scanCode() {

      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

        }

        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')

          // that.distinguishCode(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

      }

    }