JS realizes the function of scanning QR code with code scanning gun

this.start = new Date().getTime()
  let code = ''
  let lastTime, nextTime
  let lastCode, nextCode
  let that = this
  window.document.onkeypress = function (e) {
   if (window.event) { // IE
    nextCode = e.keyCode
   } else if (e.which) { // Netscape/Firefox/Opera
    nextCode = e.which
   }
   console.time()
   console.log('nextCode', nextCode)
   if (e.which === 13) {
    if ( code.length  < 3) return // the manual input time will not make the code length greater than 2, so only the code scanning gun will be available
    console.log(code)
    console.log ('scan code ends')
    console.timeEnd()
    that.parseQRCode (code) // get the input of the code scanning gun and do other operations
    code = ''
    lastCode = ''
    lastTime = ''
    return
   }
   nextTime = new Date().getTime()
   if (!lastTime && !lastCode) {
    console.log ('scan code begins... )
    code += e.key
   }
   If (lastcode & & lasttime & & nexttime - lasttime > 500) {// when there is a keypress event before code scanning, the prefix is prevented from missing
    console.log ('prevent missing initials... )
    code = e.key
   } else if (lastCode && lastTime) {
    console.log ('scanning code... )
    code += e.key
   }
   lastCode = nextCode
   lastTime = nextTime
  }