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?

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) {
  // ......
}

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

Underscore.js or Lodash

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

:)

You can trim that up a bit like this:

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