Child process exit codes in Node.js - Step-by-Step Execution

Concept Flow - Child process exit codes

Start parent process

↓

Spawn child process

↓

Child runs task

↓

Child finishes

↓

Child sends exit code

↓

Parent receives exit code

↓

Parent handles exit code

↓

End

The parent process starts a child process, which runs a task and then exits with a code. The parent receives this code and can act based on it.

Execution Sample

Node.js

const { spawn } = require('child_process');
const child = spawn('node', ['-e', 'process.exit(1)']);
child.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

This code spawns a child Node.js process that immediately exits with code 1, and the parent logs the exit code.

Execution Table

Step	Action	Child Process State	Exit Code	Parent Reaction
1	Parent spawns child process	Running	N/A	Waiting for exit
2	Child runs code: process.exit(1)	Exiting	1	Receives exit event
3	Parent receives exit event	Exited	1	Logs: Child exited with code 1
4	Parent finishes handling	Exited	1	Process ends or continues

💡 Child process exits with code 1, parent receives and logs it.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
child	undefined	Child process object (running)	Child process object (exiting)	Child process object (exited)	Child process object (exited)
code	undefined	undefined	1	1	1

Key Moments - 2 Insights

Why does the exit code appear only after the child process finishes?

Because the exit code is sent when the child process ends, as shown in execution_table step 2 and 3 where the parent waits until the child exits to get the code.

Can the exit code be anything other than 0 or 1?

Yes, exit codes can be any integer between 0 and 255, where 0 means success and others indicate different errors or statuses, as the child sets it before exiting (step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the child process state at step 2?

A Running

B Exited

C Exiting

D Not started

💡 Hint

Check the 'Child Process State' column at step 2 in the execution_table.

At which step does the parent receive the exit code from the child?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for when the parent logs the exit code in the execution_table.

If the child process exited with code 0 instead of 1, what would change in the execution_table?

A The 'Exit Code' column would show 0 instead of 1

B The 'Child Process State' would change to Running

C The parent would not receive an exit event

D The parent would spawn a new child process

💡 Hint

Focus on the 'Exit Code' column values in the execution_table.

Concept Snapshot

Child processes run separately from the parent.
They exit with a code (0 means success, others mean errors).
Parent listens for 'exit' event to get this code.
Use exit codes to know if child succeeded or failed.
Example: process.exit(1) exits with code 1.

Full Transcript

In Node.js, a parent process can create a child process to run tasks separately. The child process runs its code and then exits with a number called an exit code. Zero means success, any other number means some error or status. The parent listens for the child's exit event to get this code and can then decide what to do next. This flow helps the parent know if the child finished well or had problems.