如何用Node.js实现一个支持断点续传的文件上传？

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 const express = require('express'); const multer = require('multer'); const fs = require('fs'); const path = require('path');  const app = express(); const uploadDir = './uploads'; const chunkDir = './chunks';  !fs.existsSync(uploadDir) && fs.mkdirSync(uploadDir); !fs.existsSync(chunkDir) && fs.mkdirSync(chunkDir);  // 配置 multer 存储分片 const storage = multer.diskStorage({   destination: (req, file, cb) => {     const dir = path.join(chunkDir, req.body.fileHash);     !fs.existsSync(dir) && fs.mkdirSync(dir, { recursive: true });     cb(null, dir);   },   filename: (req, file, cb) => {     // 使用分片索引命名     cb(null, `part-${req.body.chunkIndex}`);   } }); const upload = multer({ storage });  // 查询已上传的分片 app.post('/upload/check', (req, res) => {   const { fileHash } = req.body;   const chunkPath = path.join(chunkDir, fileHash);   if (!fs.existsSync(chunkPath)) return res.json({ uploaded: [] });    fs.readdir(chunkPath, (err, files) => {     if (err) return res.status(500).send();     const uploaded = files.map(f => parseInt(f.split('-')[1]));     res.json({ uploaded });   }); });  // 接收分片 app.post('/upload/chunk', upload.single('chunk'), (req, res) => {   res.json({ success: true }); });  // 合并文件 app.post('/upload/merge', (req, res) => {   const { fileHash, fileName, totalChunks } = req.body;   const chunkPath = path.join(chunkDir, fileHash);   const filePath = path.join(uploadDir, fileName);    const writestream = fs.createWriteStream(filePath);    function appendChunk(i) {     if (i >= totalChunks) {       writeStream.end();       // 清理分片       fs.rm(chunkPath, { recursive: true }, () => {});       return res.json({ url: `/files/${fileName}` });     }     const partPath = path.join(chunkPath, `part-${i}`);     const readStream = fs.createReadStream(partPath);     readStream.pipe(writeStream, { end: false });     readStream.on('end', () => appendChunk(i + 1));   }    appendChunk(0); });