Javascript why wrap a variable or constructor in an IIFE?

Question

I saw something like this today

var Visualizer = (function() {
    function Visualizer() {
    //...
    }
    Visualizer.prototype.function1 = function () {         


        

Answer 1

In case there will be a lot of Visualizers, function1 will be same instance in all of them. As discussed here IIFE method leaks.