Investigate how to support const app = fastify(); await app; in TypeScript

[Ethan-Arrowood]

Related to #2376

See the test in the PR.

How do we type this behavior?

If its not possible (which I sorta expect but don't know for sure), then we should provide some type-cast option like:

// untested code ahead do not assume this is the solution :-) 

const _app = fastify()

const app = _app as unknown as () => Promise<FastifyInstance>

await app; // resolves FastifyInstance

[kbkk]

@Ethan-Arrowood Could you explain why wouldn't it work?

You can await anything in JS/TS, not just promises - it'll just return the awaited value.

So, given the above, this compiles/works just fine:

import * as fastify from "fastify";

const _app = fastify.fastify()

const app = await _app;

I also see no use for this

[mcollina]

The fastify instance is a Thenable, it implements a then() method that will be called by await. This custom method will load all the plugins registered so far.

[Ethan-Arrowood]

Understandable, but if I'm not mistake, this issue is specific to the promise behavior thenable aspect of the main fastify function. As a TS dev, if I await something I'm expecting that it's type is a promise as well

[fox1t]

This is really a great point @Ethan-Arrowood and I somehow agree with you.

[L2jLiga]

@Ethan-Arrowood correct me if I'm wrong but can't we use types union?

See #2428