Object.hasOwn()

obj

The JavaScript object instance to test.

prop

The String name or Symbol of the property to test.

true if the specified object has directly defined the specified property. Otherwise false

The Object.hasOwn() method returns true if the specified property is a direct property of the object — even if the property value is null or undefined. The method returns false if the property is inherited, or has not been declared at all. Unlike the in operator, this method does not check for the specified property in the object's prototype chain.

It is recommended over Object.prototype.hasOwnProperty() because it works for objects created using Object.create(null) and with objects that have overridden the inherited hasOwnProperty() method. While it is possible to workaround these problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() is more intuitive.

The following code shows how to determine whether the example object contains a property named prop.

const example = {};
Object.hasOwn(example, "prop"); // false - 'prop' has not been defined

example.prop = "exists";
Object.hasOwn(example, "prop"); // true - 'prop' has been defined

example.prop = null;
Object.hasOwn(example, "prop"); // true - own property exists with value of null

example.prop = undefined;
Object.hasOwn(example, "prop"); // true - own property exists with value of undefined

The following example differentiates between direct properties and properties inherited through the prototype chain:

const example = {};
example.prop = "exists";

// `hasOwn` will only return true for direct properties:
Object.hasOwn(example, "prop"); // true
Object.hasOwn(example, "toString"); // false
Object.hasOwn(example, "hasOwnProperty"); // false

// The `in` operator will return true for direct or inherited properties:
"prop" in example; // true
"toString" in example; // true
"hasOwnProperty" in example; // true

To iterate over the enumerable properties of an object, you should use:

const example = { foo: true, bar: true };
for (const name of Object.keys(example)) {
  // …
}

But if you need to use for...in, you can use Object.hasOwn() to skip the inherited properties:

const example = { foo: true, bar: true };
for (const name in example) {
  if (Object.hasOwn(example, name)) {
    // …
  }
}

The elements of an Array are defined as direct properties, so you can use hasOwn() method to check whether a particular index exists:

const fruits = ["Apple", "Banana", "Watermelon", "Orange"];
Object.hasOwn(fruits, 3); // true ('Orange')
Object.hasOwn(fruits, 4); // false - not defined

This section demonstrates that hasOwn() is immune to the problems that affect hasOwnProperty. Firstly, it can be used with objects that have reimplemented hasOwnProperty():

const foo = {
  hasOwnProperty() {
    return false;
  },
  bar: "The dragons be out of office",
};

if (Object.hasOwn(foo, "bar")) {
  console.log(foo.bar); // true - re-implementation of hasOwnProperty() does not affect Object
}

It can also be used to test objects created using Object.create(null). These do not inherit from Object.prototype, and so hasOwnProperty() is inaccessible.

const foo = Object.create(null);
foo.prop = "exists";
if (Object.hasOwn(foo, "prop")) {
  console.log(foo.prop); // true - works irrespective of how the object is created.
}

• Polyfill of Object.hasOwn in core-js
• Object.prototype.hasOwnProperty()
• Enumerability and ownership of properties
• Object.getOwnPropertyNames()
• for...in
• in
• JavaScript Guide: Inheritance revisited