base64 encode/decode: btoa() and nodejs Buffer work different way?

in nodejs terminal:

> var s = String.fromCharCode(0x00,0x2B,0x20,0xFC);
> var t = new Buffer(s).toString('base64');
> console.log(s);
+ ü
> console.log(t);
ACsgw7w=
> 

string is String.fromCharCode(0x00,0x2B,0x20,0xFC) , the base64 string is "ACsgw7w="

and in Espruino Web IDE left-hand terminal:

>var s = String.fromCharCode(0x00,0x2B,0x20,0xFC);
="\x00+ \xFC"
>var t = btoa(s);
="ACsg/A=="

string is String.fromCharCode(0x00,0x2B,0x20,0xFC) , the base64 string is "ACsg/A=="

Why the same string gets different base64 encoded string in Espruino and nodejs?