const circleRadius = 50;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background('white');

  fill('black');

  // Calculate the distance from the mouse position to the center of the circle

  if (dist(mouseX, mouseY, circleCenter[0], circleCenter[1]) < circleRadius) {

    // if the distance is less than the radius, we're within the circle

    noFill();

    stroke('black');

  }

  // p5's circle() function uses diameter, not radius, so we multiply by 2

  circle(circleCenter[0], circleCenter[1], circleRadius * 2);

}