Decoding and encoding Typed Array in JS giving different result than original value

Does anyone know why this happens

    const encoder = new TextEncoder()
    const decoder = new TextDecoder()
    const array = new Uint8Array([200])
    console.log(encoder.encode(decoder.decode(array))) // prints [239, 191, 189]

As far as I know, decoding and encoding anything should give me the exact same value. Should not it just print [200] back?

>Solution :

You are decoding [200] which is an invalid UTF-8 sequence and results in �

const decoder = new TextDecoder()
const array = new Uint8Array([200])
console.log(decoder.decode(array)) // prints �

Since the binary representation of 200 is 11001000, the decoder thinks this is a two-byte UTF-8 character:

In UTF-8 encoding, a byte starting with 110 indicates that it’s the first byte of a two-byte encoded character.

Then when you try to encode � you just get the following output [239, 191, 189] or 0xEF 0xBF 0xBD in hexadecimal

const encoder = new TextEncoder()
const array = new Uint8Array([])
console.log(encoder.encode("�"))