Writing quality full-stack TypeScript

Apply these principles when writing or reviewing TypeScript code.

Make impossible states unrepresentable

Use the type system to make invalid states fail at compile time. Fewer reachable states = easier code to read and change.

Branded types

Brand primitives so they can't be mixed up. Validate once at the boundary; downstream code trusts the type.

type PhoneNumber = string & { __brand: "PhoneNumber" };

function parsePhone(input: string): PhoneNumber {
  if (!/^\+?\d{10,15}$/.test(input)) throw new Error(`Invalid: ${input}`);
  return input as PhoneNumber;
}