Annotate function boundaries and let TypeScript infer the rest

TypeScript infers most local types correctly. Writing const count: number = 0 adds noise without adding safety. Annotations earn their place at boundaries, where they act as a contract:

• function parameters, which TypeScript cannot infer,
• return types of exported functions, so a change inside the function cannot silently change what callers receive,
• exported constants and public object shapes.

An explicit return type also moves errors to the right place. Without it, a mistake in the function body shows up as a confusing error in some caller. With it, the error points at the line that broke the contract.

// The return type is the contract callers depend on
export function parsePrice(input: string): number | null {
  const value = Number.parseFloat(input); // inferred: number
  return Number.isFinite(value) ? value : null;
}