Any variable defined in a function (or block) with the same name as a global variable takes precedence over the global variable, shadowing it.

This prints undefined:

var name = 'Roger'

function run() {
  console.log(name)
  var name = 'Flavio'
}

run()

and this raises an error ReferenceError: name is not defined:

let name = 'Roger'

function run() {
  console.log(name)
  let name = 'Flavio'
}

run()

Lessons this unit:

0:	Introduction
1:	Global scope
2:	Function scope
3:	Block scope
4:	▶︎ Shadowing
5:	Hoisting
6:	Closures
7:	An issue with `var` variables and loops
8:	The event loop