javascript - How to call a function inside of a variable inside of another function? -

for example:

function x() {     var = 'hello';     var b = y(a); /*how think work*/     document.getelementbyid("greet").innerhtml=b; }  function y(bill) {     var c = bill+' billy!';     return; } 

i can't figure out! i've seen in code, how i've written it, before think code beyond outdated. since written in like.... 2000 think? want variable = bill in function y , put bill+' billy' output saying. hello billy! actual code lot more complex , has lot going on fudges up.

change to:

function y(bill) {     var c = bill+' billy!';     return c; }