扫码枪输入的时间间隔一般在8毫秒，偶尔会有16毫秒，而键盘输入一般都在80毫秒以上，因此我设定当输入间隔小于等于30毫秒时，判断为扫码枪输入,但是这段代码会丢失扫码枪扫描的第一个字符，如何判断第一个字符是扫码枪输入还是手动输入的？

window.onload = function(e){
            var code = "";
            var lastTime,nextTime;
            var lastCode,nextCode;

            document.onkeypress = function(e) {
                nextCode = e.which;
                nextTime = new Date().getTime();

                if(lastCode == null && lastTime == null) {
                    // 此处应该怎么判断
                } 
                if(lastCode != null && lastTime != null && nextTime - lastTime <= 30) {
                    code += String.fromCharCode(nextCode); 
                } else if(lastCode != null && lastTime != null && nextTime - lastTime > 100){
                    code = "";
                }

                lastCode = nextCode;
                lastTime = nextTime;
            }

            this.onkeypress = function(e){
                if(e.which == 13){
                    console.log(code);
                    code = "";
                }
            } 
        }