What's the correct way to test for existence of a property on a JavaScript Object?

Question

I have a custom Javascript object that I create with new, and assign properties to based on creation arguments:

function MyObject(argument) {
    if (argument) {
        this.prop = "foo";
    }
}
var objWithProp = new MyObject(true); // objWithProp.prop exists
var objWithoutProp = new MyObject(false); // objWithoutProp.prop does not exist

What's the correct way to test for the existence of the prop property of the objects? I've seen the following ways used, but I'm not sure if any of these ways is the best way:

if (obj.prop) {}
if (obj.hasOwnProperty("prop")) {}
if ("prop" in obj) {}

Specifically, I'm only interested in testing if the property is explicitly defined for this object, not in the prototype chain. In addition, the value will never be set to null or undefined, but it could be something like an empty object or array. However, if you want to include what the correct way is if those could be the case, feel free.

Alex Martelli · Accepted Answer · 2009-05-22 04:14:38Z

35

hasOwnProperty is exactly what you're looking for, since you specify "if the property is explicitly defined for this object, not in the prototype chain". Per https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Object/hasOwnProperty , "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 down the object's prototype chain." -- seems to exactly match your requirement!

answered May 22, 2009 at 4:14

Alex Martelli

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Comments

farzad · Accepted Answer · 2013-02-13 02:13:21Z

If you are looking for a property defined in an object, you can use hasOwnProperty method of the object. like this:

myObject = new MyObject();
// some code
if ( myObject.hasOwnProperty('prop') ) {
    // prop exists
}

but this is only to know if such a property is defined in object itself, but not its parents. so if such property is inherited by the object, you can not test its existence like this.

the other way is to test the property against undefined value. like this:

if ( myObject.prop !== undefined ) {
    // prop exists
}

remember to use the !== operator instead of != because != will not differ between null and undefined, but !== does. so if your object has a property but the value is null, != will not help you. so this test:

if ( myObject.prop ) {
}

might have wrong results if "prop" has a false or null value. but by comparing to undefined with !== operator, you can be sure that null/false values will not confuse you.

Blo · Accepted Answer · 2016-04-04 14:50:27Z

4

You can check the existence of a variable as follows:

if ( typeof variable === 'undefined' || variable === null ) {
    // Do stuff
}

It's also can be used for properties.

edited Apr 4, 2016 at 14:50

Blo

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answered Apr 4, 2016 at 12:14

voltento

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1 Comment

Jan Molnár Over a year ago

Please note this answer relies on "the value will never be set to null or undefined", usually this is not the case and the answer would be incorrect. E.g. when var obj = {prop: undefined} then typeof obj.prop would return undefined as well.

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