Benefit of using Object.hasOwnProperty vs. testing if a property is undefined

Question

Since hasOwnProperty has some caveats and quirks (window / extensive use in Internet Explorer 8 issues, etc.):

Is there any reason to even use it? If simply testing if a property is undefined, is it better justified and more simplistic?

For example:

var obj = { a : 'here' };  if (obj.hasOwnProperty('a')) { /* do something */ }  if (obj.a !== undefined) { /* do something */ } // Or maybe (typeof (obj.a) !== 'undefined') 

I'd prefer to be using the most cross-browser friendly, and up to date methodology.

I've also seen this prototype overwritten for hasOwnProperty, which works, but I'm not sold on its usefulness...

if (!Object.prototype.hasOwnProperty) {     Object.prototype.hasOwnProperty = function(prop) {         var proto = this.__proto__ || this.constructor.prototype;         return (prop in this) && (!(prop in proto) || proto[prop] !== this[prop]);     }; } 

Answer 1

hasOwnProperty does not check for undefined values. It only checks if a property is assigned to the object even if is undefined:

var obj = { a : undefined }; obj.hasOwnProperty("a") // true obj.a === undefined     // true obj.hasOwnProperty("b") // false obj.b === undefined     // true 

Answer 2

The hasOwnProperty method checks that a property is assigned to the object directly.

So, if property 'a' is in the prototype, hasOwnProperty will filter that.

function NewClass() {} NewClass.prototype = { a: 'there' }; var obj = new NewClass();  if (obj.hasOwnProperty('a')) { /* Code does not work */ } if (obj.a !== undefined) { /* Code works */ } 

So, hasOwnProperty is safer in many cases.