How to properly handle incoming TCP/UDP data

No, but .set seems to fit:

a = new Uint8Array([1,2,3,4,5,6,7,8,9,10]);
b = new Uint8Array(7); // 0,0,0,0,0,0,0
c = new Uint8Array(a.buffer, 2, 5); // [3,4,5,6,7]
b.set(c, 2); // set b with the contents of c starting from index 2
b; // [0,0,3,4,5,6,7]

Source: https://www.espruino.com/Performance