acces to switch statement parametre value from a global variable

am learning javascript by making a little project , I have little probleme of understanding well global variables , in that example when I put the variable = result ; from the randomvariable() function , on the switch parametre of test() function it doesn't work

  function randomvariable () {
    var myarray = new Array;

    myarray = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
    randomvariable  = Math.floor(Math.random() * 0);
    result = (myarray[randomvariable]);  

    }

function test() {


switch (result) {
    case 0:
        alert("haahhaha");
        break; 
}   
}