CODEWITHSUNDEEP
javascriptnode.js
StarFox
StarFox

Reputation: 141

Converting a number to a 4 bytes string

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.

Upvotes: 2

Views: 2942

Answers (3)

loganfsmyth
loganfsmyth

Reputation: 161457

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>

Upvotes: 4

Adam
Adam

Reputation: 36703

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]

Upvotes: 2

Iłya Bursov
Iłya Bursov

Reputation: 24146

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

Upvotes: 2

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