Weird behaviour in buffer on node v4.0.0

[Junkern]

var length = 256;
var buf = new Buffer(length);
for (var i = 0; i < length; ++i) {
        buf[i] = i;
}
var string = buf.toString();
for(var i = 0; i < string.length; i++) {
    console.log("i",i, string[i], string.charCodeAt(i));
}

results in the following output:

i 123 { 123
i 124 | 124
i 125 } 125
i 126 ~ 126
i 127 ⌂ 127
i 128 ? 65533
i 129 ? 65533
i 130 ? 65533
i 131 ? 65533
i 132 ? 65533

I cannot quite get my head around the fact, that the charCodeAt() suddenly changes from 127 to 65533. I also took a look at the source code of buffer.js and node_buffer.cc, but could not find code that explains this behaviour. Is this behavior intended? If so, could someone explain why?

[fatfisz]

These are invalid characters and they are substituted with 0xFFFD (see: http://www.fileformat.info/info/unicode/char/0fffd/index.htm).
That happens because the default encoding in Buffer.toString is UTF-8.

[targos]

What @fatfisz said and the fact that Buffer#toString defaults to UTF-8 encoding.
See also https://en.wikipedia.org/wiki/UTF-8

I'm closing this because it's not a technical issue with node.
If you have more questions, you are welcome to ask them in https://github.com/nodejs/help/issues