TypeScript type-system validation

type-validation.ts

type IPv4Tuple = [number, number, number, number];
type IPv4String = `${number}.${number}.${number}.${number}`;

/**
 * TypeScript validates up to a point.
 * But we still need runtime validation and a type guard.
 **/
const createIpAddress = (values: number[]): IPv4String => {
  if (!isValidIpAddress(values)) {
    throw new Error(`Invalid IP address: ${values.join('.')}`);
  }
  
  // Type safety is guaranteed while unpacking the tuple
  const [first, second, third, fourth] = values;
  
  // We cannot use `Array.prototype.join()` because `string` does not satisfy `IPv4String`
  return `${first}.${second}.${third}.${fourth}`;
}

const isValidIpAddress = (list: number[]): list is IPv4Tuple =>
  list.length === 4
  && list.every(isInteger)
  && list.every((value) => isInRange(value, 0, 255));

const isInteger = (value: number): boolean =>
  Math.floor(value) === value;

const isInRange = (value: number, min: number, max: number): boolean =>
  value >= min && value <= max;