Node.js provides a built-in module called child_process that allows you to create and control child processes. These child processes can be used to run system commands, execute scripts, or spawn other Node.js processes.
When working with child processes, it’s important to handle events that occur during their lifecycle. This can include events such as the child process exiting, receiving an error, or emitting output.
In this blog post, we will explore how to handle events in child processes in Node.js.
Table of Contents
Spawning a Child Process
To create a child process in Node.js, we can use the spawn method from the child_process module. This method returns a ChildProcess object that represents the spawned process.
Here’s an example of how to spawn a child process:
const { spawn } = require('child_process');
const child = spawn('ls', ['-l']);
// Add event listeners here
In this example, we are spawning a child process that runs the ls -l command. Next, let’s see how to listen for events emitted by the child process.
Listening for Events
exit event
The exit event is emitted when the child process exits. It is triggered when the child process finishes running or is terminated. You can listen for this event by adding an event listener to the ChildProcess object.
child.on('exit', (code, signal) => {
console.log(`Child process exited with code ${code}`);
});
In this example, we are logging the exit code when the child process exits. The code parameter represents the exit code of the child process, while the signal parameter indicates the signal that caused the process to terminate (if applicable).
error event
The error event is emitted when the child process encounters an error. This can include issues with spawning the process, running the command, or other runtime errors. It’s important to handle this event to catch and handle any errors that occur.
child.on('error', (err) => {
console.error('Error occurred:', err.message);
});
In this example, we are logging the error message when the child process encounters an error. The err parameter contains an Error object representing the error that occurred.
message event
The message event is only emitted when using the fork method to spawn a child process. It allows communication between the parent and child processes using the send method and the process.on('message') event listener.
To send a message from the child process:
process.send({ data: 'Hello from child process' });
To listen for messages in the parent process:
child.on('message', (message) => {
console.log('Received message:', message);
});
In this example, we are sending a message from the child process using process.send and logging it in the parent process.
Conclusion
Handling events in child processes is crucial for ensuring proper control and error handling. By listening for events like exit, error, and message, you can effectively manage and interact with child processes in your Node.js applications.
Remember to add event listeners after spawning a child process to capture and handle events as needed. This way, you can create more robust and reliable applications with Node.js.