JS vs Python byte array encoding

Question

I am trying to convert a byte array into a string and then send it over a socket to a remote server. I have successfully prototyped the code in Python and am trying to migrate it to Javascript.

For some reason, there is a discrepancy in the last character between the two languages.

Python Code

def make_checksum(data):
    num = 0x00
    for num2 in data:
        num = (num + num2) & 0xFF
    return num

data = [0x56, 0x54, 0x55, 0x3E, 0x28, 0x00, 0x08, 
        0x00, 0x03, 0x01, 0x46, 0x00, 0x00, 0x00, 0xC0]
message = bytearray(data + [make_checksum(data)])

Javascript

function checksum(data) {
    let res = 0x00
    for (let i = 0; i < data.length; ++i) {
        res = (res + data[i]) & 0xFF
    }
    return String.fromCharCode(res)
}

let data = new Int8Array([0x56, 0x54, 0x55, 0x3E, 0x28, 0x00,
           0x08, 0x00, 0x03, 0x01, 0x46, 0x00, 0x00, 0x00, 0xC0])
let message = String.fromCharCode(...data) + checksum(data)

I think this might have something to do with the difference between ascii and UTF.

Should the javascript be doing checksum(data) instead of checksum(0xC0)? Should the python be doing bytearray(data + [make_checksum(data)])? — mgilson
– mgilson, Commented Sep 7, 2017 at 3:33
If message is bytes, you shouldn’t represent it with a string in JavaScript. Leave it as a Uint8Array. — Ry-
– Ry- ♦, Commented Sep 7, 2017 at 3:34
do we talk about node or about browser JS here? You can't represent any byte array as string because not every byte array is valid unicode. — Lux
– Lux, Commented Sep 7, 2017 at 3:37
@DovBenyominSohacheski: How are you posting it to a server? Please show that code. — Ry-
– Ry- ♦, Commented Sep 7, 2017 at 6:28
@DovBenyominSohacheski: stream.write accepts a buffer. Buffer.concat([Buffer.from([0x56, 0x54, 0x55, 0x3E, 0x28, 0x00, 0x08, 0x00, 0x03, 0x01, 0x46, 0x00, 0x00, 0x00, 0xC0]), checksum(data)]), where checksum also returns a Buffer. — Ry-
– Ry- ♦, Commented Sep 7, 2017 at 6:30

Dov Benyomin Sohacheski · Accepted Answer · 2017-09-07 09:46:00Z

2

With the help of @Ryan I was able to solve the issue by using a Buffer instead of an unsigned array.

Code

function checksum(data) {
    let res = 0x00
    for (let i = 0; i < data.length; ++i) {
        res = (res + data[i]) & 0xFF
    }
    return Buffer.from([res])
}

let data = Buffer.from([0x56, 0x54, 0x55, 0x3E, 0x28, 0x00, 
                  0x08, 0x00, 0x03, 0x01, 0x4F, 0x00, 0x00, 0x00])
console.log(Buffer.concat([data, checksum(data)]))

answered Sep 7, 2017 at 9:46

Dov Benyomin Sohacheski

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