Does JavaScript have the interface type (such as Java’s ‘interface’)?

Does JavaScript have the interface type (such as Java’s ‘interface’)?

I’m learning how to make OOP with JavaScript. Does it have the interface concept (such as Java’s interface)?
So I would be able to create a listener…


Solution 1:

There’s no notion of “this class must have these functions” (that is, no interfaces per se), because:

  1. JavaScript inheritance is based on objects, not classes. That’s not a big deal until you realize:
  2. JavaScript is an extremely dynamically typed language — you can create an object with the proper methods, which would make it conform to the interface, and then undefine all the stuff that made it conform. It’d be so easy to subvert the type system — even accidentally! — that it wouldn’t be worth it to try and make a type system in the first place.

Instead, JavaScript uses what’s called duck typing. (If it walks like a duck, and quacks like a duck, as far as JS cares, it’s a duck.) If your object has quack(), walk(), and fly() methods, code can use it wherever it expects an object that can walk, quack, and fly, without requiring the implementation of some “Duckable” interface. The interface is exactly the set of functions that the code uses (and the return values from those functions), and with duck typing, you get that for free.

Now, that’s not to say your code won’t fail halfway through, if you try to call some_dog.quack(); you’ll get a TypeError. Frankly, if you’re telling dogs to quack, you have slightly bigger problems; duck typing works best when you keep all your ducks in a row, so to speak, and aren’t letting dogs and ducks mingle together unless you’re treating them as generic animals. In other words, even though the interface is fluid, it’s still there; it’s often an error to pass a dog to code that expects it to quack and fly in the first place.

But if you’re sure you’re doing the right thing, you can work around the quacking-dog problem by testing for the existence of a particular method before trying to use it. Something like

if (typeof(someObject.quack) == "function")
    // This thing can quack

So you can check for all the methods you can use before you use them. The syntax is kind of ugly, though. There’s a slightly prettier way:

Object.prototype.can = function(methodName)
     return ((typeof this[methodName]) == "function");

if (someObject.can("quack"))

This is standard JavaScript, so it should work in any JS interpreter worth using. It has the added benefit of reading like English.

For modern browsers (that is, pretty much any browser other than IE 6-8), there’s even a way to keep the property from showing up in

Object.defineProperty(Object.prototype, 'can', {
    enumerable: false,
    value: function(method) {
        return (typeof this[method] === 'function');

The problem is that IE7 objects don’t have .defineProperty at all, and in IE8, it allegedly only works on host objects (that is, DOM elements and such). If compatibility is an issue, you can’t use .defineProperty. (I won’t even mention IE6, because it’s rather irrelevant anymore outside of China.)

Another issue is that some coding styles like to assume that everyone writes bad code, and prohibit modifying Object.prototype in case someone wants to blindly use If you care about that, or are using (IMO broken) code that does, try a slightly different version:

function can(obj, methodName)
     return ((typeof obj[methodName]) == "function");

if (can(someObject, "quack"))

Solution 2:

Pick up a copy of ‘JavaScript design patterns‘ by Dustin Diaz. There’s a few chapters dedicated to implementing JavaScript interfaces through Duck Typing. It’s a nice read as well. But no, there’s no language native implementation of an interface, you have to Duck Type.

// example duck typing method
var hasMethods = function(obj /*, method list as strings */){
    var i = 1, methodName;
    while((methodName = arguments[i++])){
        if(typeof obj[methodName] != 'function') {
            return false;
    return true;

// in your code
if(hasMethods(obj, 'quak', 'flapWings','waggle')) {
    //  IT'S A DUCK, do your duck thang

Solution 3:

JavaScript (ECMAScript edition 3) has an implements reserved word saved up for future use. I think this is intended exactly for this purpose, however, in a rush to get the specification out the door they didn’t have time to define what to do with it, so, at the present time, browsers don’t do anything besides let it sit there and occasionally complain if you try to use it for something.

It is possible and indeed easy enough to create your own Object.implement(Interface) method with logic that baulks whenever a particular set of properties/functions are not implemented in a given object.

I wrote an article on object-orientation where use my own notation as follows:

// Create a 'Dog' class that inherits from 'Animal'
// and implements the 'Mammal' interface
var Dog = Object.extend(Animal, {
    constructor: function(name) {, name);
    bark: function() {

There are many ways to skin this particular cat, but this is the logic I used for my own Interface implementation. I find I prefer this approach, and it is easy to read and use (as you can see above). It does mean adding an ‘implement’ method to Function.prototype which some people may have a problem with, but I find it works beautifully.

Function.prototype.implement = function() {
    // Loop through each interface passed in and then check 
    // that its members are implemented in the context object (this).
    for(var i = 0; i < arguments.length; i++) {
       // .. Check member's logic ..
    // Remember to return the class being tested
    return this;

Solution 4:

JavaScript Interfaces:

Though JavaScript does not have the interface type, it is often times needed. For reasons relating to JavaScript’s dynamic nature and use of Prototypical-Inheritance, it is difficult to ensure consistent interfaces across classes — however, it is possible to do so; and frequently emulated.

At this point, there are handfuls of particular ways to emulate Interfaces in JavaScript; variance on approaches usually satisfies some needs, while others are left unaddressed. Often times, the most robust approach is overly cumbersome and stymies the implementor (developer).

Here is an approach to Interfaces / Abstract Classes that is not very cumbersome, is explicative, keeps implementations inside of Abstractions to a minimum, and leaves enough room for dynamic or custom methodologies:

function resolvePrecept(interfaceName) {
    var interfaceName = interfaceName;
    return function curry(value) {
        /*      throw new Error(interfaceName + ' requires an implementation for ...');     */
        console.warn('%s requires an implementation for ...', interfaceName);
        return value;

var iAbstractClass = function AbstractClass() {
    var defaultTo = resolvePrecept('iAbstractClass');

    this.datum1 = this.datum1 || defaultTo(new Number());
    this.datum2 = this.datum2 || defaultTo(new String());

    this.method1 = this.method1 || defaultTo(new Function('return new Boolean();'));
    this.method2 = this.method2 || defaultTo(new Function('return new Object();'));


var ConcreteImplementation = function ConcreteImplementation() {

    this.datum1 = 1;
    this.datum2 = 'str';

    this.method1 = function method1() {
        return true;
    this.method2 = function method2() {
        return {};

    //Applies Interface (Implement iAbstractClass Interface)
    iAbstractClass.apply(this);  // .call / .apply after precept definitions


Precept Resolver

The resolvePrecept function is a utility & helper function to use inside of your Abstract Class. Its job is to allow for customized implementation-handling of encapsulated Precepts (data & behavior). It can throw errors or warn — AND — assign a default value to the Implementor class.


The iAbstractClass defines the interface to be used. Its approach entails a tacit agreement with its Implementor class. This interface assigns each precept to the same exact precept namespace — OR — to whatever the Precept Resolver function returns. However, the tacit agreement resolves to a context — a provision of Implementor.


The Implementor simply ‘agrees’ with an Interface (iAbstractClass in this case) and applies it by the use of Constructor-Hijacking: iAbstractClass.apply(this). By defining the data & behavior above, and then hijacking the Interface’s constructor — passing Implementor’s context to the Interface constructor — we can ensure that Implementor’s overrides will be added, and that Interface will explicate warnings and default values.

This is a very non-cumbersome approach which has served my team & I very well for the course of time and different projects. However, it does have some caveats & drawbacks.


Though this helps implement consistency throughout your software to a significant degree, it does not implement true interfaces — but emulates them. Though definitions, defaults, and warnings or errors are explicated, the explication of use is enforced & asserted by the developer (as with much of JavaScript development).

This is seemingly the best approach to “Interfaces in JavaScript”, however, I would love to see the following resolved:

  • Assertions of return types
  • Assertions of signatures
  • Freeze objects from delete actions
  • Assertions of anything else prevalent or needed in the specificity of the JavaScript community

That said, I hope this helps you as much as it has my team and I.

Solution 5:

Hope, that anyone who’s still looking for an answer finds it helpful.

You can try out using a Proxy (It’s standard since ECMAScript 2015):

latLngLiteral = new Proxy({},{
    set: function(obj, prop, val) {
        //only these two properties can be set
        if(['lng','lat'].indexOf(prop) == -1) {
            throw new ReferenceError('Key must be "lat" or "lng"!');

        //the dec format only accepts numbers
        if(typeof val !== 'number') {
            throw new TypeError('Value must be numeric');

        //latitude is in range between 0 and 90
        if(prop == 'lat'  && !(0 < val && val < 90)) {
            throw new RangeError('Position is out of range!');
        //longitude is in range between 0 and 180
        else if(prop == 'lng' && !(0 < val && val < 180)) {
            throw new RangeError('Position is out of range!');

        obj[prop] = val;

        return true;

Then you can easily say:

myMap = {}
myMap.position = latLngLiteral;

Solution 6:

You need interfaces in Java since it is statically typed and the contract between classes should be known during compilation. In JavaScript it is different. JavaScript is dynamically typed; it means that when you get the object you can just check if it has a specific method and call it.