asynchronously configure a feathers application

asyncBuilder.js

// ============================================================================
// Author: Arleigh Dickerson
// (adapted from https://github.com/eliaslfox/lazy-chain)
// ============================================================================
const _ = require('lodash');
const Deferred = require('./Deferred');

const pipe = Symbol('asyncBuilder.lazyHander.pipe');

const lazyHandler = app => ({
  get(methods, methodName) {
    switch (methodName) {
      case pipe:
        return async (obj) => {
          for (const {name, args} of methods) {   // eslint-disable-line
            if (name === 'configure') {
              await obj[name].call(app, ...args); // eslint-disable-line
            } else {
              obj[name](...args);
            }
          }
          return obj;
        };
      default:
        methods.push({ name: methodName });
        return (() => {
          const i = methods.length - 1;
          return (...args) => {
            methods[i].args = args;
          };
        })();
    }
  },
  set() {
    throw new Error(
      'invalid proxy handler call `set` in asyncBuilder.lazyHander'
    );
  },
});


function build(app, chain) {
  const deferred = new Deferred();

  const ready = deferred.promise;

  const start = (() => {
    let starting = false;
    return () => {
      if (!starting) {
        app.emit('beforeStart');
        starting = chain[pipe](app)
          .then(_.partialRight(_.tap, (res) => { // success interceptor
            deferred.resolve(res);
            app.emit('afterStart');
          }))
          .catch(_.partialRight(_.tap, (e) => { // failure interceptor
            deferred.reject(e);
          }));
      }
      return ready;
    };
  })();

  Object.defineProperties(app, {
    ready: { value: ready },
    start: { value: start },
  });

  return app;
}

module.exports = function asyncBuilder(app) {
  const lazyProxy = new Proxy([], lazyHandler(app));
  const buildHandler = {
    get(target, name) {
      switch (name) {
        case 'build':
          return () => build(app, lazyProxy);
        default:
          return lazyProxy[name];
      }
    },
  };
  return new Proxy({}, buildHandler);
};