If a promise in the array is rejected, Promise.all (which returns a promise) rejects with (supposedly) the first rejected promise, but that's extremely hard to predict if you have two promises which could reject.

Serializing a promise to localStorage; I get what you're trying to do, you want to have a worker pick up exactly where it left off when you left the app. This is where a service worker would help you. I suppose you could write some kind of durable mailbox a la Akka, only running in the browser.