場景：
掃碼槍掃碼，獲取信息，js代碼如何獲取掃描信息呢？

原理：
掃碼槍輸入會觸發(fā)鍵盤輸入事件。
掃碼槍輸入的時間間隔一般在8毫秒，偶爾會有16毫秒，而鍵盤輸入一般都在80毫秒以上。

        var keycode = "";
        var lastTime=null;
        var nextTime=null;
        var lastCode=null;
        var nextCode=null;

        document.onkeydown=function(e){
            //兼容性處理
            if(window.event){
                nextCode = e.keyCode
            } else if(e.which){
                nextCode = e.which
            }
            
            //獲取當前時間
            nextTime = new Date().getTime();
              
            if(nextCode==13 && keycode!= "" && nextTime - lastTime <= 30){//回車字符
                console.log(keycode);
                keycode = "";
                lastCode = null;
                lastTime = null;
            }else{//此處可以增加限制nextCode的種類例如數(shù)字
                if(lastCode == null && lastTime == null){//初始字母
                    keycode = String.fromCharCode(nextCode);
                }else if(lastCode != null && lastTime != null && nextTime - lastTime <= 30){
                    keycode += String.fromCharCode(nextCode);
                }else{//手動輸入
                    keycode = "";
                    lastCode = null;
                    lastTime = null;
                }
                lastCode = nextCode;
                lastTime = nextTime;
            }
        }