hasownproperty.md

hasOwnProperty

To check whether an object has a property defined on itself and not somewhere on its prototype chain, it is necessary to use the hasOwnProperty method which all objects inherit from Object.prototype.

Note: It is not enough to check whether a property is undefined. The property might very well exist, but its value just happens to be set to undefined.

hasOwnProperty is the only thing in JavaScript which deals with properties and does not traverse the prototype chain.

// Poisoning Object.prototype
Object.prototype.bar = 1;
var foo = {goo: undefined};

foo.bar; // 1
'bar' in foo; // true

foo.hasOwnProperty('bar'); // false
foo.hasOwnProperty('goo'); // true

Only hasOwnProperty will give the correct and expected result. See the section on for in loops for more details on when to use hasOwnProperty when iterating over object properties.

hasOwnProperty as a Property

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 hasOwnProperty from the Object
// prototype for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

In Conclusion

Using hasOwnProperty is the only reliable method to check for the existence of a property on an object. It is recommended that hasOwnProperty be used in many cases when iterating over object properties as described in the section on for in loops.