logic of variable in javascript

Question

I have some lines of code : why print 'undefined' and 'B' on my browser . Can you explain for me?

var x = 'A';

function func(){
  document.write(x);
  var x = "B";
  document.write(x);
};

func();

Answer 1

It's JavaScript Hoisting. In your example x inside that function is hoisted. In JavaScript, variable declarations are only hoisted, NOT their initializations.

Update:

Regarding ES6, const and let declarations are not hoisted.