Why debugging skills matter in Node.js - Challenge Your Understanding

Challenge - 5 Problems

🔧 Debug

intermediate

Identify the error in this Node.js asynchronous code

What error will this Node.js code produce when run?

Node.js

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return data;
}

fetchData().then(result => console.log(result));

console.log(data);

A ReferenceError: data is not defined

B TypeError: fetch is not a function

C SyntaxError: Unexpected token 'await'

D No error, logs the data correctly

💡 Hint

Look at where the variable 'data' is used outside the async function.

The variable data is declared inside the async function fetchData and is not accessible outside it. Trying to log data outside causes a ReferenceError.

❓ component_behavior

intermediate

What happens when a Node.js server callback throws an error?

Consider this Node.js HTTP server code. What will happen if the callback throws an error?

Node.js

import http from 'http';

const server = http.createServer((req, res) => {
  if (req.url === '/error') {
    throw new Error('Test error');
  }
  res.end('Hello World');
});

server.listen(3000);

A The server crashes and stops running

B The error is caught and logged automatically

C The server ignores the error and continues normally

D The client receives a 500 Internal Server Error response

💡 Hint

Think about unhandled exceptions in Node.js event callbacks.

Throwing an error inside the server callback without a try-catch causes the Node.js process to crash, stopping the server.

📝 Syntax

advanced

Which option correctly uses optional chaining in Node.js?

Which code snippet correctly uses optional chaining to safely access a nested property?

A const value = obj.prop?.subProp;

B const value = obj?.prop?.subProp;

C const value = obj?.prop?.subProp();

D const value = obj?.prop.subProp;

💡 Hint

Optional chaining must be used before each property that might be undefined.

Option B safely accesses subProp only if obj and prop exist. Other options either miss optional chaining on a property or incorrectly use it on a function call.

❓ state_output

advanced

What is the output of this Node.js event emitter code?

What will this code print to the console?

Node.js

import EventEmitter from 'events';

const emitter = new EventEmitter();

emitter.on('start', () => {
  console.log('Started');
  emitter.emit('process');
});

emitter.on('process', () => {
  console.log('Processing');
});

emitter.emit('start');

A Processing

B Processing\nStarted

C Started

D Started\nProcessing

💡 Hint

Events can trigger other events synchronously.

The 'start' event handler logs 'Started' and then emits 'process', which logs 'Processing'. So both lines print in order.

🧠 Conceptual

expert

Why is debugging asynchronous code in Node.js challenging?

Which reason best explains why debugging asynchronous Node.js code can be difficult?

A Because asynchronous code runs in parallel threads causing race conditions

B Because asynchronous code always causes memory leaks

C Because asynchronous callbacks can execute out of order making stack traces less clear

D Because Node.js does not support debugging tools for asynchronous code

💡 Hint

Think about how asynchronous callbacks affect the flow of execution and error tracing.

Asynchronous callbacks may run later and out of the original call order, making stack traces harder to follow and causing confusion during debugging.