Java byteArray equivalent in JavaScript

Question

I am trying to determine what encoding scheme will give me the numbers -1 or -40 (the starting numbers for the file) for a jpeg file type.

A rest api I'm working on is expecting a byte array that looks like [-1, 94, 43, 34, etc]. In node.js I can have the byte array as hex, or any other encoding type, but I seem not able to get -1 or -40 for the starting value whichever encoding scheme I try.

In the documentation I saw an example in Java which uses the "toByteArray()" function, which seems to get the starting values (-1 or -40). Can anyone help me?

See typed arrays? http://stackoverflow.com/a/12332802/461499 — Rob Audenaerde, Jun 12 '15 at 07:53

score 3 · Accepted Answer · answered Jun 12 '15 at 08:19

3

If I correctly understand your question, you can use Buffer to get file contents, then read the bytes from it into your array.

Java byte type is a signed 8-bit integer, the equivalent in Node.js Buffer is buf.readInt8().

So you can read the required amount of bytes from Buffer into your array using buf.readInt8()

Or just convert it into Int8Array:

new Int8Array(new Buffer([100, 200, 255, 1, 32]));

answered Jun 12 '15 at 08:19

zlumer

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Hi zulmer thnxs for the response, so the value passed to the Buffer object would it be the size of the file var buf = new Buffer(filesize). Also for readInt8(offset), the offset value should be 0? – Raj Jun 12 '15 at 16:42
If you need to get jpeg file contents, use [`fs`](https://nodejs.org/api/fs.html) module, more specifically `fs.readFile()` or `fs.readFileSync`. It will provide you with `Buffer` with file contents. `offset` is the amount of bytes to skip, e.g. `offset=0` will return the first byte, `offset=1` will return the second byte, etc. – zlumer Jun 12 '15 at 17:50
perfect readInt8() does give the expected byte when offset is set to 0, how can get all the byte values with this function do i just set readInt8(0, true), not sure about this – Raj Jun 13 '15 at 15:44
to rephrase the above comment, i can run a loop to get all the byte values but is there a function that just returns all the values – Raj Jun 13 '15 at 16:21

score 0 · Answer 2 · answered Jun 12 '15 at 08:18

0

You're looking for Buffer. It allows reading/writing of variously sized binary values and random access.

answered Jun 12 '15 at 08:18

Alex Taylor

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score 0 · Answer 3 · edited May 23 '17 at 12:14

I would recommend you to change the logic of identifying a JPEG file using magic numbers from the first 4 bytes, your -40 logic could be insufficient.

FFD8FFE0 should be the first 4 bytes of a jpeg file

Therefore following Node.js program could be helpful

var fs = require('fs');
fs.open("c:\\aa1.jpg", 'r', function(err, doc){
    var check = "FFD8FFE0";
    var headerLength = ( check.length / 2 );

    var byteBuffer = new Buffer(headerLength);
    for(var i=0;i<headerLength;i++)
      byteBuffer[i] = 0x00;

    var head = "";

    fs.read(doc, byteBuffer, 0, headerLength, 0, function(err, num) {
      for(var i=0;i<headerLength;i++)
        head += byteBuffer[i].toString(16);

      if(head.toLowerCase() == check.toLowerCase())
        console.log('It is a JPEG file');
      else
        console.log('It is not a JPEG file');
    });
});

check for err and doc null check is not included in this program

Refer How to identify contents of a byte[] is a jpeg?

score 0 · Answer 4 · answered Nov 01 '21 at 22:45

You can also use TypeArray.from() method on all Typed Arrays to convert between normal arrays:

const values = [-1, 94, 43, 34]

const byteArr = Int8Array.from(values)
const original = Array.from(byteArr)

console.log({ byteArr, original })

Bonus: This works in node and also the browser

Java byteArray equivalent in JavaScript

4 Answers4

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