Object.prototype.hasOwnProperty()

prop

The String name or Symbol of the property to test.

Returns true if the object has the specified property as own property; false otherwise.

The hasOwnProperty() method returns true if the specified property is a direct property of the object — even if the 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.

The method can be called on most JavaScript objects, because most objects descend from Object, and hence inherit its methods. For example Array is an Object, so you can use hasOwnProperty() method to check whether an index exists:

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

The method will not be available in objects where it is reimplemented, or on objects created using Object.create(null) (as these don't inherit from Object.prototype). Examples for these cases are given below.

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

let example = {};
example.hasOwnProperty('prop');   // false

example.prop = 'exists';
example.hasOwnProperty('prop');   // true - 'prop' has been defined

example.prop = null;
example.hasOwnProperty('prop');   // true - own property exists with value of null

example.prop = undefined;
example.hasOwnProperty('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';

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

The following example shows how to iterate over the enumerable properties of an object without executing on inherited properties.

let buz = {
  fog: 'stack'
};

for (let name in buz) {
  if (buz.hasOwnProperty(name)) {
    console.log('this is fog (' +
      name + ') for sure. Value: ' + buz[name]);
  }
  else {
    console.log(name); // toString or something else
  }
}

Note that the for...in loop only iterates enumerable items: the absence of non-enumerable properties emitted from the loop does not imply that hasOwnProperty itself is confined strictly to enumerable items (as with Object.getOwnPropertyNames()).

JavaScript does not protect the property name hasOwnProperty; an object that has a property with this name may return incorrect results:

let foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // reimplementation always returns false

The recommended way to overcome this problem is to instead use Object.hasOwn() (in browsers that support it). Other alternatives include using an external hasOwnProperty:

let foo = { bar: 'Here be dragons' };

// Use Object.hasOwn() method - recommended
Object.hasOwn(foo, "bar");  // true
  
// Use the hasOwnProperty property from the Object prototype
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

// Use another Object's hasOwnProperty
// and call it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

Note that in the first two cases there are no newly created objects.

Objects created using Object.create(null) do not inherit from Object.prototype, making hasOwnProperty() inaccessible.

let foo = Object.create(null);
foo.prop = 'exists';
foo.hasOwnProperty("prop");  // Uncaught TypeError: foo.hasOwnProperty is not a function

The solutions in this case are the same as for the previous section: use Object.hasOwn() by preference, otherwise use an external object's hasOwnProperty().

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