George Jempty

Posted on May 14, 2018

hasOwnProperty considered harmful

#javascript #performance #coding #programming

A lot of Javascript developers automatically reach for hasOwnProperty when looping over an object with for...in; case in point something I read on reddit recently:

    for (var k in this.generalKeyHandling) {
        if (this.generalKeyHandling.hasOwnProperty(k) && this.down.indexOf(k) > -1) {
        // etc.

The object however had just been defined in an immediately preceding literal, therefore it would not have any inherited properties, which is the point of using hasOwnProperty. Not only is using it in this case therefore unnecessary, but a JSPerf seems to indicate it could be up to 10 times slower.

If object (literals) you define can somehow get hijacked such that they have inhertited properties by the time you want to iterate over them, you have bigger problems that using hasOwnProperty merely masks. So, when thinking about using it, to quote some advice from a lost episode of the Andy Griffith show: "Aunt Bea, I gotta cookin' tip for ya: DON'T!"

Top comments (6)

Joshua Johnson • May 15 '18

hasOwnPropery is meant as a way to ensure that you are only iterating over values that belong to the object you are checking the property against. Meaning that you won't use cpu to process information on properties that live on the parent of the object you inherited from.

Also, I'm not sure what you mean by "hijacked"? I'm not sure how that plays into your argument about the object growing or not to use hasOwnPropery?

Comment deleted

Joshua Johnson • May 16 '18 • Edited

Sorry man, I'm still not understanding the message you are trying to convey. Is it that when looping through object literals that don't inherit from other objects, it's not necessary to include the hasOwnProperty check?

var prop;
var A = {
    foo: 'bar',
    fooFn: function () {}
};

var B = Object.create(a);


/**
 * ensures that you are only looping through
 * properties that belong only to B. Will not
 * execute code on properties from A.
 */
for (prop in B) {
    if (b.hasOwnProperty(prop)) {
        console.log(b[prop]);
    }
}

/**
 * will execute code on not only properties on B
 * but it will also loop through properties on A.
 */
for (prop in B) {
    console.log(b[prop]);
}

Guillaume Martigny • May 19 '18

hasOwnPropery is a great way to prevent you from looping though the object prototype. Sorry, but I can't see any reason not to.

Where I agree with you, it's that you perform an unnecessary check. The best IMO, it not to avoid hasOwnPropery, but to prefer other loops like for( ... of ... ) or Object.keys().forEach().

Victor Vincent • May 16 '18

Usually I use

for (const key of Object.keys(obj)) {
    console.log(key)
}

Also you can seal and/or freeze objects if that's the desired effect.

Alain Van Hout • May 15 '18

In this regard, any idea how hasOwnPropery compares to Object.keys?

DEV Community

hasOwnProperty considered harmful

Top comments (6)

Read next

Undetectable Backdoors in Outsourced Machine Learning Models: A Theoretical Vulnerability

2563. Count the Number of Fair Pairs

Software knowledge

Building a Highly Reactive Page in React: A Deep Dive into Gladiator Crash