The ES6 way to IIFE

script.js

'use  strict'; // V8 only can only use block scopes in strict mode so far.

var a = 42; // Variables outside may be defined in any way you want.

{ // The block starts. Yeah, we can just start a new block anywhere we want.
    let a = 'Oh, hi!'; // Define a block scoped variable using let.
    console.log(a); // Use it.
} // The block ends, and all the block scoped variables dies. 

console.log(a); // Left untouched.