Why do some variables declared using let inside a function become available in another function, while others result in a reference error?

Question

I can\'t understand why variables act so strange when declared inside a function.

1. In the first function I declare with let the variabl

Answer 1

It's because of when you don't use let or var then variable is getting declare on the fly, better you declare like following.

let a = 10;
let b = 10;
let c = 10;