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(),
      })),
    };
  }

}

This allows to use promisified version of the @grpc-js client with all types preserved. This method is the least invasive, since it preserves the original signatures for client and its calls, only the callback is removed and still leaves the original non-promisified methods under $ property.

Of course, the promisification only works for unary calls.

I got a Element implicitly has an 'any' type because expression of type 'string | symbol' can't be used to index type 'Client'. No index signature with a parameter of type 'string' was found on type 'Client'. error on line 19, target[descriptor].

I don't know if I had a wrong typescript setting.

You should explicitly use generate_package_definition if you use https://www.npmjs.com/package/grpc_tools_node_protoc_ts . Example: grpc_tools_node_protoc --plugin=protoc-gen-ts=../../node_modules/.bin/protoc-gen-ts --ts_out=generate_package_definition:... ...

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.