile((err, content) => {
  if (err) {
    console.log('Error:', err.message);
  } else {
    console.log('Content:', content);
  }
});

A Content: File not found

B Content: null

C Error: null

D Error: File not found

💡 Hint

Check how the error object is passed and accessed.

The callback receives an Error object as the first argument, so the if branch runs and prints the error message.

❓ component_behavior

advanced

Behavior of nested error-first callbacks

Consider this nested callback code. What will be logged?

Node.js

function step1(cb) {
  setTimeout(() => cb(null, 'Step1 done'), 30);
}
function step2(cb) {
  setTimeout(() => cb(new Error('Step2 failed'), null), 20);
}

step1((err, res1) => {
  if (err) {
    console.log('Error in step1:', err.message);
  } else {
    step2((err, res2) => {
      if (err) {
        console.log('Error in step2:', err.message);
      } else {
        console.log('Success:', res2);
      }
    });
  }
});

A Error in step1: Step2 failed

B Error in step2: Step2 failed

C Success: Step2 failed

D Success: Step1 done

💡 Hint

Look carefully at which callback receives the error.

step1 succeeds, so step2 runs and returns an error, triggering the error log for step2.

📝 Syntax

advanced

Identify the syntax error in error-first callback usage

Which option contains a syntax error in the error-first callback pattern?

Node.js

function getData(callback) {
  setTimeout(() => {
    callback(null, 'OK');
  }, 10);
}

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

A getData((err data) => { console.log(data); });

B getData((err, data) => { console.log(data); });

C getData(function(err, data) { console.log(data); });

D getData((error, result) => { console.log(result); });

💡 Hint

Check the arrow function parameter list syntax.

Option A misses a comma between parameters, causing a syntax error.

🔧 Debug

expert

Debugging incorrect error handling in callback

What is the main problem with this error-first callback usage?

Node.js

function processData(callback) {
  setTimeout(() => {
    callback(null, 'Done');
  }, 10);
}

processData((err, data) => {
  if (!err) {
    console.log('Error:', err.message);
  } else {
    console.log('Success:', data);
  }
});

A It logs 'Error: Cannot read property message of null' because err is null but accessed as an object.

B It logs 'Success: Done' correctly.

C It throws a syntax error due to wrong if condition.

D It never logs anything because callback is not called.

💡 Hint

Check how err is tested and used inside the callback.

The code checks if !err (meaning no error), then tries to access err.message, causing a runtime error because err is null.