Why is the parenthesis missing in this anonymous function call? [duplicate]

Question

I am reading this book and it has this code example

function getFunction() {
    var result = [];
    for (var i = 0; i < 10; i++) {
        result[i] = function(num) {
            return function() {
                console.log("this is " + num);
            }
        }(i);
    };

    return result;
}

It is working OK but why the anonymous function here not wrapped in parenthesis like this (function(...))(i); ? And in which cases can parenthesis be omitted in anonymous function?

Answer 1

Since the syntax for function declarations and function expressions is identical, JS tells which one you are using from the code around the function.

To stop it being a function declaration you need to use it in an expression. Wrapping it in parenthesis will will do that but preceding it with a = will too (as will a host of other operators). Since there is a = here, the parenthesis are unnecessary.