How to determine whether an object has a given property in JavaScript

How to determine whether an object has a given property in JavaScript

How can I determine whether an object x has a defined property y, regardless of the value of x.y?
I’m currently using
if (typeof(x.y) !== ‘undefined’)

but that seems a bit clunky. Is there a better way?

Solutions/Answers:

Solution 1:

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) {
  // ......
}

Solution 2:

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

Solution 3:

Underscore.js or Lodash

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

🙂

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Solution 4:

You can trim that up a bit like this:

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

Solution 5:

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.

Solution 6:

Since question was regarding clunkiness of property checking, and one regular usecase for that being validation of function argument options objects, thought I’d mention a library-free short way of testing existence of multiple properties.
Disclaimer: It does require ECMAScript 5 (but IMO anyone still using IE8 deserves a broken web).

function f(opts) {
  if(!["req1","req2"].every(opts.hasOwnProperty, opts)) {
      throw new Error("IllegalArgumentException");
  }
  alert("ok");
}
f({req1: 123});  // error
f({req1: 123, req2: 456});  // ok