jQuery Mortgage Amortization Calculator First Month Not Calculating-CodePudding

My mortgage calculator is working fine except for the first month.

I have created a codepen for testing. For example, the start date is this month, this year. The loan amount is 300,000, the interest rate is 3% and the term is 30 years.

Month 1 balance should be the total balance minus the Principal but the principal is not getting subtracted until the 2nd month.

function calculate_amortization(loan_amount, monthly_mortgage, monthly_interest_rate, length_of_mortgage) {
    var month = parseInt($('#month').val());
    var year = parseInt($('#year').val());

    var tableData = "<tr> \
                    <th>Month</th> \
                    <th>Payment</th> \
                    <th>Principal</th> \
                    <th>Interest</th> \
                    <th>Balance</th> \
                    </tr>";

    // Initializing the empty totals
    var total_mortgage = parseFloat(0);
    var total_principal = parseFloat(0);
    var total_interest = parseFloat(0);

    for (i = length_of_mortgage; i > 1; i--) {
        var monthly_interest = parseFloat(loan_amount * monthly_interest_rate).toFixed(DIGIT_PRECISION);
        var monthly_principal = parseFloat(monthly_mortgage - monthly_interest).toFixed(DIGIT_PRECISION);
        total_mortgage = parseFloat(total_mortgage)   parseFloat(monthly_mortgage);
        total_principal = parseFloat(total_principal)   parseFloat(monthly_principal);
        total_interest = parseFloat(total_interest)   parseFloat(monthly_interest);
        // var monthStr = convert_month(month);
        var monthStr = convert_month(month);
        var tablerow = "<tr> \
            <td>"   monthStr   " "   year   "</td> \
            <td>$"   localeString(monthly_mortgage)   "</td> \
            <td>$"   localeString(monthly_principal)   "</td> \
            <td>$"   localeString(monthly_interest)   "</td> \
            <td>$"   localeString(parseFloat(loan_amount).toFixed(DIGIT_PRECISION))   "</td> \
            </tr>";

        tableData = tableData   tablerow;

        if (month == 12) {
            month = 1;
            year  ;
        }
        else {
            month  ;
        };

        loan_amount = parseFloat(loan_amount - monthly_principal).toFixed(DIGIT_PRECISION);
    };

    tablerow = "<tr> \
                    <td></td> \
                    <td></td> \
                    <td><strong>$"   localeString(parseFloat(total_mortgage).toFixed(DIGIT_PRECISION))   "</strong></td> \
                    <td><strong>$"   localeString(parseFloat(total_principal).toFixed(DIGIT_PRECISION))   "</strong></td> \
                    <td><strong>$"   localeString(parseFloat(total_interest).toFixed(DIGIT_PRECISION))   "</strong></td> \
                    </tr>";
    tableData = tableData   tablerow;
    $('h3#amortization-header').html('Amortization Schedule');
    $('#total_interest').val(localeString(parseFloat(total_interest).toFixed(DIGIT_PRECISION)));
    $('table#amortization').html(tableData);
}

CodePudding user response：

You don't calculate the loan amount until the end of the loop - after the first month is put into the table.

Move this line:

loan_amount = parseFloat(loan_amount - monthly_principal).toFixed(DIGIT_PRECISION);`

To just after the monthly_principal declaration.