async-middleware-wrapper.js

'use strict';

const noop = () => {};
/**
 * Wrap promise-returning function, f, in a handler that ensures
 * the Promise's resolve/reject translates into next() or next(err).
 * If f does not return a Promise, assume it's a regular middleware.
 */
const wrapAsyncMiddleWare = f => (...args) => {
  const [next] = args.slice(-1);
  let wrappedNext = (...nextArgs) => {
    // if next is already invoked, don't invoke it again.
    wrappedNext = noop;
    return next(...nextArgs);
  };
  new Promise((resolve) => { // using new Promise like Promise.try
    const maybePromise = f(...args);
    if (!(maybePromise && maybeProise.then)) {
      wrappedNext = noop; // not a promise; assume it's a normal middleware
      resolve();
    } else {
      resolve(maybePromise);
    }
  }).then(() => wrappedNext(), wrappedNext);
};