Why this matters
The production reason.
Convert and validate user input before math so empty strings and NaN do not leak into UI. This matters because small implementation habits compound across every page, component, and handoff in a production codebase.
Use the pattern where the behavior is owned, keep the API small, and make the result easy for another developer to verify later.
The goal is not clever code. The goal is predictable code that survives new content, new teammates, and real user traffic.
Copy-ready example
// Parse numbers explicitly
// Apply this pattern in the smallest file that owns the behavior.
const beforeShipping = [
"Name the intent clearly",
"Handle the empty or error state",
"Keep the change easy to test",
];