using protobuf-ts and @grpc-js client with promises

[josefschabasser]

I tried to promisify @grpc-js clients using this gist: https://gist.github.com/smnbbrv/f147fceb4c29be5ce877b6275018e294 and keep running into the same issue as https://gist.github.com/smnbbrv/f147fceb4c29be5ce877b6275018e294?permalink_comment_id=4178884#gistcomment-4178884.

What can I do about it?

[timostamm]

This doesn't seem to be an issue with protobuf-ts, so I'm converting this into a discussion.

[josefschabasser]

Yeah, sorry about that. I'm fairly new to TypeScript and down't know how to properly type this.

[timostamm]

I'm not new to TypeScript, and wouldn't want to type this. 😅

Did you look at @protobuf-ts/grpc-transport? This gives you a promise-based client. Under the hood, it uses @grpc/grpc-js.

[josefschabasser]

The thing is: we want to move from JS to TS and our whole architecture is based on @grpc/grpc-js. I don't want to break everything.

[josefschabasser]

Alright, I got it working:

import type { Client, ServiceError, CallOptions, ClientUnaryCall, Metadata } from '@grpc/grpc-js'

type OriginalCall<T, U> = (
  request: T,
  metadata: Metadata,
  options: Partial<CallOptions>,
  callback: (err: ServiceError | null, res?: U) => void,
) => ClientUnaryCall

type PromisifiedCall<T, U> = (
  request: T,
  metadata?: Metadata,
  options?: Partial<CallOptions>,
) => Promise<U>

export type PromisifiedClient<C> = { $: C } & {
  [prop in Exclude<keyof C, keyof Client>]: C[prop] extends OriginalCall<infer T, infer U>
    ? PromisifiedCall<T, U>
    : never
}

export function promisifyClient<C extends Client>(client: C) {
  return new Proxy(client, {
    get: (target, descriptor) => {
      const key = descriptor as keyof PromisifiedClient<C>

      if (key === '$') return target

      const func = target[key]
      if (typeof func === 'function')
        return (...args: unknown[]) =>
          new Promise((resolve, reject) =>
            func.call(
              target,
              ...[...args, (err: unknown, res: unknown) => (err ? reject(err) : resolve(res))],
            ),
          )
    },
  }) as unknown as PromisifiedClient<C>
}

Usage:

import { credentials } from '@grpc/grpc-js'

import { PGetImportantStuffArguments } from './_generated/test'
import { TestClient } from './_generated/test.grpc-client'
import { promisifyClient } from './grpc_promisify'

const client = promisifyClient(new TestClient('localhost:51623', credentials.createInsecure()))
const args = PGetImportantStuffArguments.create({})

client
  .getImportantStuff(args)
  .then(console.log)
  .catch(console.error)

[josefschabasser]

Replacing

            func.call(
              target,
              ...[...args, (err: unknown, res: unknown) => (err ? reject(err) : resolve(res))],
            ),

with

            Reflect.apply(func, target, [
              ...args,
              (err: unknown, res: unknown) => (err ? reject(err) : resolve(res)),
            ]),

works great, too.

[smnbbrv]

Thanks for adapting my gist, I also use your version now :)

[asperling]

Nice, thanks both of you. I had to wrap

            func.call(
              target,
              ...[...args, (err: unknown, res: unknown) => (err ? reject(err) : resolve(res))],
            ),

in a body ({ … }) to fix an implicit any on return though.

[asperling]

Also, whenever I use the client, the return type of the client calls is unknown. So const stuff = client.getImportantStuff(args) would be of type unknown.