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ViewsMath.log instead of recursive method - flaw in my formula
I just got solved this problem: Conversion from VBA to JS - wrong output (thanks to Terry Lennox!)
But I'd rather would like to do the same calculation with Math.log in stead of the recursive method, my customer suggested.
I am having the following code:
var amount = parseFloat($('#input_loan_amount').val()); //value: 250000
var initial_fee = parseFloat($('#input_initial_fee').val()); //value: 3499
var monthly_fee = parseFloat($('#input_monthly_fee').val()); //value: 99
var monthly = parseFloat($('#input_monthly_interest').val()); //value: 0.41
var rate = parseFloat($('#input_installment').val()); //value:3499
$('#result_albert').html('Calculating ...');
if (amount > 0) {
amount = amount + initial_fee;
if (isNaN(monthly) == true) {
monthly = 0;
}
if (isNaN(rate) == true) {
rate = 0;
}
monthly = monthly / 100;
var monthly2 = Math.round(monthly * 100) / 100;
var months = -(Math.log(1 - (monthly * amount / (rate - monthly_fee)))) / Math.log(1 + monthly);
months = -(Math.log(Math.round((1 - (monthly * amount / (rate - monthly_fee))) * 100) / 100) / Math.log(Math.round((1 + monthly) * 100) / 100));
}
The number before the dot is correct, but the decimals are not correct. I reckon, my formula months= is not quite correct, but my mathematic skills are not high enough ...
·
Juan Pablo Isaza
1 answers
Answer question0
We can use the formula explained here: https://math.stackexchange.com/questions/3185889/how-do-i-solve-for-n-number-of-periods-in-a-loan-repayment-formula
To get us the number of months required or duration, this is basically what the NPER formula will give us in Excel.
// Using the formula n = log(A/(A-rP))
// A: monthly payment
// P: principal
// r: rate
function nper(A, P, r) {
return Math.log(A/(A-r*P))/Math.log1p(r);
}
function calculate() {
var amount = parseFloat($('#input_loan_amount').val()); //value: 250000
var initial_fee = parseFloat($('#input_initial_fee').val()); //value: 3499
var monthly_fee = parseFloat($('#input_monthly_fee').val()); //value: 99
var monthly = parseFloat($('#input_monthly_interest').val()); //value: 0.41
var rate = parseFloat($('#input_installment').val()); //value:3499
amount=amount+initial_fee;
monthly_payment = rate - monthly_fee;
monthly=monthly/100;
// Using formula from https://math.stackexchange.com/questions/3185889/how-do-i-solve-for-n-number-of-periods-in-a-loan-repayment-formula
const durationMonths = nper(monthly_payment, amount, monthly);
const roundedDuration = Math.ceil(durationMonths);
const runningAdm = roundedDuration * monthly_fee;
$('#durationOutput').html(roundedDuration);
$('#runningAdm').html(runningAdm);
}
calculate()<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js" integrity="sha512-894YE6QWD5I59HgZOGReFYm4dnWc1Qt5NtvYSaNcOP+u1T9qYdvdihz0PPSiiqn/+/3e7Jo4EaG7TubfWGUrMQ==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
<label for="input_loan_amount">Loan amount:</label>
<input type="text" id="input_loan_amount" name="input_loan_amount" value="250000">
<br>
<label for="input_initial_fee">Initial fee:</label>
<input type="text" id="input_initial_fee" name="input_initial_fee" value="3499">
<br>
<label for="input_monthly_fee">Monthly fee:</label>
<input type="text" id="input_monthly_fee" name="input_monthly_fee" value="99">
<br>
<label for="input_monthly_interest">Monthly interest (%):</label>
<input type="text" id="input_monthly_interest" name="input_monthly_interest" value="0.41">
<br>
<label for="input_installment">Fixed price:</label>
<input type="text" id="input_installment" name="input_installment" value="3499">
<br>
<br>
<bold>Duration (months): </bold><i id="durationOutput"></i><br>
<bold>Running Adm.: </bold><i id="runningAdm"></i><br>
<br><br>
<button type="button" onclick="calculate()">Calculate</button>
·
Juan Pablo Isaza
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