yep, that's a first!

i dont see how an ad blocker could possibly interfere with the variables, the code is evaluated server side, not client side

That is not true.

Array indices are based on some offset from the first address. A shift basically means setting a new first address.

Of course, this is assuming a standard implementation of the array data structure. If you implemented an array with a stack, then it will obviously be O(n) instead of O(1).

And if you really want to talk about worst case time complexities, a push could actually be an O(n) operation even for the standard implementation. It just depends on memory allocated.

ops[operation] || function() { return null; }
You need this function for other operation values beside "+-*/" and it would return null.
For example, if you get "m" or "w" as an operation, the compiler will give errors since m or w are not a method so OR function is needed to return null.

I might be wrong, but (ops[operation] || function() {return null;}) seems to return a function from ops object or a function that returns null if no such operation is defined. This is possible because ops[operation] returns undefined for unexistent members, which in turns forces function () { return null; } to be evaluated and returned.

Basically, if the operation exists in the ops object I call its function with the num1 and num2 arguments.

If the operation does not exist in ops I default (using ||) to a function which returns null. num1 and num2 are also provided to the default function but are ignored.