Modern JavaScript testing focuses heavily on testing public outputs and user behavior rather than chasing line-by-line or branch-by-branch coverage. This is because tests that check final outputs survive when you rewrite internal logic or change how a loop works, and they already catch the majority of regressions. Despite this, if you write complex financial calculations, security rules, or state machines where an unhandled condition cannot be tolerated, then a green test on the final value i...