let a = [];

function setup() {
  createCanvas(400, 400);
  for (let i = 0; i < 10; i++) {
    a[i] = i * 20;
  }
}

function draw() {
  background(220);
  for (let i = 0; i < a.length; i++) {
    rect(a[i], 20, 20, 20);
  }
}

function mousePressed() {
   for (let i = 0; i < 10; i++) {
    if (mouseX > a[i] && mouseX < a[i] + 20 && mouseY > 20 && mouseY < 20 + 20) {
       a.splice(i, 1);
    }
    }
}