Dealing with uint8_t in javascript

Question

G'day peoples,

I'm using MavLink to obtain GPS information. One of the message types is GPS_STATUS which is described by MavLink using a series of uint8_t[20].

If I run the following code:

console.log(' sat prn: ' + message.satellite_prn);
console.log(' sat prn: ' + JSON.stringify(message.satellite_prn));
console.log(' sat prn: ' + JSON.stringify(new Uint8Array(message.satellite_prn)));

I get the following output:

 sat prn: <weird charcaters...>
 sat prn: "\u0002\u0005\n\f\u000f\u0012\u0015\u0019\u001a\u001b\u001d\u001e\u001f\u0000\u0000\u0000\u0000"
 sat prn: {"BYTES_PER_ELEMENT":1,"buffer":{"byteLength":0},"length":0,"byteOffset":0,"byteLength":0}

So obviously it's not working. I need a means to get the int value of each element.

I found this https://developer.mozilla.org/en-US/docs/JavaScript/Typed_arrays?redirectlocale=en-US&redirectslug=JavaScript_typed_arrays

Which made me think I would be able to do the following:

satellite_prn = Array.apply([], new Uint8Array(message.satellite_prn));
satellite_prn.length === 20;
satellite_prn.constructor === Array;

But when I stringify it via JSON it reports [], I presume this is an empty array.

Anyone know how I can do this? I know that the data is an array of 20 unsigned 8 bit integers. I just need to know how to access or parse them.

Note: I'm using node.js, but that shouldn't affect what I'm doing. This is why I'm using console.log, as it's avail in node.js.

Answer 1

Two issues with your code:

1. message.satellite_prn is a string not an array
2. Unit8Array needs to be loaded with .set

---

To get an array of numbers from message.satellite_prn, do this:

var array = message.satellite_prn.map(function(c) { return c.charCodeAt(0) })

To load an ArrayBuffer, do this:

var buffer = new ArrayBuffer(array.length);
var uint8View = new Uint8Array(buffer);
uint8View.set(array);

Ideally you wouldn't need to go through the string. If you are obtaining the data from an up-to-date implementation of XMLHttpRequest, such as xhr2, you can set:

responseType = "arraybuffer"

Answer 2

Looks like the problem is my understanding on how to deal with binary data within javascript. I found that I can just do the following to get the data as base 10 via charCodeAt().

The following is the code that allowed me to iterate through the 20 unsigned 8 bit integers and get each value as a decimal:

for(var i = 0, l = message.satellite_prn.length; i < l; i++) {
    console.log(' Satellite ' + i + ', id: ' +
        parseInt(message.satellite_prn.charCodeAt(i), 10)
    );
}

I suspect that there may be a function that allows me to convert the binary data into an array, but for an unknown reason Uint8Array didn't appear to be working for me. However the above does.