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Easy to use function overloads for JavaScript
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Simple function overloading for JavaScript

Have you ever wished there was a nice way to handle method overloading from within JavaScript? We certainly have and decided to come up with a solution. Meet Functionality, a drop in solution to method overloading which avoids many of the pitfalls of competing solutions.


It's always nice to see what you're getting into, so here's a quick example of how you can use Functionality. The following is an excerpt from Iridium showing how the Model.find method is implemented using Functionality.

var fn = require('functionality');

Model.prototype.find = fn
	.first(function() {
		// Any generic setup stuff you want to be available within the handlers
		this.promise = Q.defer();

		this.addCallback = function(callback) {
			this.promise.then(function(result) { 
					return callback(null, result); 
				function(err) { 
					return callback(err); 
	.on(String, fn.gobble(), function(query) {
		this.args[0] = { _id: query };
	}).on(Object, fn.opt(Function), function(query, callback) {
		this.query = query;
		this.project = null;
		this.options = {
			cache: true,
			wrap: true

		if(callback) this.addCallback(callback);
	}).on(Object, Object, fn.opt(Function), function(query, project, callback) {
		this.query = query;
		this.project = project;
		this.options = {
			cache: true,
			wrap: true

		if(callback) this.addCallback(callback);
	}).on(Object, Object, Object, fn.opt(Function), fn.gobble(), function(query, project, options, callback) {
		this.query = query;
		this.project = project;
		this.options = options;

		_.defaults(options, {
			cache: true,
			wrap: true

		if(callback) this.addCallback(callback);
	}).or(function() {
		throw new Error('No overload of "find" matches the arguments you provided');
	}).then(function() {
		// Actually perform the DB query
		this.context.collection.find(this.query, this.project, this.options, (function(err, result) {
			if(err) this.promise.reject(err);

		return this.promise.promise;

How It Works


Functionality's API is very simple to get used to, it is primarily built around the on(mask, handler) and or(handler) methods, but there are a number of helpers available which allow you to do some very cool stuff with ease.


Registers a handler which is called before any processing

This method lets you register a handler which is executed before any overload resolution takes place, it's great for setting up your handler's execution environment.

var delay = fn.first(function() {
	this.deferred = Q.defer();
}).on(String, function(s) {
	return Q.delay(1000).thenResolve(s);
}).on(Number, String, function(s, n) {
	return Q.delay(n).thenResolve(s);

fn.on(mask, mask, handler)

Registers a handler for calls which match the given mask

This method is the backbone of Functionality, it will register a handler which is triggered whenever the method is called with a set of parameters which match. You can create your own masks using fn.mask(handler), or use any type's constructor as a validator. For more details about how masks work, take a look at the Masks section.


Registers a handler which is called if no overloads matched

This method allows you to override the default handler which is triggered when the provided arguments don't match any of the available options. The default one will throw an error explaining that there are no overloads available which satisfy the requirements, but if you want to do something else this is the way you'd do it.

var beAwesome = fn.on(function() {
}).or(function(arg1, arg2, arg3) {
	console.log("You're about %d times too awesome for this!", arguments.length);


Registers a handler which is called after the overload handlers are finished

This method allows you to override the default finalization method (which is responsible for returning the result of using return ... within any of your on() handlers) to run some other custom logic. We use it internally when there's a large overlap in the way a function works for different inputs, using the on() handlers to simply prepare their inputs for this master function.

You can access the return value of the matched handler function using this.result from within the handler function.

var inspect = fn.on(String, function(s) {
	return s;
}).on(Number, function(n) {
	return n.toString;
}).on(Object, function(o) {
	return JSON.stringify(n);
}).on(Array, function(arr) {
	return JSON.stringify(arr);
}).then(function() {


Compiles the overloads into a callable function

Compiles a Functionality overload into a function which can be called as normal, it'll ensure that your current this is transferred into all handlers as this.context and that all arguments are processed correctly.

var sayHi = fn.on(function() {
	return "Hi!";

sayHi(); // => "Hi!"

fn...execute(context, arguments)

Runs the overloaded function, the same as calling fn.compile().apply(context, arguments)

This method lets you manually trigger the execution of an overloaded method within the given context (this.context within handlers) and with a specific set of arguments. It's the same as calling fn.compile().apply(context, arguments) and can be useful for cases where you want to use your overloaded method as a logical segment within a parent method.

function replace(format, placeholders) {
	fn.on(String, function(s) {
		format = s;
		placeholders = {};
	}).on(String, Object, function(s, p) {
		format = s;
		placeholders = p;
	}).execute(this, arguments);

	return format.replace(/:([\w_]+)/g, function(match, id) {
		return placeholders.hasOwnProperty(id) ? placeholders[id] : match;

fn.mask(name, handler)

Creates a new mask option for use in your functions

This method lets you create your own selection logic for a function's arguments, you should provide a function of the form function(value) {} or function(value, next) if you want control over when the function yields control to the next validator. Your function should return true if the validator matches, and if you're accepting next as a parameter you need to remember to return the result of that too.

var between1and10 = fn.mask("Between1And10", function(value) {
	return typeof value == 'number' && value > 1 && value < 10;

var isEmpty = fn.mask("isEmpty", function(value, next) {
	return typeof value == 'string' && !value.length && next();

var startsWith1 = fn.mask("Starts With",  function(value, next) {
	return value === 1;


Marks an argument as optional (may be null or undefined)

You can use this to wrap any other mask option, making the option optional (can be null or undefined). It works great for optional trailing arguments which you want to ensure are of a specific type, like an optional callback.

var fn = fn.on(String, fn.opt(Function), function(path, callback) {
	if(callback) return callback(path);
	return path;


Acts as the logical inverse of the given mask (except for undefined)

This method lets you easily reverse the behaviour of a specific mask for all defined values. So you can easily say fn.not(String) to match everything except a string.

var print = fn.on(fn.not(String), function(o) {
	this.args[0] = JSON.stringify(o);
}).on(String, function(s) {
}).on(String, String, function(format, s) {
	console.log(format, s);


Consumes all arguments including and following the current one

Sometimes you just care about the first few arguments and really couldn't care about the rest (the way JavaScript usually works). In this case, gobble() is the method you want to use. It effectively validates whether there's a value in its position or not, and forces Functionality to ignore any remaining values (and validators, if they appear after it). We use it in cases where you want to mutate a specific parameter before re-running the overload resolution (the same as re-calling the method).

var fn = fn.on(String, fn.gobble(), function(id) {
	this.args[0] = { _id: id };
}).on(Object, fn.opt(Function), function(query, callback) {
	// Some funky stuff
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