Object.create()

The Object.create() static method creates a new object, using an existing object as the prototype of the newly created object.

Syntax

Object.create(proto)
Object.create(proto, propertiesObject)

Parameters

proto
- : The object which should be the prototype of the newly-created object.
propertiesObject
- : If specified and not undefined, an object whose enumerable own properties specify property descriptors to be added to the newly-created object, with the corresponding property names. These properties correspond to the second argument of Object.defineProperties.

Return value

A new object with the specified prototype object and properties.

Exceptions

TypeError
- : Thrown if proto is neither null nor an Object.

Examples

Classical inheritance with Object.create()

Below is an example of how to use Object.create() to achieve classical inheritance. This is for a single inheritance, which is all that JavaScript supports.

// Shape - superclass
function Shape() {
  this.x = 0;
  this.y = 0;
}

// superclass method
Shape.prototype.move = function (x, y) {
  this.x += x;
  this.y += y;
  console.info("Shape moved.");
};

// Rectangle - subclass
function Rectangle() {
  Shape.call(this); // call super constructor.
}

// subclass extends superclass
Rectangle.prototype = Object.create(Shape.prototype, {
  // If you don't set Rectangle.prototype.constructor to Rectangle,
  // it will take the prototype.constructor of Shape (parent).
  // To avoid that, we set the prototype.constructor to Rectangle (child).
  constructor: {
    value: Rectangle,
    enumerable: false,
    writable: true,
    configurable: true,
  },
});

const rect = new Rectangle();

console.log("Is rect an instance of Rectangle?", rect instanceof Rectangle); // true
console.log("Is rect an instance of Shape?", rect instanceof Shape); // true
rect.move(1, 1); // Logs 'Shape moved.'

Note that there are caveats to watch out for using create(), such as re-adding the constructor property to ensure proper semantics. Although Object.create() is believed to have better performance than mutating the prototype with Object.setPrototypeOf, the difference is in fact negligible if no instances have been created and property accesses haven't been optimized yet. In modern code, the class syntax should be preferred in any case.

Using propertiesObject argument with Object.create()

Object.create() allows fine-tuned control over the object creation process. The object initializer syntax is, in fact, a syntax sugar of Object.create(). With Object.create(), we can create objects with a designated prototype and also some properties. Note that the second parameter maps keys to property descriptors — this means you can control each property's enumerability, configurability, etc. as well, which you can't do in object initializers.

o = {};
// Is equivalent to:
o = Object.create(Object.prototype);

o = Object.create(Object.prototype, {
  // foo is a regular data property
  foo: {
    writable: true,
    configurable: true,
    value: "hello",
  },
  // bar is an accessor property
  bar: {
    configurable: false,
    get() {
      return 10;
    },
    set(value) {
      console.log("Setting `o.bar` to", value);
    },
  },
});

// Create a new object whose prototype is a new, empty
// object and add a single property 'p', with value 42.
o = Object.create({}, { p: { value: 42 } });

With Object.create(), we can create an object with null as prototype. The equivalent syntax in object initializers would be the __proto__ key.

o = Object.create(null);
// Is equivalent to:
o = { __proto__: null };

By default properties are not writable, enumerable or configurable.

o.p = 24; // throws in strict mode
o.p; // 42

o.q = 12;
for (const prop in o) {
  console.log(prop);
}
// 'q'

delete o.p;
// false; throws in strict mode

To specify a property with the same attributes as in an initializer, explicitly specify writable, enumerable and configurable.

o2 = Object.create(
  {},
  {
    p: {
      value: 42,
      writable: true,
      enumerable: true,
      configurable: true,
    },
  },
);
// This is not equivalent to:
// o2 = Object.create({ p: 42 })
// which will create an object with prototype { p: 42 }

You can use Object.create() to mimic the behavior of the new operator.

function Constructor() {}
o = new Constructor();
// Is equivalent to:
o = Object.create(Constructor.prototype);

Of course, if there is actual initialization code in the Constructor function, the Object.create() method cannot reflect it.