How javascript || (or) operator works

Question

In the following JavaScript statement:

var a = true;
a = a || b;

Will the a variable have an unneeded reasignment to it's own value?

Answer 1

Yes it will assign to a. This sort of thing probably wouldn't even be optimised in a compiled language.

It won't however waste time evaluating b however as it knows the result already. Something like this happens when a = a || b is run:

if a
    a = a
else
    a = b

EDIT:

To follow up what icktoofay said "it will not significantly impact performance.", it is simply setting a (boolean) variable which is one of the simplest operations that can occur. It will make little difference even if you're assigning to something more significant like a function or array as it will be assigning to a reference of the item, not creating it again.

Here is a performance comparison of doing nothing vs assigning to self (jsPerf link) thanks to @bfavaretto for setting it up.