TextEncoder and TextDecoder not perfect inverses of each other

Question

I received this answer to my previous question about encoding strings. My hope in asking that question was to get some reversible way of shifting between a string and its representation as an array of bytes like in Python 3.

I ran into a problem with one particular Uint8Array though:

var encoder = new TextEncoder();
var decoder = new TextDecoder(encoder.encoding);
var s = [248, 35, 45, 41, 178, 175, 190, 62, 134, 39];
var t = Array.from(decoder.decode(encoder.encode(Uint8Array(s)));

I expected the value of t to be [248, 35, 45, 41, 178, 175, 190, 62, 134, 39]. Instead, it is [239, 191, 189, 35, 45, 41, 239, 191, 189, 239, 191, 189, 239, 191, 189, 62, 239, 191, 189, 39]. The person who posted the answer was temporarily suspended from the site, so I cannot resolve this through commenting on his answer.

Answer 1

change var t = Array.from(decoder.decode(encoder.encode(Uint8Array(s))); to
var t = JSON.parse('['+decoder.decode(encoder.encode(new Uint8Array(s)))+']');.

var encoder = new TextEncoder();
var decoder = new TextDecoder(encoder.encoding);
var s = [248, 35, 45, 41, 178, 175, 190, 62, 134, 39];
var t = JSON.parse('['+decoder.decode(encoder.encode(new Uint8Array(s)))+']');
console.log(t);