simple upload node server without 3rd library

upload-server-without-3rd-library.js

// some code from <https://stackoverflow.com/questions/46629086/node-js-file-upload-server-without-third-party-module>

var http = require('http');
var fs = require('fs')
var buffer = require('buffer')

function parseForm(boundary, data){
  var form = {};
  var delimiter = Buffer.from("\r\n--" + boundary);
  var body = extract(data, "--" + boundary + "\r\n");
  var CR = Buffer.from("\r\n\r\n");
  var i = 0;
  var head;
  var name;
  var filename;
  var value;
  var obj;

  if (body) {
      while (i !== -1){
          [head, i] = extract(body, i, CR);
          name = extract(head, '; name="', '"').toString();
          filename = extract(head, '; filename="', '"').toString();
          [value, i] = extract(body, i, delimiter);
          if (name){
              obj = filename ? {filename, value} : {value};
              if (form.hasOwnProperty(name)){ // multiple 
                  if (Array.isArray(form[name])){
                      form[name].push(obj);
                  } else {
                      form[name] = [form[name], obj];
                  }
              } else {
                  form[name] = obj;
              }
          }
          if (body[i] === 45 && body[i + 1] === 45) break; // "--"
          if (body[i] === 13 && body[i + 1] === 10){
              i += 2; // "\r\n"
          } else {
              //error
          }
      }
  }
  return form;
}

function extract(arr, start, end){
  if (!arr) {
    return
  }
  var useIndex = typeof start === "number",
      i,
      j;
  if (useIndex){
      i = start;
     if (end){
          j = arr.indexOf(end, i);
          return (j === -1) ? ["", -1] : [ (i === j) ? "" : arr.slice(i, j), j + end.length];
      } else return arr.slice(i);
  } else {
      i = arr.indexOf(start);
      if (i !== -1){
          i += start.length;
          if (end){
              j = arr.indexOf(end, i);
              if (j !== -1) return arr.slice(i, j);
          } else return arr.slice(i);
      }
      return "";
  }
}

http
  .createServer(function(req, res) {
    if (
      req.url === '/upload' 
      && 
      req.method === 'POST'
      && 
      req.headers['content-type'].indexOf('multipart/form-data') > -1
    ) {
      let boundary = req.headers['content-type'].match(/boundary=(.*)/)[1]
      let data = new buffer.Buffer(parseInt(req.headers['content-length'], 10))
      let pos = 0
      req.on('data', chunk => {
        data.fill(chunk, pos)
        pos += chunk.length
      })

      req.on('end', () => {
        var parseData = parseForm(boundary, data)
        var file = parseData.file
        fs.createWriteStream(file.filename).end(file.value)
        res.end('ok');
        });
    } else {
      // show a file upload form
      res.writeHead(200, { 'content-type': 'text/html' });
      res.end(
        '<form action="/upload" enctype="multipart/form-data" method="post">' +
          '<input type="file" name="file" multiple="multiple"><br>' +
          '<input type="submit" value="Upload">' +
          '</form>'
      );
    }

  })
  .listen(8080);