Converting a number to a 4 bytes string

Question

How can I convert a number like 5 into a 4 bytes string that would resemble something like this if it were to be entered manually.

var size = "\00\00\00\05";

The number is the size of a packet I need to send through a socket and the first 4 bytes of the packet needs to be the size of the packet (without counting the first 4 bytes obviously).

var size = "\00\00\00\05";
var packet = size + "JSON_STRING";

Let me know if I'm not explaining myself properly I have never used node.js before but I'm getting the hang of it.

Thanks for your answers I'm having issues with the par of my script that is supposed to send the TCP packet, I'll come back to you probably tomorrow to find out which method is better. — StarFox
– StarFox, Commented Feb 10, 2015 at 23:04

loganfsmyth · Accepted Answer · 2015-02-10 22:57:14Z

4

Use Node's Buffer rather than a string. Strings are for characters, Buffers are for bytes:

var size = 5;
var sizeBytes = new Buffer(4);
sizeBytes.writeUInt32LE(size, 0);

console.log(sizeBytes);
// <Buffer 05 00 00 00>

edited Feb 10, 2015 at 22:57

answered Feb 10, 2015 at 22:32

loganfsmyth

162k31 gold badges349 silver badges259 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Iłya Bursov · Accepted Answer · 2015-02-10 22:08:27Z

2

One obvious variant is:

var size = 5;
var b1 = size & 0xff;
var b2 = (size>>>8) & 0xff;
var b3 = (size>>>16) & 0xff;
var b4 = (size>>>24) & 0xff;
var sizeString = String.fromCharCode(b1, b2, b3, b4);

very important note - depending on architecture order of b1-b4 could be different, it is called Endianness and probably you need b4, b3, b2, b1

answered Feb 10, 2015 at 22:08

Iłya Bursov

24.4k4 gold badges36 silver badges68 bronze badges

Comments

Adam · Accepted Answer · 2015-02-10 22:13:41Z

2

You can do this using JavaScript Typed arrays, this allows actual byte level control rather than using characters within Strings...

Create an ArrayBuffer

var ab = new ArrayBuffer(4);

Use DataView to write an integer into it

var dv = new DataView(ab);
dv.setInt32(0, 0x1234, false); // <== false = big endian

Now you can get your ArrayBuffer as an unsigned byte array

var unit8 = new Uint8Array(ab);

Output

console.log(unit8);
[0, 0, 18, 52]

answered Feb 10, 2015 at 22:13

Adam

36.9k9 gold badges104 silver badges145 bronze badges

Collectives™ on Stack Overflow

Converting a number to a 4 bytes string

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related