Javascript to calculate a monthly loan payment

[Michael Tucker]

I found this javascript example & tried to adapt it but seems I did not get it right.

I am trying to calculate a monthly payment into the field that is this script (shown below).

The number of years of the loan, the principal balance of the loan & the annual percentage rate are already inputted manually in the same form. The script references those data points & does math to find the monthly payment.

Perhaps it could be done without javascript with a calculation field but the calculation is not simple, and this existing javascript example (meant for a web form) seemed like it should be workable if only it were formatted for tapforms. A few parts included in the original example (commented out) I am not trying to get to work at this time, just the part that concerns the loan payment amount.

function calculatePayment() {

// Source: https://www.oreilly.com/library/view/javascript-the-definitive/0596000480/ch01s08.html
// 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;

var principal = record.getFieldValue(‘fld-a37b001914c9476e85ff6e72dc5f7e41’);
var interest = record.getFieldValue(‘fld-ad2e6a56376f462fa750fee0eca40902’) / 100 / 12;
var payments = record.getFieldValue(‘fld-3aaee77c789642dbb64633cac5b9b210’) * 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)) {

x = 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 {
x = “”;
// 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;
record.setFieldValue(scr_TP_MortgagePayment,round(x));
form.saveAllChanges();
}

calculatePayment();

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