I have the following code that works fine for me, and it have been used succesfully multiple times in my application that I’m working on:

function rotate(position, angle, axis) {
    if (angle !== 0) {
        const x = position.x;
        const y = position.y;
        const z = position.z;
        const sin = Math.sin(angle);
        const cos = Math.cos(angle);

        if (axis === 'X') {
            position.x = x * cos + z * sin;
            position.z = -x * sin + z * cos;
        } else if (axis === 'Y') {
            position.y = y * cos - z * sin;
            position.z = y * sin + z * cos;
        } else if (axis === 'Z') {
            position.x = x * cos - y * sin;
            position.y = x * sin + y * cos;
        }
    }

    return position;
}

function applyAngle(position, angles) {
    position = rotate(position, angles.x, 'X');
    position = rotate(position, angles.y, 'Y');
    position = rotate(position, angles.z, 'Z');

    return position;
}

I want to create an “addAngles(a, b)” function where I can simply add angles together. I have written a unit test for this:

const testAngleA = {
    x: Math.random() * Math.PI * 2,
    y: Math.random() * Math.PI * 2,
    z: Math.random() * Math.PI * 2,
};

const testAngleB = {
    x: Math.random() * Math.PI * 2,
    y: Math.random() * Math.PI * 2,
    z: Math.random() * Math.PI * 2,
};

const testAdditionResult = addAngles(testAngleA, testAngleB);

let testPositionA = {x: 0, y: 1, z: 0};
let testPositionB = {x: 0, y: 1, z: 0};

testPositionA = applyAngle(testPositionA, testAdditionResult);

testPositionB = applyAngle(testPositionB, testAngleA);
testPositionB = applyAngle(testPositionB, testAngleB);

let strA = JSON.stringify(testPositionA);
let strB = JSON.stringify(testPositionB);

if(strA === strB) {
    console.log('succeed', strA, strB);
} else {
    console.log('failed', strA, strB);
}

I have tried a lot of things (Quaternion multiplication, matrix multiplication) but I can’t make it. Chat gpt, Geminy and Claude is also struggeling to solve it 🙂