Overloading curried functions in TypeScript

overloading-curried-functions.ts

function sum(x: number): (y: number) => number;
function sum(x: number, y: number): number;
function sum(x: number, y?: number): ((y: number) => number) | number {
  if (y === undefined) {
    return (y: number) => x + y;
  }

  return x + y;

  // This also works:
  // switch (y) {
  //   case undefined:
  //     return (y: number) => y + x;
  //   default:
  //     return x + y;
  // }
}