Making counter using closure and self-invoking function

Question

I'm wondering why this code doesn't work,

var uniqueInteger = function() {
    var counter = 0;
    return function() { return counter++; }
};

console.log(uniqueInteger()());  // 0
console.log(uniqueInteger()());  // 0
console.log(uniqueInteger()());  // 0 
console.log(uniqueInteger()());  // 0

and this code does. The only difference is making the function self invoked instead of invoking it in console.log()

var uniqueInteger = (function() {
    var counter = 0;
    return function() { return counter++; }
}());

console.log(uniqueInteger());  // 0
console.log(uniqueInteger());  // 1
console.log(uniqueInteger());  // 2 
console.log(uniqueInteger());  // 3

I'm very new to JS so please excuse my noobness. Thanks!

Answer 1

The second code created a closure with var counter = 0 only once since when defining uniqueInteger, it called a function that returned a function where initialization is done. The first code creates var counter = 0 every time you call it.

Note that with the first code you can do:

ui = uniqueInteger();
console.log(ui()); // 0
console.log(ui()); // 1
ui2 = uniqueInteger();
console.log(ui()); // 2
console.log(ui2()); // 0
console.log(ui()); // 3
console.log(ui2()); // 1