JavaScript local and global variable confusion

It is not replacing the global variable. What is happening is called "variable hoisting". That is, var myname; gets inserted at the top of the function. Always initialize your variables before you use them.

Try this:

var myname = "initial";

function c() {
    alert(myname);
    myname = "changed";
    alert(myname);
}

c();

In JavaScript, the variable declarations are automatically moved to the top of the function. So, the interpreter would make it look more like this:

var myname = "initial"
function c(){
    var myname;
    // Alerts undefined
    alert(myname);
    myname = "changed";
    // Alerts changed
    alert(myname);
}
c();

This is called hoisting.

Due to hoisting and the fact that the scope for any variable is the function it's declared in, it's standard practice to list all variables at the top of a function to avoid this confusion.