Question

58

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

rated 0 times [ 65] [ 7] / answers: 1 / hits: 21463 / 11 Years ago, mon, june 17, 2013, 12:00:00

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]);

    };

}

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beatriceisabelad

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2 Years ago

answered 11 Years ago aileent · Accepted Answer

As further information to the answer given by Pavel Gruba, and the polyfil that you supplied:

To the best of my knowledge, there is no good way to polyfil hasOwnProperty for browsers that do not support it natively. I have seen quite a few different ones in the wild and they all produce false positives or negatives. If I have absolutely no alternative then this is what I created for my use, but it also suffers false positives and negatives. According to MSDN.

Supported in the following document modes: Quirks, Internet Explorer 6
standards, Internet Explorer 7 standards, Internet Explorer 8
standards, Internet Explorer 9 standards, Internet Explorer 10
standards. Also supported in Windows Store apps.

JavaScript

function is(x, y) {

    if (x === y) {

        if (x === 0) {

            return 1 / x === 1 / y;

        }



        return true;

    }



    var x1 = x,

        y1 = y;



    return x !== x1 && y !== y1;

}



function hasOwnProperty(object, property) {

    var prototype;



    return property in object && (!(property in (prototype = object.__proto__ || object.constructor.prototype)) || !is(object[property], prototype[property]));

}



function NewClass() {}

NewClass.prototype = {

    a: 'there'

};



var obj = new NewClass();



if (obj.hasOwnProperty("a")) {

    console.log("has property")

}



if (hasOwnProperty(obj, "a")) {

    console.log("has property")

}

On JSFiddle.