[TS] Basic support for functions in factories

[NLincoln]

There's a small limitation of the current types in that we can't use functions within factory definitions. This fixes it so that we can.

This should fix miragejs/examples#9 by making it so that we don't need to use the property-getter workaround

[samselikoff]

Thank you!

[timbakkum]

Hi, when will this be released? Latest version is from september.
We are migrating to TypeScript and use mirage factories for our unit tests as well, so we'd like to make them type-safe.

[samselikoff]

@timbakkum Looks like master is breaking due to some failures with the TS type tests. If you have time and could look into those that would definitely help me get a release out faster!

[wagenet]

I don't think this is correct. We have code like this:

export default Factory.extend({
  // This has to come before `id` so we have a value for `this.sha`
  sha(n: number): string {
    return `ABC${n}`;
  },

  id() {
    return `OirVnUtLCe_0:${this.sha}`;
  }
}

And previously, this.sha would report that it was a string. Now it claims to be string | ((n: number) => string) but when I add a debugger, it's still a string. It looks like the this is a special object that is created by iterating over each object in the hash passed to Factory.extend in order. This also means that the order of the properties matters as defining id will lead to the value of this.sha being undefined when id is called.

[wagenet]

Ok, I see what's happening here now. I think this code is actually correct, but the this in the factory methods is not what is inferred.