Table of Contents
You can find source code here: https://github.com/aelassas/watcher
Introduction
When working on Node.js applications, you often need to restart your server after some changes. Well, this would be annoying if you have to manually restart your server each time you do some modifications in your source code. Thankfully, there are some utilities available that let you automatically restart your server when changes are detected. And the most famous one is nodemon. nodemon is a tool that helps develop Node.js based applications by automatically restarting the node application when file changes in the directory are detected. The purpose of this tip is not to reinvent the wheel, but to show you how you can create your own utility that monitors for any changes in your Node.js application and automatically restart the server in just a few lines of code.
Setting up the Project
The first step will be to initialize our Node.js project:
npm init
Then, we will have to update package.json in order to add support of ES6 by setting module as type:
{
"name": "watcher",
"type": "module",
"version": "1.0.0",
"author": "Akram El Assas"
}
Then, we will install development dependencies:
npm i -D @types/node
-
@types/node: To have autocomplete in Visual Studio Code
Server
We will create a simple web server server.js as follows:
import { createServer } from 'http'
const PORT = 8888
createServer((_, res) => {
res.writeHead(200, { 'Content-Type': 'text/plain' })
res.write('Hello World!')
res.end()
}).listen(PORT)
console.log('HTTP server is running on Port', PORT)
Then, we will create a watcher in order to restart the server each time changes are detected on the parent folder of the server and in its subfolders through the following command:
node watcher.js server.js
Watcher
The watcher will restart the server each time changes are detected.
First and foremost, we'll need to retrieve the command-line arguments. In Node.js, we can have access to command-line arguments through process.argv. The process.argv property returns an array containing the command-line arguments passed when the Node.js process was launched. The first element will be execPath. The second element will be the path to the JavaScript file being executed. The remaining elements will be any additional command-line arguments.
If we run the following command:
node watcher.js server.js
process.argv will be as follows:
[
'C:\\Program Files\\nodejs\\node.exe',
'C:\\dev\\watcher\\src\\watcher.js',
'server.js'
]
The first element is the path to Node.js executable. The second element is the path to watcher.js. And the last element is server.js. Thus, we can start our code by declaring the first and third elements as follows:
const [node, _, file] = process.argv
We then need to create a function that starts a child process that launches Node.js with the specified file as argument which in our case is server.js. To do so, we will use spawn method from child_process module. The child_process.spawn() method spawns a new process using the given command, with command-line arguments in args. The advantages of using spawn method is that we can redirect stdout and stderr of the child process to the parent process using pipe method. pipe method is used to attach a writable stream to a readable stream so that it consequently switches into flowing mode and then pushes all the data that it has to the attached writable stream. The source code of our function will look like follows:
import { spawn } from 'child_process'
const [node, _, file] = process.argv
const spawnNode = () => {
const childProcess = spawn(node, [file])
childProcess.stdout.pipe(process.stdout)
childProcess.stderr.pipe(process.stderr)
childProcess.on('close', (code) => {
if (code !== null) {
process.exit(code)
}
})
return childProcess
}
First of all, we spawn a child Node.js process with the given file argument. Then, we redirect stdout and stderr of the child process to the parent process using pipe method. Then, when the child process is closed, we exit the parent process with the same exit code. The process.exit() method instructs Node.js to terminate the process synchronously with an exit status of code. If code is omitted, exit uses either the success code 0 or the value of process.exitCode if it has been set. Node.js will not terminate until all the exit event listeners are called. And finally, we return the child process.
Now, we need to detect changes on the parent folder of the file and its subfolders. And each time a change related to a JavaScript file is detected, we will kill the child process and spawn the child process again. To do so, we will use watch method from fs/promises module. The fs/promises.watch() method returns an async iterator that watches for changes on filename, where filename is either a file or a directory. We will create a watcher on the parent folder of the file. Then, we will iterate through watcher. We will ignore node_modules folder and each time a change is detected on a JavaScript file, we will kill the child process and spawn it again as follows:
let childProcess = spawnNode()
const watcher = watch(dirname(file), { recursive: true })
for await (const event of watcher) {
if (
!event.filename.includes('node_modules') &&
event.filename.endsWith('.js')
) {
childProcess.kill('SIGKILL')
childProcess = spawnNode()
}
}
The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the SIGTERM signal. SIGKILL signal cannot be caught, blocked, or ignored and forces the child process to stop. See signal(7) for a list of available signals.
That's it! We have finished our own nodemon in just a few lines of code.
Last but not least, we need to add the start and dev scripts to our package.json as follows:
{
"name": "watcher",
"type": "module",
"version": "1.0.0",
"scripts": {
"start": "node server.js",
"dev": "node watcher.js server.js"
},
"author": "Akram El Assas",
"devDependencies": {
"@types/node": "^18.11.17"
}
}
In order to start our application, just type the following command:
npm run dev
Now if we run our application and make changes to server.js, the server will be restarted automatically. We no longer need to stop and start the server manually.
Bringing the Pieces Together
We set up our own nodemon in just a few lines of code. Now, The whole source code of our watcher.js looks like follows:
import { spawn } from 'child_process'
import { watch } from 'fs/promises'
import { dirname } from 'path'
const [node, _, file] = process.argv
const spawnNode = () => {
const childProcess = spawn(node, [file])
childProcess.stdout.pipe(process.stdout)
childProcess.stderr.pipe(process.stderr)
childProcess.on('close', (code) => {
if (code !== null) {
process.exit(code)
}
})
return childProcess
}
let childProcess = spawnNode()
const watcher = watch(dirname(file), { recursive: true })
for await (const event of watcher) {
if (
!event.filename.includes('node_modules') &&
event.filename.endsWith('.js')
) {
childProcess.kill('SIGKILL')
childProcess = spawnNode()
}
}
This is just a simple example but you can imagine other situations where you monitor changes on video files and each time a change is detected, a conversion child process (ffmpeg) is launched.
You can also implement other options such as:
- Ignoring specific files or directories
- Watching specific directories
- Monitoring multiple directories
- Specifying extension watch list
- Delaying restarting
- Running executables other than Node.js such as Python, Ruby, make, etc.
- and so on...
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