Object.prototype.hasOwnProperty()

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).

obj.hasOwnProperty(prop)

prop

The String name or Symbol of the property to test.

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

All descendants of Object inherit 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 for a property in the object's prototype chain. If an Object is an Array, hasOwnProperty method can check whether an index exists.

;(function(hasOwnProp) { // IIFE with guarding semicolon
  function hasOwnProperty(name) {
    if (this == null) { throw new TypeError("Cannot convert undefined or null to object!") }
    var O = Object(this), key = String(name);
    var _proto = O.__proto__ || O.constructor.prototype || {}; // Object.prototype
    return key in O && ( !(key in _proto) || O[key] !== _proto[key] );
  }
  if (!hasOwnProp) {
    try {
      Object.defineProperty( Object.prototype, 'hasOwnProperty', {
        enumerable: false, configurable: true, writable: true,
        value: hasOwnProperty
      });
    } catch (e) { // Object.defineProperty isn't supported
      Object.prototype['hasOwnProperty'] = hasOwnProperty;
    }
  }
})(Object.prototype.hasOwnProperty);

hasOwnProperty returns true even if the value of the property is null or undefined.

o = new Object();
o.propOne = null;
o.hasOwnProperty('propOne');   // returns true
o.propTwo = undefined;
o.hasOwnProperty('propTwo');   // returns true

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

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

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

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

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.

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