An implementation of the interface system in Javascript that takes a design form similar to other popular object-oriented languages such as Java, C# and PHP.

interfaceDeveloper.js

/* An implementation of the interface system in Javascript that takes a design form
 similar to other popular object-oriented languages such as Java, C# and PHP. */
Function.prototype.implements = function(){
	var class = this;
	
	if(arguments.length < 1) {
		throw new Error("Expecting interface as argument but found none.");
	}
	
	for (var i =0; i<arguments.length; i++){
		var interface = arguments[i], interfaceType = typeof interface;
		var foundInterface = false;
		
		if(interfaceType !== 'object') {
			throw new Error("Expecting interface type 'object' but found '" + interfaceType + "'");
		}
		
		for(var method in interface) {
			var foundMethod = false;
			for(var definedMethod in class) {
				if (definedMethod === method){
					foundMethod = true;
					break;
				}
			}
			if (!foundMethod){
				for(var definedMethod in class.prototype) {
					if (definedMethod === method){
						foundMethod = true; /* found the method within the class prototype */
						break;
					}
				}
			}
			if (!foundMethod) {
				throw new Error("Expecting interface method '" + method + "'");
			}
		}
	}
}

/* Interface Declaration */
var interfacePerson = {
	getName: function(){},
	getAge: function(){}
}
var interfaceEmployee = {
	getSalary: function(){}
}

/* Class declaration */
var Developer = function(opts){
	var salary = typeof opts !== 'undefined' && typeof opts.salary !== 'undefined' ? opts.salary : 0; 

	this.getSalary = function(){
		return salary
	}
}
Developer.prototype = {
	getName: function(){
		return this.name;
	},
	getAge: function(){
		return this.age;
	}
}

/* throws no argument error */
//Developer.implements();

/* throws type error */
//Developer.implements('String');

/* throws 'interface not found' error */
//Developer.implements(['getName','getAge']);

/* this will fail because getSalary is not defined in the prototype object of Developer */
//Developer.implements(interfacePerson,interfaceEmployee);

/* this will pass. The methods in Developer implements the interface interfacePerson */
Developer.implements(interfacePerson);

var enotionz = new Developer();