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up vote 73 down vote favorite
17

If I want to test of object x has a defined property y regardless of x.y's value, is there a better way that the rather clunky:

if ( typeof(x.y) != 'undefined' ) ...

?

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5 Answers

up vote 163 down vote accepted

Object has property:

If you are testing for properties that are on the object itself (not a part of its prototype chain) you can use .hasOwnProperty():

if (x.hasOwnProperty('y')) { 
  // ......
}

Object or its prototype has a property:

You can use the in operator to test for properties that are inherited as well.

if ('y' in x) {
  // ......
}
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That's much nicer. Thank you gnarf. Is there a better reference to be using than developer.mozilla.org/en/Core_Javascript_1.5_Reference? –  fsb Dec 12 '09 at 21:54
1  
@fsb - stackoverflow.com/questions/61012/… --- MDC is favored amongst the stack overflow community it seems, I personally use whatever ends up first on google :) –  gnarf Dec 12 '09 at 21:57
4  
if you want to include inherited properties, use the in operator, eg if('y' in x) –  Christoph Dec 12 '09 at 22:17
2  
Or even better — Object.prototype.hasOwnProperty.call(x, 'y'), so that property named "hasOwnProperty" would not conflict with inspection process ;) –  kangax Dec 24 '09 at 14:03
    
I get "TypeError: invalid 'in' operand" for some of elements I receive from getElementByTagName("*"). So what do I do now?.. –  mvmn Feb 15 '13 at 15:14
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up vote 34 down vote

If you want to know if the object physically contains the property @gnarf's answer using hasOwnProperty will do the work.

If you're want to know if the property exists anywhere, either on the object itself or up in the prototype chain, you can use the in operator.

if ('prop' in obj) {
  // ...
}

Eg.:

var obj = {};

'toString' in obj == true; // inherited from Object.prototype
obj.hasOwnProperty('toString') == false; // doesn't contains it physically
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I'd like to mark this and gnarf's answer as "accepted" because they combine to provide the tools I need. But it's not allowed to mark more than one answer thus. Anyway, thanks! –  fsb Dec 12 '09 at 22:57
    
@fsb: You are welcome! –  CMS Dec 13 '09 at 5:00
1  
The for in operator is the best for this ... –  SecretService Jul 19 '13 at 19:48
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up vote 5 down vote

You can trim that up a bit like this:

if ( x.y !== undefined ) ...
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4  
That would fail with x = {y:undefined} –  James Dec 12 '09 at 21:59
9  
Does anyone need to distinguish between "not defined" and "defined to be undefined?" –  darkporter Dec 12 '09 at 22:25
    
@darkporter I do sometimes ;) –  SecretService Jul 19 '13 at 19:48
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up vote 3 down vote

One feature of my original code

if ( typeof(x.y) != 'undefined' ) ...

that might be useful in some situations is that it is safe to use whether x exists or not. With either of the methods in gnarf's answer, one should first test for x if there is any doubt if it exists.

So perhaps all three methods have a place in one's bag of tricks.

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You could always use (x && x.hasOwnProperty('y')) or (x && 'y' in x) –  gnarf Aug 4 '10 at 21:50
    
I agree, testing for x should be a separate case on it's own. Also yields better error reporting. –  b01 Aug 18 '11 at 14:44
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up vote 2 down vote

Underscore.js

if (_.has(x, "y")) ...

:)

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Does this work on both arrays and objects? –  Viktor Sehr Nov 3 '13 at 0:48
    
Nope. It's just an alias for Object.prototype.hasOwnProperty.call(x, "y"). For arrays I think you might want Array.prototype.indexOf, _.indexOf, or _.contains –  nackjicholson Nov 3 '13 at 22:13
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