Factory Function doesn't update output?

Hi, I’m new to JS and coding in general. Trying to get a better grasp of objects and factory functions. Can someone help explain why the factory function below doesn’t update the output when a property is changed?

//this is a factory function. it is a function that returns a new object
function circleArea(radius = 10) {
    return {
        radius: radius,
        get area(){
            return radius ** 2 * Math.PI;

// this is an object made from Factory function 
const a = circleArea(12);

a.radius = 10;
//this function doesn't update the area when radius is changed

You’ve just modified it’s radius, and I think the whole func hasn’t been called fully to update what is dependent of radius.
The best way is to const circle1 = circleArea(10);