Question

I encountered the method o.propertyIsEnumerable(x) in Javascript code. I understand it as a synonym for the x in o construct. Is there a difference? If so, could you show when to use the first construct and when to use the second construct on some practical example?

var o = {};
o.x = 1;
o.y = 2;

if ("x" in o) {
 // some code
}

if (o.propertyIsEnumerable(x)) {
  // some code
}

Answer 1

The easiest way to work out things like this is to follow the algorithms in the spec. This is what it tells us about propertyIsEnumerable:

• Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.

• If desc is undefined, return false.

• Return the value of desc.[[Enumerable]].

As you can see, it will call the [[GetOwnProperty]] internal method of the object in question. That simply returns the value of the specified property from the object itself (not from anything in its prototype chain).

Now let’s have a look at the in operator:

Return the result of calling the [[HasProperty]] internal method of rval with argument ToString(lval).

And if you look at the [[HasProperty]] internal method:

Let desc be the result of calling the [[GetProperty]] internal method of O with property name P

And here you can see the difference. The in operator results in the use of the [[[GetProperty]] internal method][4], instead of the [[GetOwnProperty]] method, and that means it will find properties on objects down the prototype chain.

The other major difference is that you can define non-enumerable properties on an object (with the Object.defineProperty method). If you define a non-enumerable property, it will be returned by the in operator, but obviously not by the propertyIsEnumerable method. Here’s a fiddle that demonstrates the difference.