var a = 2;
(function IIFE(global){
var a =3
console.log(a)//3
console.log(global.a)//2
})(window);
console.log(a);//2
let a = 2;
(function IIFE(global){
var a =3
console.log(a)//3
console.log(global.a)//undefine undefined
})(window);
console.log(a);//2 let const does not bind variables to window objects even if they are defined in the global environment.
you can find a basic book
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