ArrayBuffers are used to transport raw data and several new APIs rely on them, including WebSockets, Web Intents 2](https://www.html5rocks.com/en/tutorials/file/xhr2/) and WebWorkers. However, because they recently landed in the JavaScript world, sometimes they are misinterpreted or misused.
Semantically, an ArrayBuffer
is simply an array of bytes viewed through a specific mask.
This mask, an instance of
ArrayBufferView,
defines how bytes are aligned to match
the expected structure of the content. For example, if you know that the bytes
in an ArrayBuffer represent an array of 16-bit unsigned integers, you just wrap
the ArrayBuffer in a Uint16Array view and you can manipulate its elements
using the brackets syntax as if the Uint16Array was an integer array:
// suppose buf contains the bytes [0x02, 0x01, 0x03, 0x07]
// notice the multibyte values respect the hardware endianess, which is little-endian in x86
var bufView = new Uint16Array(buf);
if (bufView[0]===258) { // 258 === 0x0102
console.log("ok");
}
bufView[0] = 255; // buf now contains the bytes [0xFF, 0x00, 0x03, 0x07]
bufView[0] = 0xff05; // buf now contains the bytes [0x05, 0xFF, 0x03, 0x07]
bufView[1] = 0x0210; // buf now contains the bytes [0x05, 0xFF, 0x10, 0x02]
One common practical question about ArrayBuffer is how to convert a String to
an ArrayBuffer and vice-versa. Since an ArrayBuffer is, in fact, a byte array,
this conversion requires that both ends agree on how to represent the characters
in the String as bytes. You probably have seen this "agreement" before: it is
the String's character encoding (and the usual "agreement terms" are, for
example, Unicode UTF-16 and iso8859-1). Thus, supposing you and the other party
have agreed on the UTF-16 encoding, the conversion code could be something like:
function ab2str(buf) {
return String.fromCharCode.apply(null, new Uint16Array(buf));
}
function str2ab(str) {
var buf = new ArrayBuffer(str.length*2); // 2 bytes for each char
var bufView = new Uint16Array(buf);
for (var i=0, strLen=str.length; i < strLen; i++) {
bufView[i] = str.charCodeAt(i);
}
return buf;
}
Note the use of Uint16Array. This is an ArrayBuffer view that aligns bytes of
the ArrayBuffers as 16-bit elements. It doesn't handle the character encoding
itself, which is handled as Unicode by String.fromCharCode and
str.charCodeAt.
A popular StackOverflow
question about this
has a highly voted answer with a somewhat convoluted solution to the conversion:
create a FileReader to act as a converter and feed a Blob containing the
String into it. Although this method works, it has poor readability and I
suspect it is slow. Since unfounded suspicions have driven many mistakes in the
history of humanity, let's take a more scientific approach here. I have
jsperf'ed the two methods
and the result confirms my suspicion and you check out the demo here.
In Chrome 20, it is almost 27 times faster to use the direct ArrayBuffer manipulation code on this article than it is to use the FileReader/Blob method.