What does charCodeAt(...) & 0xff accomplish?

Question

i'm not sure what 0xFF does here... is it there just to make sure that the binary code is 8bit long or has something to do with the signed/unsigned encoding? ty.

var nBytes = data.length, ui8Data = new Uint8Array(nBytes);

for (var nIdx = 0; nIdx < nBytes; nIdx++) {
  ui8Data[nIdx] = data.charCodeAt(nIdx) & 0xff;
}

XHR.send(ui8Data);

Answer 1

You're right with your first guess. It takes only the least significant 8 bits of what's returned by data.charCodeAt.

charCodeAt will return a value in the range of 0..65536. This code truncates that range to 0..255. Effectively, it's taking each 16-bit character in the string, assuming it can fit into 8 bits, and throwing out the upper byte.

[6 years later edit] In the comments, we discovered a few things: you're questioning the code for the MDN polyfill for sendAsBinary. As you came to understand, the least significant byte does come first in little-endian systems, while the most significant byte comes first in big-endian systems. Given that this is code from MDN, the code certainly does what was intended - by using FileReader.readAsBinaryString, it stores 8bit values into a 16bit holder. If you're worried about data loss, you can tweak the polyfill to extract the other byte using sData.charCodeAt(nIdx) && 0xff00 >> 8.