waiter.ts

export const waiter: {
  <T, R, A extends any[]>(
    w:Waitable<T>, done:(x:T, ...args:A) => Promise<R>|R
  ): (...args:A) => Promise<R>;
  <R, A extends any[], T=void|undefined>(
    w:Waitable<T>, done:(...args:A) => Promise<R>|R
  ): (...args:A) => Promise<R>;
} = <T, R, A extends any[]>(w:Waitable<T>, done:{
  (x:T, ...args:A):Promise<R>|R;
  (...args:A):Promise<R>|R;
}) => async (...args:A) => await w.wait(
  x => x === undefined ? done(...args) : done(x, ...args)
);