var sum = 0
    var num
    num = prompt("Enter new value, or 0 to end")
    while (num != 0) {
    	sum = sum + parseInt(num)
    	num = prompt("Enter new value, or 0 to end")
    }
    alert("Sum = " + sum)

   **** UPDATE ****

    var sum = 0
    var num = prompt("Enter new value, or 0 to end")
    while (num != 0) {
    	sum = sum + parseInt(num)
    	num = prompt("Enter new value, or 0 to end")
    }
    alert("Sum = " + sum)