JavaScript: reading 3 bytes Buffer as an integer

Question

Let's say I have a hex data stream, which I want to divide into 3-bytes blocks which I need to read as an integer.

For example: given a hex string 01be638119704d4b9a I need to read the first three bytes 01be63 and read it as integer 114275. This is what I got:

var sample = '01be638119704d4b9a';
var buffer = new Buffer(sample, 'hex');
var bufferChunk = buffer.slice(0, 3);
var decimal = bufferChunk.readUInt32BE(0);

The readUInt32BE works perfectly for 4-bytes data, but here I obviously get:

RangeError: index out of range
  at checkOffset (buffer.js:494:11)
  at Buffer.readUInt32BE (buffer.js:568:5)

How do I read 3-bytes as integer correctly?

Rudie · Accepted Answer · 2019-10-30 19:10:07Z

44

If you are using node.js v0.12+ or io.js, there is buffer.readUIntBE() which allows a variable number of bytes:

var decimal = buffer.readUIntBE(0, 3);

(Note that it's readUIntBE for Big Endian and readUIntLE for Little Endian).

Otherwise if you're on an older version of node, you will have to do it manually (check bounds first of course):

var decimal = (buffer[0] << 16) + (buffer[1] << 8) + buffer[2];

edited Oct 30, 2019 at 19:10

Rudie

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answered Jun 18, 2015 at 9:44

mscdex

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3 Comments

kmachnicki Over a year ago

Thanks! Must've missed that readUIntBE function. It works perfectly.

Leif Neland Over a year ago

To save a few microseconds: var decimal = (((buffer[0] << 8) + (buffer[1]) << 8) + buffer[2];

Denis Gladkiy Over a year ago

Little endian variant: (bytes[2] << 16) | (bytes[1] << 8) | bytes[0]

Ernest Okot · Accepted Answer · 2019-05-11 17:53:05Z

4

I'm using this, if someone knows something wrong with it, please advise;

const integer = parseInt(buffer.toString("hex"), 16)

answered May 11, 2019 at 17:53

Ernest Okot

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3 Comments

Josh Peak Over a year ago

This worked great for me. I just want text categories to semi randomly get assigned a colour number from a fixed palette of colours.

Ernest Okot Over a year ago

Nice! @JoshPeak

iTayb Over a year ago

This is perfect.

BlackMamba · Accepted Answer · 2015-06-18 09:48:14Z

-3

you should convert three byte to four byte.

function three(var sample){
    var buffer = new Buffer(sample, 'hex');

    var buf = new Buffer(1);
    buf[0] = 0x0;

    return Buffer.concat([buf, buffer.slice(0, 3)]).readUInt32BE();
}

You can try this function.

answered Jun 18, 2015 at 9:48

BlackMamba

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