title: pipeAsyncFunctions tags: function,promise,intermediate

Performs left-to-right function composition for asynchronous functions.

Use Array.prototype.reduce() and the spread operator (...) to perform function composition using Promise.then(). The functions can return a combination of normal values, Promises or be async, returning through await. All functions must accept a single argument.

type PromiseReturn = (v: any) => Promise<any>;
const pipeAsyncFunctions = (...fns: PromiseReturn[]) => (arg: any) =>
  fns.reduce((p, f) => p.then(f), Promise.resolve(arg) as PromiseLike<any>);

const sum = pipeAsyncFunctions(
  (x) => x + 1,
  (x) => new Promise((resolve) => setTimeout(() => resolve(x + 2), 1000)),
  (x) => x + 3,
  async (x) => (await x) + 4
);
(async () => {
  console.log(await sum(5)); // 15 (after one second)
})();