PHP variable scope: JS variable scope

Please look at the code first:
var scope = "global";
function f () {
alert (scope);//dispaly "undefined" not "global"
var scope = "local";//vaeiable initized Here,but defined everywhere
alert (scope);//display "Local"
}
f (); why?
Local variables are defined throughout the function body, which means that global variables with the same name are hidden throughout the function body. Although the local variable is defined throughout the function body, it is not initialized in the execution of the Var statement, so the above function f and the following function are equivalent
function f () {
var scope;//local variable in function header declaration
alert (scope); Here the variable is defined, but the value is "undefined"
Scope = "local"; Initializing variables and assigning values
alert (scope);//Here the variable has a value
}
This article links http://www.cxybl.com/html/wyzz/JavaScript_Ajax/20121203/34637.html