Clean TypeScript Modules

Modules should read as small, cohesive units. Keep related code close, expose one clear concept, and avoid abstractions that do not carry real responsibility.

M1: Keep Declarations Close To Use

Declare variables near the code that needs them. A value defined at the top of a function but used much later forces the reader to remember stale context.

// Bad - formatter is irrelevant until much later
function sendDigest(users: User[], now: Date) {
  const formatter = new Intl.DateTimeFormat("en-US");

  const activeUsers = users.filter((user) => user.active);
  const recipients = activeUsers.map((user) => user.email);
  const subject = `Digest for ${recipients.length} users`;