Spawn vs Fork child process in Node.Js

require('child_process').spawn() starts sending back data from the child process in a stream as soon as the child process starts executing. When you run this command, it send a system command that will run on its own process rather than executing code within your node process. In this no new V8 instance will be created and only one copy of the node module will be active on the processor. It is used when you want the child process to return large amount of data to Node.

Example:

child.js

	var i = 0;
	setInterval(function() {
	if (i++ % 2) {
	console.log('I\'m the child!');
	} else {
	console.error('I\'m the child!');
	}
	}, 1500);

spawn.js

	var spawn = require('child_process').spawn;
	var child = spawn('node', ['child.js']);
	child.stdout.on('data', function (data) {
	console.log('stdin:', data.toString());
	});
	child.stderr.on('data', function (data) {
	console.log('child stderr: ', data.toString());
	});
	child.on('close', function (code) {
	if (code !== 0) {
	console.log('child process exited with code ' + code);
	}
	});
	setTimeout(function() {
	throw new Error('ZEPELE');
	}, 5000);

require('child_process').fork() is a special instance of spawn thats runs a new instance of the V8 engine. Which actually means you are creating multiple workers running on the same Node code base for different task.

fork.js

	var fork = require('child_process').fork; var child = fork('child.js');
	child.on('close', function (code) {
	if (code !== 0) {
	console.log('child process exited with code ' + code);
	}
	});
	setTimeout(function() {
	throw new Error('ZEPELE');
	}, 5000);

why undefined and null coexist in java script(Node js).

down vote

The basic difference is that undefined and null represent different concepts. If only null was available, you would not be able to determine whether null was set intentionally as the value or whether the value has not been set yet unless you used cumbersome error catching: eg var a; a == null; // This is true a == undefined; // This is true; a === undefined; // This is true; However, if you intentionally set the value to null, strict equality with undefined fails, thereby allowing you to differentiate between null and undefined values: var b = null; b == null; // This is true b == undefined; // This is true; b === undefined; // This is false; basically if we just declare a variable in js its type is undefined var b; console.log(typeof b);// must print undefined but if we declare a object and initialize with null its type is 'object' not undefined var b = null; console.log(typeof b) // must print object not undefined.