node.js中的流是不可重用的。读或写的时候，他们会消耗。也就是说，一旦从流中读取数据一次，就不能再次读取它。因此，不能使用同一个流来处理多个请求
我写了一个从youtube获取音频的例子：

const express = require('express');
const ytdl = require('ytdl-core');
const Stream = require('stream');

let buffer;
let v_id;

const app = express();

app.get("/ytid/:videoId", function (req, res) {
  let inputVideoID = req.params.videoId;
  getAudio(inputVideoID)
  .then((buffer) => {
    const stream = new Stream.PassThrough();
    stream.end(buffer);
    stream.pipe(res);
  })
  .catch(err => {
    console.log(err);
    res.status(500).send(err.message);
  });
});

async function getAudio(videoID) {
  if (videoID !== v_id || !buffer) {
    console.log(`Audio ID: ${videoID}`);
    const stream = ytdl(videoID, {
      quality: "lowestaudio",
      filter: "audioonly",
    });
    buffer = await streamToBuffer(stream);
    v_id = videoID;
  }

  return buffer;
}

function streamToBuffer(stream) {
  return new Promise((resolve, reject) => {
    const chunks = [];
    stream.on('data', chunk => chunks.push(chunk));
    stream.on('end', () => resolve(Buffer.concat(chunks)));
    stream.on('error', reject);
  });
}

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server is listening on port: ${PORT}`));

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