Why is x undefined in inner scope? [duplicate]

Answer 1

Because of the compiler, even if you initiate a var down below the code, the compiler send it to the top, just like var x;, so it first initiate as undefined "x" before running console.log, that's why is such a good practice to initate all vars you're going to use first thing in the function, so these mistakes won't happen.

Answer 2

Variable hoisting. The actual code is executed like this.

var x = 1;
(function() {
    var x; // x = undefined
    console.log(x);
    x = 2;
})();

Edit: On Mr Lister's advice, a bit on variable hoisting. From MDN (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var):

"Variable declarations, wherever they occur, are processed before any code is executed. The scope of a variable declared with var is its current execution context, which is either the enclosing function or, for variables declared outside any function, global."