javascript - What does charCodeAt(...) & 0xff accomplish? - Stack Overflow

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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);

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);

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Share Improve this question Follow edited Dec 23, 2013 at 20:06 Scott Mermelstein 15.4k44 gold badges4949 silver badges7777 bronze badges asked Dec 23, 2013 at 19:29 RayRay 10311 silver badge44 bronze badges 5

• 0xFF is hex for 255. It's masking the charCodeAt(nIdx) with 255 as a way of "selecting" the lower 8 bits. – vcsjones Commented Dec 23, 2013 at 19:30
• There's more to this question than the title implies. The "duplicate" question has nothing to do with bitmasking. – Scott Mermelstein Commented Dec 23, 2013 at 19:45
• It may be a duplicate of this question, though: stackoverflow./questions/20486551/… – Scott Mermelstein Commented Dec 23, 2013 at 19:51
• @ScottMermelstein If there is more to it, help us out by editing the title to go over exactly what the OP is asking? Based on all the avialable text in the question; the questions appear to be duplicates. If they aren't, the best thing to do would be to edit the question to show those differences so it can be re-opened and the question he actually wants to be answered can be answered. – George Stocker Commented Dec 23, 2013 at 20:02
• @GeorgeStocker Really? You don't see a difference between "What does 0X mean? I saw it mentioned in class" and someone posting code using "& 0xff" asking what it does? Hang on, I'll edit the title. – Scott Mermelstein Commented Dec 23, 2013 at 20:05

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1 Answer 1

Sorted by: Reset to default 10

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 ments, 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 e first in little-endian systems, while the most significant byte es 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.

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