SOURCE

function getCheckSum(payload) {
    var checkSum = 0;
    for (i = 0; i < payload.length-1; i++) {
        checkSum = checkSum + parseInt(payload[i]);
    }
    return checkSum&0xff;
}
var cmd= [0xFF,0x55,0x04,0xa8,0x0f,0x00,0x00,0x0f];
checkSum=getCheckSum(cmd)
console.log(checkSum.toString(16))