In Javascript what is more efficient, deleting an element or setting it to undefined explicitly

Question

Say I have an Associative Array Foo, with key bar and value xyz.

 console.log(Foo['bar']);

 >> xyz

 delete Foo['bar'];
 console.log Foo['bar'];

 >> undefined

 Foo['bar'] = 'xyz';
 console.log(Foo['bar']);

 >> xyz

 Foo['bar'] = undefined;
 console.log (Foo['bar']);

 >> undefined

My question is, which of the two is more efficient, do they differ in any way? Is there a scenario when I should use one over the other?

Thanks!

Results:

Thank you to everyone for helping out and showing me jsperf. Setting it to undefined appears to be (relatively) significantly faster then delete, although all the caveats pointed out below are also very interesting (in fact, I will probably be using delete a lot going forward to avoid future errors out of leftfield).

Answer 1

I didn't benchmark the performance of those operations (as I mentioned in a comment, just create a little benchmark on http://www.jsperf.com), but I'll lose some words on the difference.

You will always be good on deleteing properties, wheras setting them to undefined or null will let people and/or code hang, which check with the IN operator.

Like

if( 'bar' in Foo ) { }

will still return true if you set Foo.bar to undefined. It won't if you go with delete Foo.bar.

Answer 2

Be aware that deleting a property from an object will replace that property with one of the same name if one exists on the prototype chain.

Setting the property to null or undefined will simply mask it.