Question

I’m trying to create method to append buffers.

Here’s code, which takes very strange result:

   var offset = 0
   var a = new Buffer(0)
   var b = new Buffer('test')

   offset = a.length
   a.length += b.length
   a.copy(b, offset)
   console.log(a.toString())
   // everything works normaly
   // returns: test

   b = new Buffer('hello')
   offset = a.length
   a.length += b.length
   a.copy(b, offset)
   console.log(a.toString())
   // code the same
   // but returns: test<Buff
   // nor: testhello

   // at the third time code doesn't works and raise error: targetStart out of bounds

What do I do wrong?

Answer 1

:EDIT:

Got confused by the question. I thought he is going to handle buffers of same type.

Buffers cant be resized as they are defined by a fixed size in Node.js so the best option is to use a function like this

   function concat(buffers) {

    if (!Array.isArray(buffers)) {
      buffers = Array.prototype.slice.call(arguments);
    }

    var buffersToConcat = [], length = 0;
    buffers.forEach(function (buffer) {
        if (!Buffer.isBuffer(buffer)) {
          buffer = new Buffer(buffer);
        }
        length += buffer.length;
        buffersToConcat.push(buffer);
    });

    var concatinatedBuffer = new Buffer(length), index = 0;
    buffersToConcat.forEach(function (buffer) {
      buffer.copy(concatinatedBuffer, index, 0, buffer.length);
      index += buffer.length;
    });

    return concatinatedBuffer;
  }

What were you doing wrong?
You were trying to manipulate the length of a fixed size allocation in memory by the
+= method. Hence it threw exception beacause += operator obviously didnt re-allocated memory

*What are we doing here *

Pretty simple we are just making a new buffer out of the argument buffers.