Node.js Best Practice Exception Handling

README.md

Node.js Best Practice Exception Handling

Code examples from this stack overflow answer.

async.js

var divide = function(x,y,next) {
	// if error condition?
	if ( y === 0 ) {
		// "throw" the error safely by calling the completion callback
		// with the first argument being the error
		var err = new Error("Can't divide by zero");
		next(err);
	}
	else {
		// no error occured, continue on
		next(null, x/y);
	}
};

divide(4,2,function(err,result){
	// did an error occur?
	if ( err ) {
		// handle the error safely
		console.log('4/2=err', err);
	}
	else {
		// no error occured, continue on
		console.log('4/2='+result);
	}
});

divide(4,0,function(err,result){
	// did an error occur?
	if ( err ) {
		// handle the error safely
		console.log('4/0=err', err);
	}
	else {
		// no error occured, continue on
		console.log('4/0='+result);
	}
});

domains.js

var d = require('domain').create();
d.on('error', function(err){
	// handle the error safely
	console.log(err);
});

// catch the uncaught errors in this asynchronous or synchronous code block
d.run(function(){
	// the asynchronous or synchronous code that we want to catch thrown errors on
	var err = new Error('example');
	throw err;
});

events.js

// Definite our Divider Event Emitter
var events = require('events');
var Divider = function(){
	events.EventEmitter.call(this);
};  require('util').inherits(Divider, events.EventEmitter);

// Add the divide function
Divider.prototype.divide = function(x,y){
	// if error condition?
	if ( y === 0 ) {
		// "throw" the error safely by emitting it
		var err = new Error("Can't divide by zero");
		this.emit('error', err);
	}
	else {
		// no error occured, continue on
		this.emit('divided', x, y, x/y);
	}

	// Chain
	return this;
};

// Create our divider and listen for errors
var divider = new Divider();
divider.on('error', function(err){
	// handle the error safely
	console.log(err);
});
divider.on('divided', function(x,y,result){
	console.log(x+'/'+y+'='+result);
});

// Divide
divider.divide(4,2).divide(4,0);

/* Outputs:
4/2=2
[Error: Can't divide by zero]
*/

sync.js

// Define divider as a syncrhonous function
var divideSync = function(x,y) {
	// Prepare
	var result;

	// if error condition?
	if ( y === 0 ) {
		// "throw" the error safely by returning it
		result = new Error("Can't divide by zero");
	}
	else {
		// no error occured, continue on
		result = x/y;
	}

	// Return
	return result;
};

// Divide 4/2
var result;
result = divideSync(4,2);
// did an error occur?
if ( result instanceof Error ) {
	// handle the error safely
	console.log('4/2=err', result);
}
else {
	// no error occured, continue on
	console.log('4/2='+result);
}

// Divide 4/0
result = divideSync(4,0);
// did an error occur?
if ( result instanceof Error ) {
	// handle the error safely
	console.log('4/0=err', result);
}
else {
	// no error occured, continue on
	console.log('4/0='+result);
}

trycatch.js

// catch the uncaught errors in this synchronous code block
// try catch statements only work on synchronous code
try {
	// the synchronous code that we want to catch thrown errors on
	var err = new Error('example');
	throw err;
} catch (err) {
	// handle the error safely
	console.log(err);
}

uncaught.js

// catch the uncaught errors that weren't wrapped in a domain or try catch statement
// do not use this in modules, but only in applications, as otherwise we could have multiple of these bound
process.on('uncaughtException', function(err) {
	// handle the error safely
	console.log(err);
});

// the asynchronous or synchronous code that we want to catch thrown errors on
var err = new Error('example');
throw err;