SOURCE

function getCheckSum(payload) {
    var checkSum = 0;
    for (i = 0; i < payload.length-1; i++) {
        checkSum = checkSum + parseInt(payload[i]);
    }
    return checkSum&0xff;
}
var cmdArr = [0xFF,0x55,0x04,0x3C,0x01,0X01,0x00,0x3E];
checkSum = getCheckSum(cmdArr);
console.log(checkSum.toString(16))