Handling child process errors in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a child process in Node.js?

A child process is a separate process created by a Node.js program to run tasks independently, allowing the main program to continue running without waiting.

beginner

How do you listen for errors from a child process in Node.js?

You listen for the 'error' event on the child process object to catch errors like failure to start the process.

intermediate

What does the 'exit' event on a child process indicate?

The 'exit' event fires when the child process ends. It provides the exit code and signal, which help determine if the process ended successfully or with an error.

intermediate

Why should you handle both 'error' and 'exit' events when working with child processes?

Because 'error' catches startup failures, while 'exit' tells you if the process ended with an error code. Handling both ensures you catch all problems.

intermediate

What is a common way to handle errors from a child process spawned with spawn()?

Attach listeners to 'error' and 'exit' events, check the exit code, and handle any error messages from stderr to respond properly.

Which event should you listen to catch if a child process fails to start?

A 'close'

B 'exit'

C 'error'

D 'message'

The 'error' event is emitted if the child process fails to start or there is a problem spawning it.

What does an exit code of 0 usually mean for a child process?

A Process ended successfully

B Process crashed

C Process was killed by a signal

D Process is still running

An exit code of 0 means the process finished without errors.

Which Node.js method creates a child process that allows streaming input/output?

A fork()

B exec()

C execFile()

D spawn()

spawn() creates a child process with streams for stdin, stdout, and stderr.

If a child process emits an 'exit' event with a non-zero code, what does it mean?

A The process was paused

B The process ended with an error

C The process is waiting for input

D The process started successfully

A non-zero exit code usually indicates the process ended due to an error.

Which event can you use to detect when a child process has fully closed all stdio streams?

A 'close'

B 'exit'

C 'error'

D 'disconnect'

The 'close' event fires after the process ends and all stdio streams are closed.

Explain how to properly handle errors when spawning a child process in Node.js.

Think about events and exit codes.

Describe the difference between the 'error' and 'exit' events on a child process.

One is about starting, the other about ending.

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