Why function "return UNDEFINED" ?

Question

var str = prompt();
var arr = str.split('');
var num = arr.length;

for (var k=0;k<=num;k++) {
var x = arr[k];
document.write(getString(x));
}

function getString(x){
var div = document.createElement("div");
div.className = "className";
var oText = document.createTextNode(x);
div.appendChild(oText);
document.body.appendChild(div);
}

Answer 1

// Because "x" is a variable parameter and he must need a "return" keyword to show the value of this variable, otherwise, he doesn't work.

Answer 2

Because you haven't defined a return value at the end of your function.