Wait for callbacks to finish and access their results in TypeScript

result_pipe.ts

#! /usr/bin/env node
// Copyright (c) 2025 Sebastian Pipping <[email protected]>
// SPDX-License-Identifier: 0BSD

import { Sema } from "async-sema";

// eslint-disable-next-line @typescript-eslint/no-require-imports
import assert = require("node:assert");

class ResultPipe<T> {
  private sema: Sema;
  private results: T[];

  public constructor(count: number) {
    this.sema = new Sema(count);
    this.results = new Array<T>();
    for (let i = 0; i < count; i++) {
      this.sema.acquire();
    }
  }

  public push(result: T): void {
    this.results.push(result);
    this.sema.release();
  }

  public async pop(): Promise<T> {
    await this.sema.acquire();
    const result = this.results.shift();
    assert(result !== undefined); // because of how .push works above
    return result;
  }
}

async function asyncMain() {
  const p = new ResultPipe<number>(2);

  console.log("Before callback");

  setTimeout(function () {
    console.log("Inside callback A");
    p.push(11);
  }, 2000);

  setTimeout(function () {
    console.log("Inside callback B");
    p.push(22);
  }, 1000);

  console.log("Waiting for callbacks to finish...");

  const firstResult = await p.pop();
  const secondResult = await p.pop();

  console.log(
    `After both callbacks, got ${firstResult} first and then ${secondResult}.`,
  );
}

asyncMain();