mrjacobbloom / iced-defer

A utility for JavaScript Promises, inspired by IcedCoffeeScript's "defer" keyword

iced-defer

A failed attempt at a utility for JavaScript Promises, inspired by IcedCoffeeScript's "defer" keyword.

The goal was to sidestep the "parallel arrays" required by Promise.all:

async function initialize() {
  const [
    { data: foo }, // rename response.data => foo using destructuring
    { data: bar },
    { data: baz },
  ] = await Promise.all([
    request('foo.json'),
    request('bar.json'),
    request('baz.json'),
  ]);
  render(foo, bar, baz);
}

The approach I was playing with was to use an object that collects promises and returns an object with named properties:

import { Defer } from 'iced-defer';
async function initialize() {
  const defer = Defer<{ data: Date }>();
  request('foo.json').then(defer('foo'));
  request('bar.json').then(defer('bar'));
  request('baz.json').then(defer('baz'));
  const { foo, bar, baz } = await defer.all();
  render(foo.data, bar.data, baz.data);
}

However, this doesn't work when promises reject, because .then() callbacks only run when the promise resolves. Some possible alternatives:

• Add a new method to Promise.prototype. Ex. somePromise.register(defer('someKey'))
• defer() could return a [resolveCallback, rejectCallback] tuple. Ex. somePromise.then(...defer('someKey'))
• Only provide defer.register(). Ex. defer.register(somePromise, 'someKey'). This works but at that point the library is no easier to use than a vanilla array :/
• Also attach to promise.catch(). Ex. somePromise.then(defer('someKey')).catch(defer('someKey')). This is clunky though
• Not viable: Attach to promise.finally() instead. This doesn't work because the result/error aren't passed to .finally callbacks