Introduce Assertion

Goal

Invariants the code assumes are stated explicitly; readers don't need to deduce them.

Before the refactoring

// rate must be positive
const tax = base * rate;

After the refactoring

if (rate <= 0) throw new Error('rate must be positive');
const tax = base * rate;

Savings

Bugs that violate the invariant fail loudly at the source instead of bubbling out as mysterious downstream errors.

Note

Assertions used as control flow couple production behavior to debug-mode invariants — keep them as runtime contracts that should never fire.