Promisify @grpc-js service client with typescript

promisified-grpc-client.ts

import { Client, ServiceError, Metadata, CallOptions, ClientUnaryCall } from '@grpc/grpc-js';
import { Message } from 'google-protobuf';

type OriginalCall<T, U> = (request: T, metadata: Metadata, options: Partial<CallOptions>, callback: (error: ServiceError, res: U) => void) => ClientUnaryCall;

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

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

export function promisify<C extends Client>(client: C): Promisified<C> {
  return new Proxy(client, {
    get: (target, descriptor) => {
      let stack = '';

      // this step is required to get the correct stack trace
      // of course, this has some performance impact, but it's not that big in comparison with grpc calls
      try { throw new Error(); } catch (e) { stack = e.stack; }

      if (descriptor === '$') {
        return target;
      }

      return (...args: any[]) => new Promise((resolve, reject) => target[descriptor](...[...args, (err: ServiceError, res: Message) => {
        if (err) {
          err.stack += stack;
          reject(err);
        } else {
          resolve(res);
        }
      }]));
    },
  }) as unknown as Promisified<C>;
}

usage-example.ts

export class SearchService {

  private searchServiceClient: Promisified<SearchServiceClient>;

  constructor(private config: Config) {
    const { host, grpcPort } = this.config.services.shopCore;

    this.searchServiceClient = promisify(new SearchServiceClient(`${host}:${grpcPort}`, ChannelCredentials.createInsecure()));
  }

  async search(query: string, limit: number): Promise<SearchResult> {
    const request = new SearchRequest().setQuery(query);
    const response = await this.searchServiceClient.search(request);

    return {
      items: response.getResultsList().map(item => ({
        name: item.getName(),
        url: item.getUrl(),
      })),
    };
  }

}

Fix strict mode (and better naming): timostamm/protobuf-ts#345 (comment)

This was a big help.

I needed to make a small change to Promisified<C> to make it work on clients that have both unary and streaming calls. I changed the : never at the end to : any.

Lovely bit of code, thank for for this. :) I modified it to take non-unary calls, for metadata:

type Callback<T> = (err: ServiceError | null, reply: T) => void;

type Call<T, A1> = (message: A1, callback: Callback<T>) => UnaryResponse;
type Promisified<T, A1> = (message: A1) => Promise<T>;
type CallWithMetadata<T, A1> = (message: A1, metadata: Metadata, callback: Callback<T>) => UnaryResponse;
type PromisifiedWithMetadata<T, A1> = (message: A1, metadata: Metadata) => Promise<T>;

export type PromisifiedClient<T> = { $: T } & {
  [K in keyof T]
    : T[K] extends Call<infer T, infer A1> ? Promisified<T, A1>
    : T[K] extends CallWithMetadata<infer T, infer A1> ? PromisifiedWithMetadata<T, A1>
    : never;
}