Building one calculator is straightforward. Building dozens of calculators with different units, assumptions, ranges, and failure modes is where input handling becomes the real product.
While building 77 free browser-based calculators for Efficienco, I found that the formula was rarely the part that caused the most trouble. The difficult part was deciding what every input actually meant and what should happen when someone entered something unexpected.
This article covers the validation patterns that became useful across the project. It does not cover the calculation formulas themselves. The focus is everything that must happen before and after a formula runs.
1. Empty, zero, and invalid are different states
A common mistake is treating every falsy value as missing:
