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...

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"))
{
    someObject.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 for...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 for...in. 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"))
{
    someObject.quack();
}

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
}

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) {
        Dog.superClass.call(this, name);
    },
    bark: function() {
        alert('woof');
    }
}).implement(Mammal);

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;
}

##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
};

##Participants

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.

iAbstractClass

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.

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.

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.

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): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

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;

---

If you want to check via instanceof (asked by @Kamaffeather), you can wrap it in an object like so:

class LatLngLiteral {
	constructor(props)
	{
		this.proxy = new Proxy(this, {
			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;
			}
		})
		return this.proxy
	}
}

This can be done without using Proxy but instead the classes getters and setters:

class LatLngLiteral {
	#latitude;
	#longitude;

	get lat()
	{
		return this.#latitude;
	}

	get lng()
	{
		return this.#longitude;
	}
	
	set lat(val)
	{
		//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(!(0 < val && val < 90)) {
			throw new RangeError('Position is out of range!');
		}
		
		this.#latitude = val
	}
	
	set lng(val)
	{
		//the dec format only accepts numbers
		if(typeof val !== 'number') {
			throw new TypeError('Value must be numeric');
		}

		//longitude is in range between 0 and 180
		if(!(0 < val && val < 180)) {
			throw new RangeError('Position is out of range!');
		}
		
		this.#longitude = val
	}
}

abstract interface like this

const MyInterface = {
  serialize: () => {throw "must implement serialize for MyInterface types"},
  print: () => console.log(this.serialize())
}

create an instance:

function MyType() {
  this.serialize = () => "serialized "
}
MyType.prototype = MyInterface

and use it

let x = new MyType()
x.print()

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.

When you want to use a transcompiler, then you could give TypeScript a try. It supports draft ECMA features (in the proposal, interfaces are called "protocols") similar to what languages like coffeescript or babel do.

In TypeScript your interface can look like:

interface IMyInterface {
    id: number; // TypeScript types are lowercase
    name: string;
    callback: (key: string; value: any; array: string[]) => void;
    type: "test" | "notATest"; // so called "union type"
}

What you can't do:

• Define RegExp patterns for type value
• Define validation like string length
• Number ranges
• etc.

there is no native interfaces in JavaScript, there are several ways to simulate an interface. i have written a package that does it

you can see the implantation here

Javascript does not have interfaces. But it can be duck-typed, an example can be found here:

http://reinsbrain.blogspot.com/2008/10/interface-in-javascript.html

This is an old question, nevertheless this topic never ceases to bug me.

As many of the answers here and across the web focus on "enforcing" the interface, I'd like to suggest an alternative view:
> I feel the lack of interfaces the most when I'm using multiple classes that behave similarly (i.e. implement an interface).

For example, I have an Email Generator that expects to receive Email Sections Factories, that "know" how to generate the sections' content and HTML. Hence, they all need to have some sort of getContent(id) and getHtml(content) methods.

> The closest pattern to interfaces (albeit it's still a workaround) I could think of is using a class that'll get 2 arguments, which will define the 2 interface methods.

The main challenge with this pattern is that the methods either have to be static, or to get as argument the instance itself, in order to access its properties. However there are cases in which I find this trade-off worth the hassle.

class Filterable {
  constructor(data, { filter, toString }) {
    this.data = data;
    this.filter = filter;
    this.toString = toString;
    // You can also enforce here an Iterable interface, for example,
    // which feels much more natural than having an external check
  }
}

const evenNumbersList = new Filterable(
  [1, 2, 3, 4, 5, 6], {
    filter: (lst) => {
      const evenElements = lst.data.filter(x => x % 2 === 0);
      lst.data = evenElements;
    },
    toString: lst => `< ${lst.data.toString()} >`,
  }
);

console.log('The whole list:    ', evenNumbersList.toString(evenNumbersList));
evenNumbersList.filter(evenNumbersList);
console.log('The filtered list: ', evenNumbersList.toString(evenNumbersList));

I know this is an old one, but I've recently found myself needing more and more to have a handy API for checking objects against interfaces. So I wrote this: https://github.com/tomhicks/methodical

It's also available via NPM: npm install methodical

It basically does everything suggested above, with some options for being a bit more strict, and all without having to do loads of if (typeof x.method === 'function') boilerplate.

Hopefully someone finds it useful.

This is old but I implemented interfaces to use on ES6 without transpiller.

https://github.com/jkutianski/ES6-Interfaces

It bugged me too to find a solution to mimic interfaces with the lower impacts possible.

One solution could be to make a tool :

/**
@parameter {Array|object} required : method name list or members types by their name
@constructor
*/
let Interface=function(required){
	this.obj=0;
	if(required instanceof Array){
		this.obj={};
		required.forEach(r=>this.obj[r]='function');
	}else if(typeof(required)==='object'){
		this.obj=required;
	}else {
		throw('Interface invalid parameter required = '+required);
	}
};
/** check constructor instance
@parameter {object} scope : instance to check.
@parameter {boolean} [strict] : if true -> throw an error if errors ar found.
@constructor
*/
Interface.prototype.check=function(scope,strict){
	let err=[],type,res={};
	for(let k in this.obj){
		type=typeof(scope[k]);
		if(type!==this.obj[k]){
			err.push({
				key:k,
				type:this.obj[k],
				inputType:type,
				msg:type==='undefined'?'missing element':'bad element type "'+type+'"'
			});
		}
	}
	res.success=!err.length;
	if(err.length){
		res.msg='Class bad structure :';
		res.errors=err;
		if(strict){
			let stk = new Error().stack.split('\n');
			stk.shift();
			throw(['',res.msg,
				res.errors.map(e=>'- {'+e.type+'} '+e.key+' : '+e.msg).join('\n'),
				'','at :\n\t'+stk.join('\n\t')
			].join('\n'));

		}
	}
	return res;
};

Exemple of use :

// create interface tool
let dataInterface=new Interface(['toData','fromData']);
// abstract constructor
let AbstractData=function(){
	dataInterface.check(this,1);// check extended element
};
// extended constructor
let DataXY=function(){
	AbstractData.apply(this,[]);
	this.xy=[0,0];
};
DataXY.prototype.toData=function(){
	return [this.xy[0],this.xy[1]];
};

// should throw an error because 'fromData' is missing
let dx=new DataXY();

With classes

class AbstractData{
	constructor(){
		dataInterface.check(this,1);
	}
}
class DataXY extends AbstractData{
	constructor(){
		super();
		this.xy=[0,0];
	}
	toData(){
		return [this.xy[0],this.xy[1]];
	}
}

It's still a bit performance consumming and require dependancy to the Interface class, but can be of use for debug or open api.

With an interface you can implement a way of polymorphism. Javascript does NOT need the interface type to handle this and other interface stuff. Why? Javascript is a dynamically typed language. Take as example an array of classes that have the same methods:

Circle()
Square()
Triangle()

> If you want to know how polymorphism works the Book MFC of David Kruglinsky is great (written for C++)

Implement in those classes the method draw() push the instances of those classes in the array and call the draw() methods in a loop that iterates the array. That's completely valid. You could say you implemented implicitly an abstract class. Its not there in reality but in your mind you did it and Javascript has no problem with it. The difference with an real interface is that you HAVE to implement all the interface methods and that's in this case not needed.

An interface is a contract. You will have to implement all the methods. Only by making it statically you have to do that.

Its questionable to change a language like Javascript from dynamic to static. Its not mend to be static. Experienced developers have no problems with the dynamic nature of Javascript.

So the reason to use Typescript are not clear to me. If you use NodeJS together with Javascript you can build extremely efficient and cost effective enterprise websites. The Javascript/NodeJS/MongoDB combination are already great winners.

Try this: Describe the interface as a class and use @implements JSDoc to show that a given class implements the interface defined. You'll see red squiggly lines on the class name if its not implementing some properties. I tested with VSCode.

// @ts-check

// describe interface using a class
class PlainInterface {
    size = 4;
    describe() {}
    show(){ }
}

/**
 * @implements  PlainInterface 
 */
class ConcretePlain {
    size = 4;
    describe() {
        console.log('I am described')
    }
    show(){
        console.log('I am shown')
    }
}       

const conc = new ConcretePlain();
conc.describe();

No, but it has mixins. You can use Abstract sub-classss or mixins as an alternative https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#mix-ins