How to Implement Interfaces in JavaScript

Updated: April 17th, 2013

In Object-Oriented languages an interface defines a set of methods which a Class must include in order to implement the interface (otherwise, if the Class is missing the required methods, the code will fail and the interface will throw an error).

Interfaces are useful for making sure developers use the correct implementation of an API.

In JavaScript there are no true 'classic' Object-Oriented features, but through clever usage of the language you can emulate an Interface for use with a JavaScript API.

First thing you want to do is include the following JavaScript file (Interface.js) into your HTML:

/*
 * Constructor that creates a new Interface object for checking a function implements the required methods.
 * @param objectName | String | the instance name of the Interface
 * @param methods | Array | methods that should be implemented by the relevant function
 */
var Interface = function (objectName, methods) {
	// Check that the right amount of arguments are provided
	if (arguments.length != 2) {
		throw new Error ("Interface constructor called with " + arguments.length + "arguments, but expected exactly 2.");
	}
 
	// Create the public properties
	this.name = objectName;
	this.methods = [];
 
	// Loop through provided arguments and add them to the 'methods' array
	for (var i = 0, len = methods.length; i < len; i++) {
		// Check the method name provided is written as a String
		if (typeof methods[i] !== 'string') {
			throw new Error ("Interface constructor expects method names to be " + "passed in as a string.");
		}

		// If all is as required then add the provided method name to the method array
		this.methods.push(methods[i]);
	}
};
 
/*
 * Adds a static method to the 'Interface' constructor
 * @param object | Object Literal | an object literal containing methods that should be implemented
 */
Interface.ensureImplements = function (object) {
	// Check that the right amount of arguments are provided
	if (arguments.length < 2) {
		throw new Error ("Interface.ensureImplements was called with " + arguments.length + "arguments, but expected at least 2.");
	}
 
	// Loop through provided arguments (notice the loop starts at the second argument)
	// We start with the second argument on purpose so we miss the data object (whose methods we are checking exist)
	for (var i = 1, len = arguments.length; i < len; i++) {
		// Check the object provided as an argument is an instance of the 'Interface' class
		var interface = arguments[i];
		if (interface.constructor !== Interface) {
			throw new Error ("Interface.ensureImplements expects the second argument to be an instance of the 'Interface' constructor.");
		}
 
		// Otherwise if the provided argument IS an instance of the Interface class then
		// loop through provided arguments (object) and check they implement the required methods
		for (var j = 0, methodsLen = interface.methods.length; j < methodsLen; j++) {

			var method = interface.methods[j];
 
			// Check method name exists and that it is a function (e.g. test[getTheDate]) 

			// if false is returned from either check then throw an error
			if (!object[method] || typeof object[method] !== 'function') {
				throw new Error ("This Class does not implement the '" + interface.name + "' interface correctly. The method '" + method + "' was not found.");
			}
		}
	}
};

Next, in the HTML file we have an inline script that creates a new Interface instance and then passes it the methods we are expecting. The user of the API would need to implement an object literal of all the methods the interface requires (note the following code could be in a separate SCRIPT file as long as it is run after the above script):

// We pass into the Interface the name of the current Interface instance, 
// followed by an Array of the methods we are expecting to find
var test = new Interface('test', ['details', 'age']);
 
var properties = { 
	name: "Mark McDonnell", 
	actions: { 
		details: function() {
			return "I am " + this.age() + " years old.";
		},
		age: (function(birthdate) {
			var dob = new Date(birthdate),
				today = new Date(),
				ms = today.valueOf() - dob.valueOf(),

				minutes = ms / 1000 / 60,
				hours = minutes / 60,

				days = hours / 24,
				years = days / 365,

				age = Math.floor(years)					
			return function() {
				return age;
			};
		})("1981 08 30")
	}
};
 
// Create a Person constructor that will implement the above properties/methods
function Person(config) {
	// Pass in the methods we are expecting, 
	// followed by the name of the Interface instance that we're checking against
	Interface.ensureImplements(config.actions, test);
 
	this.name = config.name;
	this.methods = config.actions;
}
 
// Create a new instance of the Person constructor...
var me = new Person(properties);
 
// ...and make sure the methods are working
alert(me.methods.age());
alert(me.method.details());