How Two Hidden Bugs Broke a Browser-Based Loan Calculator Built on Excel's PMT

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A developer built a client-side loan calculator for a landing page that replicates Excel's PMT function entirely in the browser, requiring no backend. The project took two attempts after subtle bugs went unnoticed until the output involved monetary values, where even a one-cent discrepancy could undermine user trust. The calculator was designed so that user inputs never leave the browser, mirroring the same formulas used in an accompanying spreadsheet product. The live implementation is a car-refinance calculator that surfaces two key figures: the monthly payment reduction and the break-even period for closing costs. The developer also flagged that the loan payoff balance shown in lender apps is often inaccurate, advising users to call their lender for the correct figure.

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