JavaScript Setting an Object's prototype


Example

5

With ES5+, the Object.create function can be used to create an Object with any other Object as it's prototype.

const anyObj = {
    hello() {
        console.log(`this.foo is ${this.foo}`);
    },
};

let objWithProto = Object.create(anyObj);
objWithProto.foo = 'bar';

objWithProto.hello(); // "this.foo is bar"

To explicitly create an Object without a prototype, use null as the prototype. This means the Object will not inherit from Object.prototype either and is useful for Objects used for existence checking dictionaries, e.g.

let objInheritingObject = {};
let objInheritingNull = Object.create(null);

'toString' in objInheritingObject; // true
'toString' in objInheritingNull ; // false
6

From ES6, the prototype of an existing Object can be changed using Object.setPrototypeOf, for example

let obj = Object.create({foo: 'foo'});
obj = Object.setPrototypeOf(obj, {bar: 'bar'});

obj.foo; // undefined
obj.bar; // "bar"

This can be done almost anywhere, including on a this object or in a constructor.

Note: This process is very slow in current browsers and should be used sparingly, try to create the Object with the desired prototype instead.

5

Before ES5, the only way to create an Object with a manually defined prototype was to construct it with new, for example

var proto = {fizz: 'buzz'};

function ConstructMyObj() {}
ConstructMyObj.prototype = proto;

var objWithProto = new ConstructMyObj();
objWithProto.fizz; // "buzz"

This behaviour is close enough to Object.create that it is possible to write a polyfill.