Handling child process errors in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the child_process module.

Node.js

const { exec } = require('[1]');

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

Afs

Bchild_process

Chttp

Dpath

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated modules like 'fs' or 'http'.

Misspelling the module name.

The child_process module is used to create and control child processes in Node.js.

2fill in blank

medium

Complete the code to execute a shell command using exec.

Node.js

exec('ls -l', (error, stdout, stderr) => {
  if ([1]) {
    console.error(`Error: ${error.message}`);
    return;
  }
  console.log(stdout);
});

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

Aerror

Bstdout

Cstderr

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Checking stdout or stderr instead of error.

Ignoring the error argument.

The error argument indicates if the command failed. Checking it helps handle errors.

3fill in blank

hard

Fix the error in the code to properly listen for the 'error' event on a child process.

Node.js

const child = exec('node someScript.js');
child.on('[1]', (err) => {
  console.error('Child process error:', err);
});

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

Aerror

Bexit

Cclose

Ddisconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exit' or 'close' events to catch errors.

Not attaching an error listener.

The 'error' event is emitted when the child process fails to start or has an error.

4fill in blank

hard

Fill both blanks to handle errors and output from a spawned child process.

Node.js

const { spawn } = require('child_process');
const child = spawn('ls', ['-l']);
child.stdout.on('[1]', (data) => {
  console.log(`Output: ${data}`);
});
child.on('[2]', (err) => {
  console.error(`Error: ${err.message}`);
});

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

Adata

Berror

Cclose

Dexit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'close' or 'exit' to get output data.

Not listening to the 'error' event.

The 'data' event on stdout streams output, and 'error' event handles errors.

5fill in blank

hard

Fill all three blanks to create a child process, handle errors, and print output correctly.

Node.js

import { exec } from 'child_process';

exec('node -v', ([1], [2], [3]) => {
  if ([1]) {
    console.error(`Execution error: $[1].message}`);
    return;
  }
  console.log(`Node version: $[2]`);
});

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

Aerror

Bstdout

Cstderr

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of parameters.

Ignoring the error parameter.

The callback parameters are error, stdout, and stderr. We check error first, then print stdout.

Practice

1. What is the main reason to listen for the 'error' event when using Node.js child processes?

easy

A. To catch errors that happen when starting or running the child process

B. To get the output data from the child process

C. To close the child process manually

D. To restart the child process automatically

Solution

1. Step 1: Understand the purpose of the 'error' event

   The 'error' event is triggered if the child process fails to start or encounters a problem during execution.

2. Step 2: Differentiate from other events

   The 'exit' event tells when the process ends, but 'error' specifically catches startup or runtime errors.

3. Final Answer:

   To catch errors that happen when starting or running the child process -> Option A

4. Quick Check:

   'error' event = catch process startup/runtime errors [OK]

Hint: Remember: 'error' means process failed to start or crashed [OK]

Common Mistakes:

• Confusing 'error' with 'exit' event
• Thinking 'error' gives output data
• Assuming 'error' restarts the process

2. Which of the following is the correct way to listen for errors on a child process created with spawn?

easy

A. child.on('error', (err) => { console.error(err); });

B. child.error((err) => { console.error(err); });

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

D. child.catch('error', (err) => { console.error(err); });

Solution

1. Step 1: Recall the correct event listener syntax

   Node.js child processes use on method to listen for events like 'error'.

2. Step 2: Identify the correct event and method

   child.on('error', (err) => { console.error(err); }); uses child.on('error', callback), which is the proper syntax to catch errors.

3. Final Answer:

   child.on('error', (err) => { console.error(err); }); -> Option A

4. Quick Check:

   Use on('error') to catch errors [OK]

Hint: Use child.on('error', callback) to catch errors [OK]

Common Mistakes:

• Using wrong method like .error() or .catch()
• Listening to 'exit' instead of 'error' for errors
• Missing parentheses or wrong event name

3. Consider this code snippet:

const { spawn } = require('child_process');
const child = spawn('node', ['-e', "process.exit(1)"]);
child.on('exit', (code) => {
  console.log('Exit code:', code);
});
child.on('error', (err) => {
  console.error('Error:', err);
});

What will be printed when this runs?

medium

A. Exit code: 0

B. Error: Error message

C. Exit code: 1

D. No output

Solution

1. Step 1: Understand the child process command

   The child runs a Node.js command that immediately exits with code 1 using process.exit(1).

2. Step 2: Check which event triggers

   The process exits normally with code 1, so the 'exit' event fires with code 1; no 'error' event occurs.

3. Final Answer:

   Exit code: 1 -> Option C

4. Quick Check:

   process.exit(1) triggers 'exit' with code 1 [OK]

Hint: process.exit(n) triggers 'exit' with code n, no 'error' [OK]

Common Mistakes:

• Thinking exit code 1 triggers 'error' event
• Expecting 'Exit code: 0' by default
• Ignoring the 'exit' event output

4. You wrote this code to handle errors:

const { exec } = require('child_process');
const child = exec('invalidcommand');
child.on('exit', (code) => {
  if (code !== 0) console.log('Process failed');
});

Why might this code fail to detect the error properly?

medium

A. Because 'exit' event does not fire on errors

B. Because 'error' event should be used to catch command not found errors

C. Because exec does not emit any events

D. Because the callback function is missing

Solution

1. Step 1: Understand exec error handling

   When a command is invalid, exec emits an 'error' event, not just an 'exit' event with a code.

2. Step 2: Identify missing error listener

   The code listens only to 'exit', so it misses errors like 'command not found' which trigger 'error' event.

3. Final Answer:

   Because 'error' event should be used to catch command not found errors -> Option B

4. Quick Check:

   Use 'error' event to catch exec command failures [OK]

Hint: Listen to 'error' event for exec command failures [OK]

Common Mistakes:

• Assuming 'exit' event catches all errors
• Not adding 'error' event listener
• Confusing exec with spawn error handling

5. You want to run a child process and handle both startup errors and non-zero exit codes. Which code snippet correctly handles both cases?

hard

A. const child = spawn('mycmd');
child.on('close', (code) => console.log('Process closed with code', code));

B. const child = spawn('mycmd');
child.on('exit', (code) => {
if (code !== 0) console.error('Startup error:', code);
});

C. const child = spawn('mycmd');
child.on('error', (err) => console.log('Exit code:', err));

D. const child = spawn('mycmd');
child.on('error', (err) => console.error('Startup error:', err));
child.on('exit', (code) => {
if (code !== 0) console.log('Exit code:', code);
});

Solution

1. Step 1: Handle startup errors with 'error' event

   Listening to 'error' catches problems starting the process, like command not found.

2. Step 2: Handle non-zero exit codes with 'exit' event

   The 'exit' event provides the exit code; checking if it's not zero indicates failure.

3. Step 3: Verify option correctness

   const child = spawn('mycmd'); child.on('error', (err) => console.error('Startup error:', err)); child.on('exit', (code) => { if (code !== 0) console.log('Exit code:', code); }); listens to both 'error' and 'exit' properly and logs appropriate messages.

4. Final Answer:

   const child = spawn('mycmd'); child.on('error', (err) => console.error('Startup error:', err)); child.on('exit', (code) => { if (code !== 0) console.log('Exit code:', code); }); -> Option D

5. Quick Check:

   Use 'error' for startup, 'exit' for exit codes [OK]

Hint: Use both 'error' and 'exit' events to cover all errors [OK]

Common Mistakes:

• Ignoring 'error' event for startup failures
• Logging exit code as startup error
• Using only 'close' event without error handling