function a() { return 1; }
function b() { return(1); }
I tested the above code in Chrome\'s console, and both returned 1.
The same as between
var i = 1 + 1;
and
var i = (1 + 1);
That is, nothing. The parentheses are allowed because they are allowed in any expression to influence evaluation order, but in your examples they're just superfluous.
return is not a function, but a statement. It is syntactically similar to other simple control flow statements like break and continue that don't use parentheses either.