JS实现扫码枪扫描二维码功能

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 // 手动输入的时间不会让code的长度大于2，所以这里只会对扫码枪有
    console.log(code)
    console.log('扫码结束')
    console.timeEnd()
    that.parseQRCode(code) // 获取到扫码枪输入的内容，做别的操作
    code = ''
    lastCode = ''
    lastTime = ''
    return
   }
   nextTime = new Date().getTime()
   if (!lastTime && !lastCode) {
    console.log('扫码开始。。。')
    code += e.key
   }
   if (lastCode && lastTime && nextTime - lastTime > 500) { // 当扫码前有keypress事件时,防止首字缺失
    console.log('防止首字缺失。。。')
    code = e.key
   } else if (lastCode && lastTime) {
    console.log('扫码中。。。')
    code += e.key
   }
   lastCode = nextCode
   lastTime = nextTime
  }