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.

Answer 2

a will be true when a or b is true. So yes, unless you insert some more code between those lines which can affect the value of a, the lower statement will always set a to true.

Answer 3

Yes - it won't be optimised away, because JavaScript doesn't optimise. Although the underlying parser implementation could conceivably optimise, I very much doubt it would in this case, and it would be very specific to the platform implementation.