Object.prototype.hasOwnProperty()

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property.

Syntax

obj.hasOwnProperty(prop)

Parameters

prop

The String name or Symbol of the property to test.

Return value

A Boolean indicating whether or not the object has the specified property.

Description

Every object descended from Object inherits the hasOwnProperty method. This method can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check down the object's prototype chain.

Examples

Using hasOwnProperty to test for a property's existence

The following example determines whether the o object contains a property named prop:

o = new Object();
o.prop = 'exists';

function changeO() {
  o.newprop = o.prop;
  delete o.prop;
}

o.hasOwnProperty('prop');   // returns true
changeO();
o.hasOwnProperty('prop');   // returns false

Direct versus inherited properties

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

o = new Object();
o.prop = 'exists';
o.hasOwnProperty('prop');             // returns true
o.hasOwnProperty('toString');         // returns false
o.hasOwnProperty('hasOwnProperty');   // returns false

Iterating over the properties of an object

The following example shows how to iterate over the properties of an object without executing on inherit properties. Note that the for...in loop is already only iterating enumerable items, so one should not assume based on the lack of non-enumerable properties shown in the loop that hasOwnProperty itself is confined strictly to enumerable items (as with Object.getOwnPropertyNames()).

var buz = {
  fog: 'stack'
};

for (var 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
  }
}

Using hasOwnProperty as a property name

JavaScript does not protect the property name hasOwnProperty; thus, if the possibility exists that an object might have a property with this name, it is necessary to use an external hasOwnProperty to get correct results:

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

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

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

// It's also possible to use the hasOwnProperty property from the Object prototype for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

Note that in the last case there are no newly created objects.

Specifications

Browser compatibility

See also

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