JavaScript: implement let-working with var

Question

So I have a code that clone let-behavior. But I don't undestand how and why it is working. Can someone explain it?

(function() {
  var a = 2;
})()

Answer 1

let is scoped to the block it appears in.

var is scoped to the function it appears in.

By replacing a block with a function (which is immediately invoked) var is scoped to the same lines of code as let would be.

Answer 2

Where you can only use var variables but you want to ensure that a variable declaration is not going to overwrite another pre-existing variable of the same name, you can use an IIFE to scope that variable to the function. This in effect creates a "block", similar to the second example below.

var variables are scoped to functions:

 var a = 1; let b = 1; (function() { var a = 2; let b = 1; })(); console.log(a); //=> 1 (a is not overwritten because the second `var a` is function-scoped) console.log(b); //=> 1

let variables are scoped to blocks:

 let a = 1; var b = 1; { let a = 2; var b = 2; } console.log(a); //=> 1 (a is not overwritten as let is block-scoped) console.log(b); //=> 2 (b is overwritten as var is not block-scoped)

It's worth mentioning that you can redeclare a var variable, hence why it would be possible to overwrite an existing var variable of the same name. However, you cannot redeclare a let variable:

 var a = 1 // will not error as `var` does not prevent redeclaration var a = 2 let b = 1 { // will not error as `b` is not declared in this block let b = 2 } // will error as `b` is already declared in this scope let b = 3

You can read more about let and var in this question and its answers .