Find area of two intersecting circles

area.js

//reference https://en.wikipedia.org/wiki/Lens_(geometry)

function findDelta (d, r, R) {
    return (1 / 4) * Math.sqrt((-d + r + R) * (d - r + R) * (d + r - R) * (d + r + R) );
}
 
function area (d, r, R) {
    const delta = findDelta(d, r, R);
    const cosLeft = (Math.acos(((d * d) + (r * r) - (R * R)) / (2 * d * r)));
    const cosRight = (Math.acos(((d * d) - (r * r) + (R * R)) / (2 * d * R)));
    return (r * r) * (cosLeft) + (R * R) * cosRight - 2 * delta;
}
 
area(10,10,10);

area(1,10,10);

area(90,100,10);

area(20,10,10);

area(19,10,10);