Handling child process errors in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a child process emits an 'error' event?

Consider this Node.js code snippet that spawns a child process. What will be logged if the child process fails to start due to an invalid command?

Node.js

import { spawn } from 'child_process';

const child = spawn('invalid_command');

child.on('error', (err) => {
  console.log('Error event:', err.message);
});

child.on('exit', (code) => {
  console.log('Exit event with code:', code);
});

A No logs appear because the child process silently fails.

B Logs: 'Exit event with code: 1' and no error event is emitted.

C Logs both 'Error event: spawn invalid_command ENOENT' and 'Exit event with code: 0'.

D Logs: 'Error event: spawn invalid_command ENOENT' and no exit event is emitted.

💡 Hint

The 'error' event is emitted when the process cannot be spawned or killed.

When the child process fails to start (e.g., invalid command), the 'error' event is emitted with an error object describing the failure. The 'exit' event is not emitted because the process never started.

❓ state_output

intermediate

What is the value of 'exitCode' after a child process error?

Given the following code, what will be the value of 'exitCode' after the child process emits an 'error' event?

Node.js

import { spawn } from 'child_process';

let exitCode = null;
const child = spawn('badcmd');

child.on('error', () => {
  exitCode = 'error_occurred';
});

child.on('exit', (code) => {
  exitCode = code;
});

A 'error_occurred'

B 0

C null

D undefined

💡 Hint

Check which event sets the exitCode when the process fails to start.

Since the process fails to spawn, the 'error' event fires and sets exitCode to 'error_occurred'. The 'exit' event does not fire, so exitCode remains 'error_occurred'.

📝 Syntax

advanced

Which option correctly attaches an error handler to a child process?

Identify the correct syntax to handle errors from a spawned child process.

Node.js

import { spawn } from 'child_process';
const child = spawn('ls');

A child.handle('error', (err) => console.error('Error:', err));

B child.error((err) => console.error('Error:', err));

C child.on('error', (err) => console.error('Error:', err));

D child.addEventListener('error', (err) => console.error('Error:', err));

💡 Hint

Node.js child processes use EventEmitter pattern.

The correct method to listen for events on child processes is 'on'. Other methods like 'error', 'addEventListener', or 'handle' do not exist on the child process object.

🔧 Debug

advanced

Why does this child process error handler never run?

Examine the code below. Why does the error handler never log anything even if the command is invalid?

Node.js

import { spawn } from 'child_process';

const child = spawn('invalidcmd');

child.on('exit', (code) => {
  console.log('Exited with code:', code);
});

child.on('close', (code) => {
  console.log('Closed with code:', code);
});

A Because the 'exit' event only fires on successful commands.

B Because there is no 'error' event listener attached to the child process.

C Because 'close' event replaces the 'error' event.

D Because spawn automatically retries invalid commands silently.

💡 Hint

Check if the code listens for the 'error' event.

The 'error' event is emitted when the child process fails to start. Since no listener is attached for 'error', the error is unhandled and the handler never runs.

🧠 Conceptual

expert

What is the correct sequence of events when a child process fails to spawn?

Order the events emitted by a Node.js child process when the spawn command is invalid and fails immediately.

A 2 only

B 4, 2, 1, 3

C 2, 4, 1, 3

D 1, 3, 2, 4

💡 Hint

Consider which events fire when the process never starts.

When spawn fails immediately, only the 'error' event is emitted. The 'spawn', 'exit', and 'close' events do not fire because the process never started.