How do you create a new object in javascript?

Try it

Syntax

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

Parameters

proto

The object which should be the prototype of the newly-created object.

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

Object with null prototype

A new object with null prototype can behave in unexpected ways, because it doesn't inherit any object methods from Object.prototype. This is especially true when debugging, since common object-property converting/detecting utility functions may generate errors, or lose information (especially if using silent error-traps that ignore errors).

For example, the lack of Object.prototype.toString() often makes debugging intractable:

const normalObj = {}; // create a normal object
const nullProtoObj = Object.create(null); // create an object with "null" prototype

console.log(`normalObj is: ${normalObj}`); // shows "normalObj is: [object Object]"
console.log(`nullProtoObj is: ${nullProtoObj}`); // throws error: Cannot convert object to primitive value

alert(normalObj); // shows [object Object]
alert(nullProtoObj); // throws error: Cannot convert object to primitive value

Other methods will fail as well.

normalObj.valueOf(); // shows {}
nullProtoObj.valueOf(); // throws error: nullProtoObj.valueOf is not a function

normalObj.hasOwnProperty("p"); // shows "true"
nullProtoObj.hasOwnProperty("p"); // throws error: nullProtoObj.hasOwnProperty is not a function

normalObj.constructor; // shows "Object() { [native code] }"
nullProtoObj.constructor; // shows "undefined"

We can add the toString method back to the null-prototype object by simply assigning it one:

nullProtoObj.toString = Object.prototype.toString; // since new object lacks toString, add the original generic one back

console.log(nullProtoObj.toString()); // shows "[object Object]"
console.log(`nullProtoObj is: ${nullProtoObj}`); // shows "nullProtoObj is: [object Object]"

Unlike normal objects, in which toString() is on the object's prototype, the toString() method here is an own property of nullProtoObj. This is because nullProtoObj has no (null) prototype.

In practice, objects with null prototype are usually used as a cheap substitute for maps. The presence of Object.prototype properties will cause some bugs:

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return name in ages;
}

function getAge(name) {
  return ages[name];
}

hasPerson("hasOwnProperty"); // true
getAge("toString"); // [Function: toString]

Using a null-prototype object removes this hazard without introducing too much complexity to the hasPerson and getAge functions:

const ages = Object.create(null, {
  alice: { value: 18, enumerable: true },
  bob: { value: 27, enumerable: true },
});

hasPerson("hasOwnProperty"); // false
getAge("toString"); // undefined

In such case, the addition of any method should be done cautiously, as they can be confused with the other key-value pairs stored as data.

Making your object not inherit from Object.prototype also prevents prototype pollution attacks. If a malicious script adds a property to Object.prototype, it will be accessible on every object in your program, except objects that have null prototype.

const user = {};

// A malicious script:
Object.prototype.authenticated = true;

// Unexpectedly allowing unauthenticated user to pass through
if (user.authenticated) {
  // access confidential data
}

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); // Outputs, '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()

let o;

// create an object with null as prototype
o = Object.create(null);

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

// Example where we create an object with a couple of
// sample properties. (Note that the second parameter
// maps keys to *property descriptors*.)
o = Object.create(Object.prototype, {
  // foo is a regular 'value property'
  foo: {
    writable: true,
    configurable: true,
    value: "hello",
  },
  // bar is a getter-and-setter (accessor) property
  bar: {
    configurable: false,
    get() {
      return 10;
    },
    set(value) {
      console.log("Setting `o.bar` to", value);
    },
  },
});

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() cannot reflect it

// 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 } });

// by default properties ARE NOT writable,
// enumerable or configurable:
o.p = 24;
o.p;
// 42

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

delete o.p;
// false

// to specify a property with the same attributes as in an initializer
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 }

See also