Algorithm to get changes between two arrays

Question

I needed to create an algorithm which will (efficiently) take an old array and a new array and give me back the changes between the two (which items added, which removed). It ha

Answer 1

There is no undefined constant. You should check the type of the variable instead:

if (typeof o === 'undefined') o = [];

Edit:

As Tim Down showed, the property is actually defined in the standard, but as the standard doesn't define it to be constant, it's unreliable and should not be used.