Type-safe units in TypeScript

unit.ts

namespace TypeBranding {
  // This incurs no runtime cost. It is a purely compile-time construct.

  type Meters = number & { __metersBrand: any };
  type Miles = number & { __milesBrand: any };

  function Meters(i: number): Meters { return i as Meters; }
  function Miles(i: number): Miles { return i as Miles; }

  let a: Meters;
  a = 5; // does not compile
  a = Miles(5); // does not compile
  a = Meters(5); // compiles
}

namespace InterfacePlusLiteralTypes {
  interface Distance<Unit extends string> {
    value: number;
    unit: Unit;
  }

  type Meters = Distance<'meters'>;
  type Miles = Distance<'miles'>;

  function Meters(value: number): Meters { return {value, unit: 'meters'}; }
  function Miles(value: number): Miles { return {value, unit: 'miles'}; }

  let a: Meters;
  a = 5; // does not compile
  a = Miles(5); // does not compile
  a = Meters(5); // copiles
}