Copying Buffers

An important part of working with buffers is the ability to copy data from one buffer into another buffer. Node.js provides the copy(targetBuffer, [targetStart], [sourceStart], [sourceEnd]) function for Buffer objects. The targetBuffer parameter is another Buffer object, and targetStart, sourceStart and sourceEnd are indexes inside the source and target buffers.

You can also copy data from one buffer to the other by indexing them directly, as in this example:

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sourceBuffer[index] = destinationBuffer[index]

The code in Listing 5.3 illustrates three examples of copying data from one buffer to another. The first method, in lines 4–8, copies the full buffer. The next method, in lines 10–14, copies only the middle 5 bytes of a buffer. The third example, in lines 15–22, iterates through the source buffer and copies only every other byte in the buffer. Figure 5.3 shows the results.

Listing 5.3 buffer_copy.js: Various ways to copy data from one Buffer object to another

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01 var alphabet = new Buffer('abcdefghijklmnopqrstuvwxyz'),
02 console.log(alphabet.toString());
03 // copy full buffer
04 var blank = new Buffer(26);
05 blank.fill();
06 console.log("Blank: " + blank.toString());
07 alphabet.copy(blank);
08 console.log("Blank: " + blank.toString());
09 // copy part of buffer
10 var dashes = new Buffer(26);
11 dashes.fill('-'),
12 console.log("Dashes: " + dashes.toString());
13 alphabet.copy(dashes, 10, 10, 15);
14 console.log("Dashes: " + dashes.toString());
15 // copy to and from direct indexes of buffers
16 var dots = new Buffer('-------------------------'),
17 dots.fill('.'),
18 console.log("dots: " + dots.toString());
19 for (var i=0; i < dots.length; i++){
20   if (i % 2) { dots[i] = alphabet[i]; }
21 }
22 console.log("dots: " + dots.toString());

Figure 5.3 Output of buffer_copy.js, copying data from one Buffer object to another.