Object

Description

Nearly all objects in JavaScript are instances of Object; a typical object inherits properties (including methods) from Object.prototype, although these properties may be shadowed (a.k.a. overridden). The only objects that don't inherit from Object.prototype are those with null prototype, or descended from other null prototype objects.

Changes to the Object.prototype object are seen by all objects through prototype chaining, unless the properties and methods subject to those changes are overridden further along the prototype chain. This provides a very powerful although potentially dangerous mechanism to override or extend object behavior. To make it more secure, Object.prototype is the only object in the core JavaScript language that has immutable prototype — the prototype of Object.prototype is always null and not changeable.

Object prototype properties

You should avoid calling any of the Object.prototype methods directly from the instance, especially those that are not intended to be polymorphic (i.e., only its initial behavior makes sense and no descending object could override it in a meaningful way). All objects descending from Object.prototype may define a custom own property that has the same name, but with entirely different semantics from what you expect. Furthermore, these properties are not inherited by null-prototype objects. All modern JavaScript utilities for working with objects are static. More specifically:

• valueOf(), toString(), and toLocaleString() exist to be polymorphic and you should expect the object to define its own implementation with sensible behaviors, so you can call them as instance methods. However, valueOf() and toString() are usually implicitly called through type conversion and you don't need to call them yourself in your code.
• __defineGetter__(), __defineSetter__(), __lookupGetter__(), and __lookupSetter__() are deprecated and should not be used. Use the static alternatives Object.defineProperty() and Object.getOwnPropertyDescriptor() instead.
• The __proto__ property is deprecated and should not be used. The Object.getPrototypeOf() and Object.setPrototypeOf() alternatives are static methods.
• The propertyIsEnumerable() and hasOwnProperty() methods can be replaced with the Object.getOwnPropertyDescriptor() and Object.hasOwn() static methods, respectively.
• The isPrototypeOf() method can usually be replaced with instanceof, if you are checking the prototype property of a constructor.

In case where a semantically equivalent static method doesn't exist, or if you really want to use the Object.prototype method, you should directly call() the Object.prototype method on your target object instead, to prevent the object from having an overriding property that produces unexpected results.

const obj = {
  foo: 1,
  // You should not define such a method on your own object,
  // but you may not be able to prevent it from happening if
  // you are receiving the object from external input
  propertyIsEnumerable() {
    return false;
  },
};

obj.propertyIsEnumerable("foo"); // false; unexpected result
Object.prototype.propertyIsEnumerable.call(obj, "foo"); // true; expected result

Deleting a property from an object

There isn't any method in an Object itself to delete its own properties (such as Map.prototype.delete()). To do so, one must use the delete operator.

null-prototype objects

Almost all objects in JavaScript ultimately inherit from Object.prototype (see inheritance and the prototype chain). However, you may create null-prototype objects using Object.create(null) or the object initializer syntax with __proto__: null (note: the __proto__ key in object literals is different from the deprecated Object.prototype.__proto__ property). You can also change the prototype of an existing object to null by calling Object.setPrototypeOf(obj, null).

const obj = Object.create(null);
const obj2 = { __proto__: null };

An object with a 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 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.

You can also revert a null-prototype object back to an ordinary object using Object.setPrototypeOf(nullProtoObj, Object.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: