/*
Examples From
JavaScript: The Definitive Guide, Fourth Edition
Legal matters: these files were created by David Flanagan, and are
Copyright (c) 2001 by David Flanagan. You may use, study, modify, and
distribute them for any purpose. Please note that these examples are
provided "as-is" and come with no warranty of any kind.
David Flanagan
*/
JavaScript Loan Calculator
Enter Loan Information:
1)
Amount of the loan (any currency):
2)
Annual percentage rate of interest:
3)
Repayment period in years:
Payment Information:
4)
Your monthly payment will be:
5)
Your total payment will be:
6)
Your total interest payments will be:
function calculate() {
// Get the user's input from the form. Assume it is all valid.
// Convert interest from a percentage to a decimal, and convert from
// an annual rate to a monthly rate. Convert payment period in years
// to the number of monthly payments.
var principal = document.loandata.principal.value;
var interest = document.loandata.interest.value / 100 / 12;
var payments = document.loandata.years.value * 12;
// Now compute the monthly payment figure, using esoteric math.
var x = Math.pow(1 + interest, payments);
var monthly = (principal*x*interest)/(x-1);
// Check that the result is a finite number. If so, display the results
if (!isNaN(monthly) &&
(monthly != Number.POSITIVE_INFINITY) &&
(monthly != Number.NEGATIVE_INFINITY)) {
document.loandata.payment.value = round(monthly);
document.loandata.total.value = round(monthly * payments);
document.loandata.totalinterest.value =
round((monthly * payments) - principal);
}
// Otherwise, the user's input was probably invalid, so don't
// display anything.
else {
document.loandata.payment.value = "";
document.loandata.total.value = "";
document.loandata.totalinterest.value = "";
}
}
// This simple method rounds a number to two decimal places.
function round(x) {
return Math.round(x*100)/100;
}