InMemoryStream.ts

import { Writable } from 'stream';

class InMemoryStream extends Writable {
  private chunks: Uint8Array[] = [];
  private contentPromise: Promise<string>;
  constructor() {
    super();
    this.contentPromise = new Promise((resolve, reject) => {
      this.once('finish', () => { resolve(Buffer.concat(this.chunks).toString('utf8')); });
      this.once('error', err => { reject(err); });
    });
  }
  _write(chunk: any, encoding: BufferEncoding, next: Function) {
    this.chunks.push(Buffer.from(chunk));
    next();
  }
  contents() {
    return this.contentPromise;
  }
}

usageExample.ts

import fetch from 'node-fetch';

async function downloadTo(writeStream: NodeJS.WritableStream) {
  const response = await fetch('https://gist.githubusercontent.com/carpeliam/b9d48085b1556e539303fb48d82a45b6/raw/2521b5c640060e48a237f0314025dffeac501791/writeToInMemoryStream.ts');
  response.body.pipe(writeStream);
}

const writeStream = new InMemoryStream();
await downloadTo(writeStream);
const contents = await writeStream.contents();
console.log(contents);