Building a Mortgage Calculator with JavaScript: A Step-by-Step Guide

My process in creating a simple mortgage calculator using JavasScript, HTML, and CSS.

Calculating mortgage payments can be a daunting task, especially when dealing with complex financial figures. Thankfully, with the power of JavaScript, we can create a mortgage calculator that simplifies this process. In this tutorial, we'll walk through the steps involved in building a mortgage calculator using JavaScript.

Step 1: Setting Up the HTML To begin, let's set up the HTML structure for our mortgage calculator. We'll need input fields for the loan amount, loan term, and interest rate, as well as a button to trigger the calculation. Additionally, we'll include a table to display the monthly payment schedule. 

Step 2: Implementing the JavaScript Functions Next, we'll implement the JavaScript functions that power our mortgage calculator. To clear my table I created a function that would remove the data table's first child while there was a first child. This function ensures that the table is cleared before new results are displayed, providing a clean interface for users.

Step 3: Validating and Getting User Inputs To ensure accurate calculations, it's crucial to validate the user inputs. We'll implement the getValues() function to retrieve the loan amount, loan term, and interest rate from the form. This function also performs input validation to ensure the entered values are valid numbers and greater than zero.

Step 4: Calculating the Loan Payments The heart of our mortgage calculator lies in the calculation functions. We'll implement the calculateMonthlyPayment() function to calculate the monthly mortgage payment based on the loan amount, loan term, and interest rate. This function utilizes the standard formula for mortgage payment calculation: M = P * (r * (1 + r)^n) / ((1 + r)^n - 1)

Where:

• M is the monthly payment
• P is the loan amount
• r is the monthly interest rate (annual interest rate divided by 12 and converted to a decimal)
• n is the total number of monthly payments (loan term multiplied by 12)

Step 5: Calculating the Totals Next, we'll implement the calculateTotals() function to calculate the total cost of the loan and the total interest paid. This function calls the calculateMonthlyPayment() function to get the monthly payment and uses it to calculate the total cost and total interest. This function calculates the monthly payment, total cost, and total interest and returns an object containing these values. There are 2 formulas needed for this function. 

Total Cost = M * n  

• Total Cost is the total cost of the loan
• M is the monthly payment
• n is the total number of monthly payments

Total Interest = Total Cost - P

• Total Interest is the total interest paid
• Total Cost is the total cost of the loan
• P is the loan amount

Step 6: Calculating the Amortization Schedule In addition to the totals, we can calculate the amortization schedule, which displays the monthly payment breakdown. We'll implement the calculatePayments() function to calculate the payment details for each month. This function iterates over each month, calculating the interest, principal payment, total interest, and remaining balance. The details for each month are stored in an array, which is then returned.

Step 7: Displaying the Results To present the calculated results to the user, we'll implement the displayTotals() and displayPayments() functions. The displayTotals() function updates the corresponding HTML elements with the calculated totals.

Congratulations! You've successfully built a mortgage calculator using JavaScript. By following this step-by-step guide, you've learned how to implement the necessary functions for input validation, calculation, and displaying the results. Feel free to enhance the calculator further and link your live mortgage calculator below in the comments.

You can check out my live mortgage calculator here and you can also check out my GitHub repo here.

Daniel Arreola

Software Developer

I'm a dedicated web developer based in the Bay Area, California. I bring your web projects to life. Specializing in crafting clean and professional websites, I ensure a seamless user experience.