When NOT to add types

Types have a real cost

JSDoc is free at runtime, but it adds visual noise and maintenance. Every @param is one more thing that has to stay in sync with the code. For some files, the cost beats the benefit.

Skip types when

1. Tiny scripts. One-off automation, glue between two CLI tools, a 20-line cron script. Reading the code IS reading the types.
2. Prototype / spike. You're exploring an idea you might delete tomorrow. Don't pay the typing tax before knowing the shape sticks.
3. Single-use callbacks. arr.map(x => x * 2) doesn't need @param {number} x — the context tells you what x is.
4. REPL experiments. Console snippets, code golf, throwaway demos.

Reach for types when

• The function is called from 3+ places.
• The return value is consumed by code you don't own.
• The parameters could legitimately be more than one shape (union types).
• You're touching a function whose bug is "wrong arg type came in".

The shape of the rule

> Types pay for themselves at the interface, not the implementation.

Type /* lib */ exports. Skip types on /* local */ helpers a few lines below.

The task

Look at the two functions in the starter. One is a 4-line internal helper. The other is a module-exported library function called from elsewhere. Add JSDoc to the EXPORTED one only.