JS function definition : meaning of the last parentheses [duplicate]

Answer 1

Suppose we had:

function add1 (x) {
    return x+1;
}

Then we might call add1(5) to get the value 6.

But with the brackets, rather than calling add1(5), we can replace it with (function (x) {return x+1;})(5). Basically this creates a function, and applies it to the value 5 immediately, whereas when we call add1(5), we are using a named/defined function.