Object.hasOwn()

The Object.hasOwn() static method returns true if the specified object has the indicated property as its own property. If the property is inherited, or does not exist, the method returns false.

instance

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.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.

let 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:

let example = {}
example.prop = 'exists';

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

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

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

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

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

let example = { foo: true, bar: true };
for (let 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:

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

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

let foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'The dragons be out of office'
};

if (Object.hasOwn(foo, 'bar')) {
  console.log(foo.bar); //true - reimplementation 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.

let 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.hasOwnProperty()
• Enumerability and ownership of properties
• Object.getOwnPropertyNames()
• for...in
• in
• JavaScript Guide: Inheritance revisited