script.js

var $ = function (id) {
	return document.getElementById(id);
}
var investmentError=document.getElementById("investment_error");
var rateError=document.getElementById("rate_error");
var yearsError=document.getElementById("years_error");
 
var calculateClick = function () {
	var investment = parseFloat( $("investment").value );
	var annualRate = parseFloat( $("rate").value );
	var years = parseInt( $("years").value );
	// body...
	var isValid = true;
 
if (isNaN(investment) || investment < 100 || investment > 100000) {
	investmentError.innerHTML = "Must be an integer from 100 - 100,000.";
	isValid = false;
 
}
else {
 
	investmentError.innerHTML = "";
 
}
 
if (isNaN(annualRate) || annualRate < .1 || annualRate > 12) {
	rateError.innerHTML = "Must be a value from .1 - 12.";
 
	isValid = false;
 
} else {
 
	rateError.innerHTML = "";
 
}
 
 
if(isNaN(years) || years < 1 || years > 50) {
	yearsError.innerHTML = "Must be an integer from 1 - 50.";
 
	isValid = false;
 
} else {
 
	yearsError.innerHTML = "";
 
}
 
// if all entries are valid, calulate future value
 
if(isValid) {
	futureValue = investment;
	for ( i = 12; i <= years; i++ ) {
		futureValue += futureValue * annualRate / 100;
	}
	$("future_value").value = futureValue.toFixed();
}
 
}
 
var clear = function () {
 
	$("rate").value = "";
 
}
window.onload = function () {
	$("calculate").onclick = calculateClick;
	$("investment").focus();
	$("rate").ondblclick = clear;
}