How to append binary data to a buffer in node.js


I have a buffer with some binary data:

var b = new Buffer ([0x00, 0x01, 0x02]);

and I want to append 0x03.

How can I append more binary data? I'm searching in the documentation but for appending data it must be a string, if not, an error occurs (TypeError: Argument must be a string):

var b = new Buffer (256);
b.write ("hola");
console.log (b.toString ("utf8", 0, 4)); //hola
b.write (", adios", 4);
console.log (b.toString ("utf8", 0, 11)); //hola, adios

Then, the only solution I can see here is to create a new buffer for every appended binary data and copy it to the major buffer with the correct offset:

var b = new Buffer (4); //4 for having a nice printed buffer, but the size will be 16KB
new Buffer ([0x00, 0x01, 0x02]).copy (b);
console.log (b); //<Buffer 00 01 02 00>
new Buffer ([0x03]).copy (b, 3);
console.log (b); //<Buffer 00 01 02 03>

But this seems a bit inefficient because I have to instantiate a new buffer for every append.

Do you know a better way for appending binary data?


I've written a BufferedWriter that writes bytes to a file using internal buffers. Same as BufferedReader but for writing.

A quick example:

//The BufferedWriter truncates the file because append == false
new BufferedWriter ("file")
    .on ("error", function (error){
        console.log (error);

    //From the beginning of the file:
    .write ([0x00, 0x01, 0x02], 0, 3) //Writes 0x00, 0x01, 0x02
    .write (new Buffer ([0x03, 0x04]), 1, 1) //Writes 0x04
    .write (0x05) //Writes 0x05
    .close (); //Closes the writer. A flush is implicitly done.

//The BufferedWriter appends content to the end of the file because append == true
new BufferedWriter ("file", true)
    .on ("error", function (error){
        console.log (error);

    //From the end of the file:
    .write (0xFF) //Writes 0xFF
    .close (); //Closes the writer. A flush is implicitly done.

//The file contains: 0x00, 0x01, 0x02, 0x04, 0x05, 0xFF


Use concat.

7/31/2018 5:32:57 AM

Accepted Answer

Updated Answer for Node.js ~>0.8

Node is able to concatenate buffers on its own now.

var newBuffer = Buffer.concat([buffer1, buffer2]);

Old Answer for Node.js ~0.6

I use a module to add a .concat function, among others:

I know it isn't a "pure" solution, but it works very well for my purposes.

1/22/2013 3:04:37 PM

Buffers are always of fixed size, there is no built in way to resize them dynamically, so your approach of copying it to a larger Buffer is the only way.

However, to be more efficient, you could make the Buffer larger than the original contents, so it contains some "free" space where you can add data without reallocating the Buffer. That way you don't need to create a new Buffer and copy the contents on each append operation.

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