Conditional parsing

[nikeee]

What is the recommended way to parse a field when a preceding field has a specific value?

For example, take this buffer:

FF 00 00 00 AA

Where FF acts as a flag that there there is AA

Now consider this:

00 FF 00 00 BB

Having the first byte unset (but the second byte set) indicates the absence of what got assigned AA before. Instead BB means something different here.

If both are set:

FF FF 00 00 AA BB

The fields corresponding to AA and BB will be assigned.

Parsing pseudocode:

const buffer = Buffer.from("FF000000AA", "hex");

const hasFieldA = buffer[0] === 0xff;
const hasFieldB = buffer[1] === 0xff;

const result = {
  a: null,
  b: null,
};

let index = 5;
if (hasA) {
  result.a = buffer[index++];
}
if (hasB) {
  result.b = buffer[index++];
}

I have some hardware that uses this kind of format (it uses bit flags though).

Is something like this possible?

Edit:
I came up with this:

const Parser = require("binary-parser").Parser;

const parser = new Parser()
        .uint8("hasA")
        .uint8("hasB")
        .uint8("hasC")
        .uint8("hasD")
        .choice({
                tag: "hasA",
                defaultChoice: new Parser(),
                choices: {
                        0xff: new Parser().uint8("a"),
                }
        })
        .choice({
                tag: "hasB",
                defaultChoice: new Parser(),
                choices: {
                        0xff: new Parser().uint8("b"),
                }
        });

console.log(parser.parse(Buffer.from("FFFF0000AACC", "hex")));

Is this the recommeded way or is there something simpler?