Node.js - Process

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The process object in Node.js is a global object, although it is defined in process module. It is an instance of EventEmitter class. The process object provides information on current Node.js process. With the help of a number of methods and properties associated with this object, it is possible to control the current Node.js process.

Process Events

The process object is an instance of EventEmitter and emits the following events −

Example

In the following code, the beforeExit event of the process object is associated with a callback arrow function. Similarly the exit event calls another callback function. The function on beforeExit runs first, followed by the one on exit event.

process.on('beforeExit', (code) => {
   console.log('A beforeExit event occured with code: ', code);
});

process.on('exit', (code) => {
   console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

Output

This message is displayed first.
A beforeExit event occured with code:  0
Process exit event with code:  0

Note that the listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the 'exit' event listeners causing any additional work still queued in the event loop to be abandoned. In the following example, for instance, the timeout will never occur −

Example

process.on('exit', function(code) {
   // Following code will never execute.
   setTimeout(function() {
      console.log("This will not run");
   }, 0);
   
   console.log('About to exit with code:', code);
});
console.log("Program Ended");

Output

Program Ended
About to exit with code: 0

Process Methods

abort()

The abort() method immediately terminates the current process and then creates a core file.

Example

The following program prints a message to the console after every one second, and terminates the loop after 5 seconds as the abort() function is called.

const abortfunction = () => { 
   console.log('Start...'); 
  
   // It prints the message after every 1 second 
   setInterval((function() { 
      return console.log('Hello World'); 
   }), 1000); 
  
   // It calls process.abort() after 5 seconds 
   setTimeout((function() { 
      return process.abort(); 
   }), 5000); 
}; 
  
abortfunction();

Output

Start...
Hello World
Hello World
Hello World
Hello World

The termination is followed by a long core file output.

chdir()

This method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified directory does not exist).

cwd()

The process.cwd() method returns the current working directory of the Node.js process.

Example

console.log(`Starting directory: ${process.cwd()}`);
try {
   process.chdir('NewNodeApp');
   console.log(`New directory: ${process.cwd()}`);
} catch (err) {
   console.error(`chdir: ${err}`);
}

Output

Starting directory: D:\nodejs
New directory: D:\nodejs\NewNodeApp

If the directory is not found, the output will be as follows −

Starting directory: D:\nodejs
chdir: Error: ENOENT: no such file or directory, chdir 'D:\nodejs' -> 'NewDir'

exit()

This method terminates the current process synchronously with an exit status of code (default is 'success' code 0). Node.js will not terminate until all the 'exit' event listeners are called.

Example

console.log('Code running'); 
process.on('exit', function(code) { 
   return console.log(`exiting with the error code : ${code}`); 
}); 
setTimeout((function() { 
   return process.exit(0); //after 5 sec
}), 5000);

Output

Code running
exiting with the error code : 0

kill()

This method terminates the current process and sends a signal to the process identified by pid.

kill(pid[, signal])

Parameters

pid : A process ID

signal : The signal to send, either as a string or number. Default: 'SIGTERM'.

Signal names are strings such as 'SIGINT' or 'SIGHUP'.

Example

The following code obtains the pid of the current process. It waits for a long enough duration before it exits normally. In between if you emit SIGINT signal by pressing ctrl-C, the process doesn’t terminate.

const pid = process.pid;
console.log(`Process ID: ${pid}`); 
process.on('SIGHUP', () => console.log('Received: SIGHUP')); 
process.on('SIGINT', () => console.log('Received: SIGINT')); 
setTimeout(() => {}, 100000); // keep process alive 
setTimeout((function() { 
   return process.kill(pid, 'SIGINT'); //after 5 sec
}), 5000);

The terminal shows the process ID and Received: SIGINT whenever ctrlC is pressed. After another timeout of 5 seconds, the kill() method is called, which terminates the process

Process ID: 1776
Received: SIGINT

memoryUsage()

This function returns an object describing the memory usage of the Node.js process measured in bytes.

Example

console.log(process.memoryUsage());

Output

{
  rss: 24768512,
  heapTotal: 4079616,
  heapUsed: 3153848,
  external: 1097184,
  arrayBuffers: 10519
}

nextTick()

This function defers the execution of a callback function until the next Event Loop Iteration.

nextTick(callback[, ...args])

This function is an important part of the event loop in asynchronous API of Node.js. In Node.js, each iteration of an Event Loop is called a tick. To schedule a callback function to be invoked in the next iteration of the Event Loop, we use process.nextTick().

Example

console.log('start');
process.nextTick(() => {
   console.log('nextTick callback executed in next iteration');
});
console.log('scheduled');

Output

start
scheduled
nextTick callback executed in next iteration

Process properties

process.arch

The operating system CPU architecture for which the Node.js binary was compiled.

Possible values are −

• 'arm',

• 'arm64',

• 'ia32',

• 'loong64',

• 'mips',

• 'mipsel',

• 'ppc',

• 'ppc64',

• 'riscv64',

• 's390',

• 's390x',

• 'x64'

Example

console.log(`This processor architecture is ${process.arch}`);

Output

This processor architecture is x64

process.argv

This property returns an array containing the command-line arguments passed when the Node.js process was launched. You can pass arguments to the script to be executed from the command line. The arguments are stored in a an array process.argv. The 0th element in the array is the node executable, first element is the JavaScript file, followed by the arguments passed.

Save the following script as hello.js and run it from command line, pass a string argument to it from command line.

const args = process.argv;

console.log(args); 

const name = args[2];

console.log("Hello,", name);

In the terminal, enter

PS D:\nodejs> node hello.js TutorialsPoint
[ 'C:\\nodejs\\node.exe', 'c:\\nodejs\\a.js', 'TutorialsPoint' ]
Hello, TutorialsPoint

process.env

property returns an object containing the environment variables.

Example

const processEnvKeys = Object.keys(process.env);

processEnvKeys.forEach((key) => {
   console.log(`${key}: ${process.env[key]}`);
});

Output

SystemDrive: C:
SystemRoot: C:\WINDOWS
TEMP: C:\Users\mlath\AppData\Local\Temp
TMP: C:\Users\mlath\AppData\Local\Temp
USERDOMAIN: GNVBGL3
USERDOMAIN_ROAMINGPROFILE: GNVBGL3
USERNAME: mlath
USERPROFILE: C:\Users\mlath
windir: C:\WINDOWS
ZES_ENABLE_SYSMAN: 1
TERM_PROGRAM: vscode
TERM_PROGRAM_VERSION: 1.84.2
LANG: en_US.UTF-8
COLORTERM: truecolor
VSCODE_INJECTION: 1
VSCODE_NONCE: b8069509-e0f5-4bbd-aac9-fc448279b154

You can also set environment variables from the command line. Assign the values to one or more variables before the node executable name.

USER_ID=101 USER_NAME=admin node app.js

Inside the script, the environment variables are available as the properties of the process.env object

process.env.USER_ID; // "101"
process.env.USER_NAME; // "admin"

process.pid

property returns the PID of the process.

Example

const { pid } = require('node:process');

console.log(`This process is pid ${pid}`);

Output

This process is pid 16312

process.platform

This property returns a string identifying the operating system.

The possible values are −

• 'aix'

• 'darwin'

• 'freebsd'

• 'linux'

• 'openbsd'

• 'sunos'

• 'win32'

process.version

This property contains the Node.js version string.

Example

console.log(`Version: ${process.version}`);

Output

Version: v20.9.0