Why a variable defined global is undefined?

Inside a function, you're declaring var myname = "local". Even though you're doing it in the middle of the method, that variable has function scope, so it belongs to the entire function, even the code above it.

So the local variable's value is undefined before that line, and has a value after, but neither one are touching the global variable.

You have just stumbled on a js "feature" called hoisting

var myname = "global"; // global variable
function func() {
    alert(myname); // "undefined"
    var myname = "local";
    alert(myname); // "local"
}
func();

In this code when you define func the compiler looks at the function body. It sees that you are declaring a variable called myname.

Javascript Hoists variable and function declarations, by moving the declaration to the top of the function.

Because of hoisting your code is rewritten to the following.

var myname = "global"; // global variable
function func() {
   var myname; //declare local variable and assign it undefined
   alert(myname); // "undefined"
   myname = "local"; // assign local var myname to "local"
   alert(myname); // "local"
}
func();

This "Covers" the global variable. If you want access to the global variable within the scope of a function use the this keyword.

var myname = "global"; // global variable
function func() {
    var myname = "local";
    alert(this.myname); // "global"
    alert(myname); // "local"
}
func();

Note that this only works in calling a function not a method or constructor because the this keyword changes what its bound to based on how you call a function.

EDIT: For completeness

If you want to get access to global variables in any context regardless of function type then declare a global variable that by convention you never cover.

var global = this; // in global scope.
var myname = "global";
var obj = {f: function () {
    var myname = "local";
    console.log(global.myname);
}};
obj.f(); // "global"

Note that this is in method position and the this keyword refers to obj directly and therefore doesn't have myname defined.