JS 串口 16进制数据累加和取低位

//低位校验
    checkEnd(str){
        let itotal=0,len = str.length,num = 0;
        while(num<len){
            let s = str.substring(num,num+2);
            itotal += parseInt(s,16)
            num=num+2;
        }
        let mode = itotal%256;
        let shex = mode.toString(16)
        let iLen = shex.length;
        if(iLen<2){
            shex='0'+shex
        }
        return shex;
    },