How does this function determine of an object is empty?

Question

I was digging around the jQuery source code and I found that they use this little code snippet to detect if a JavaScript object is empty.

function isMyObjEmpty( obj ) {
    var name;
    for (name in obj ) {
        return false;
    }
    return true;
}

Can someone explain to me why this works? I just don't understand why this would ever return true.

Answer 1

Why it works:

This uses a for... in loop to iterate through the object's properties.

If the object has any property, it would enter the loop and return false

If the object has no properties, it would not enter the loop, and return true.

Why it doesn't:

Note that there exists a case where it does not work. for.. in loops only go through enumerable properties so technically an object can be non-empty and it would still return false. One can define a property to not be enumerable and trick this method. Here is the problematic case.

The correct thing to say is that this method checks if an object has any enumerable properties.

What the docs say:

You can find the method's documentation here.

Description: Check to see if an object is empty (contains no enumerable properties).

I personally find it odd they would call that method isEmptyObject. I think a better suited name would be hasNoEnumerableProperties.

What if you really want to check if an Object is empty?

In newer implementations of JS one can use Object.getOwnPropertyNames. getOwnPropertyNames gets all the properties, enumerable or not.

You can implement isMyObjEmpty with Object.getOwnPropertyNames(myObject).length===0. This checks that the object has no properties, enumerable or not.

This however, does not check prototypical properties though. This might, or might not be desirable behavior, you can check the discussion I've had with theshadowmonkey about it. That could be easily solved by making a call to Object.getPrototypeOf recursively and checking for properties across the prototypical chain.