Promise is undefined

Aurelia uses javascript Promise a lot and assumes it's present in the browser. It's available pretty much everywhere, except in legacy browsers such as IE11 and lower.

By default Aurelia provides polyfills for all features that it needs inside aurelia-polyfills (which is automatically included), with the notable exception of Promise. One reason is that some people have strong preferences for Promise libraries, which sometimes come with a lot of additional features, for example Bluebird, RSVP or others.

One way to fix errors is to use es6-promise-promise, which is a very slim Promise polyfill, with no features added.

plugins: [
  new ProvidePlugin({
    Promise: 'es6-promise-promise'
  })
]

What ProvidePlugin does is that each time a module references a global variable Promise, it injects an import * as Promise from "es6-promise-promise", so that the Promise implementation comes from the polyfill.

You can use other polyfills for Promise, look up their documentation for how to include them.