JavaScript Basics: Arguments and Overloading?

Arguments in JavaScript functions are optional. I might declare that a function accepts 3 arguments -- but users of the function may pass in 0, 1, 2, or 3 arguments. Question: how do I know whether all arguments were passed in? And if only some of them were, how do I know which ones were?

First question: how do I know whether all arguments were passed into a function?

JavaScript makes this easy by giving you two variables to check: arguments.length -- the number of arguments actually passed in; and function_name.length -- the number of arguments the function was expecting.

So, if your function requires that every argument be passed in, you can do something like this:

var myFunction = function( a, b, c ) {
  if ( myFunction.length !== arguments.length ){
    return;
  };
};

Second question: if I am allowing flexibility in arguments passed in, how do I know which ones were actually provided?

This one has a less satisfactory answer, as it's only a partial answer. If you only need to check the number of arguments passed in and not necessarily the type of those arguments, John Resig has posted a great solution, found at http://ejohn.org/blog/javascript-method-overloading/.

Sometimes, though, the type of those arguments is also important.

Look at this code:

var myFunction = function( name, age, subscribe ){
};

Here, I need the person's name -- but perhaps age and whether they want to subscribe to my fabulous "Investment Secrets of the Ages" are optional. I know the types expected for these arguments:

• name : string
• age : number
• subscribe : boolean

In that case, this code will help:

var myFunc = function(name , age, subscribe ){
	var args = arguments;
	var expanded_args = "";
	
	for ( var i = 0; i < myFunc.length; i++ ) {
		expanded_args += typeof args[ i ] + '_';
	};
	
	expanded_args = expanded_args.substring( 0, expanded_args.length - 1 );
	console.log( expanded_args );
	
	// overloaded functions
	var overloaded_functions = function() {
		
		return {
		
			name_age_subscribe : function() {
				// name, age, and subscribe were passed in
				var _name = args[ 0 ];
				var _age = args[ 1 ];
				var _subscribe = args[ 2 ];
				
				console.log( "Now, I'll do something with these arguments: \n" +
				"name : " + _name + "\n" +
				"age : " + _age + "\n" +
				"subscribe : " + _subscribe );
			}, 
		
			name_age : function() {
				// name and age were passed in
				var _name = args[ 0 ];
				var _age = args[ 1 ];
				
				console.log( "Now, I'll do something with these arguments: \n" +
				"name : " + _name + "\n" +
				"age : " + _age );
			},
			
			name_subscribe : function() {
				// name and subscribe were passed in
				var _name = args[ 0 ];
			 	var _subscribe = args[ 1 ];
			 
				console.log( "Now, I'll do something with these arguments: \n" +
			 	"name : " + _name + "\n" +
			 	"subscribe : " + _subscribe );
			}, 
			
			name : function() {
				var _name = args[ 0 ];
				
				console.log( "Now, I'll do something with these arguments: \n" +
				"name : " + _name );
			}					
			
		} // end return
	} (); // end overloaded_functions
	
	switch ( expanded_args ){
		case 'string_number_boolean' :
			// all arguments were passed in
			overloaded_functions.name_age_subscribe();
			break;
			
		case 'string_number_undefined' :
			// name and age only were passed in
			overloaded_functions.name_age();
			break;
			
		case 'string_boolean_undefined' :
			// name and subscribe only were passed in
			overloaded_functions.name_subscribe();
			break;
			
		case 'string_undefined_undefined' :
			// name only was passed in
			overloaded_functions.name();
			break;
			
		default : return;
	};	
};
	
// exercise the code
myFunc( 'Ann', 21, false );
myFunc( 'Bob', 31 );
myFunc( 'Carol', true );
myFunc( 'Darrell' );

For most things, frankly, this is overkill. But recently, I ran into a situation where I did need to deal with different numbers and types of functions. This is one way to do it.

This code can be downloaded from http://github.com/halhelms/Overloaded-JavaScript-Functions