<div class="container"></div>

<div class="control">
  <button class="button">Add Item</button>

  <input type="checkbox" id="contain">
  <label for="contain">contain: size</label>
</div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 5vh;
  font-family: "Exo", Arial, sans-serif;
  font-size: 20px;
  font-weight: 600;
}

.container {
  border: 1px dashed #f36;
  padding: 5px;
  margin-top: auto;
  counter-reset: item;

  display: flex;
  flex-wrap: wrap;
  gap: 5px;
  align-content: flex-end;
}

button {
  margin-top: 40px;
  border-radius: 6px;
  background-color: transparent;
  border: 4px solid #3f3d56;

  padding: 10px 20px;
  outline: none;
  color: #3f3d56;
  cursor: pointer;

  font-family: "Exo", Arial, sans-serif;
  font-size: 20px;
  font-weight: 600;
}

button:active {
  color: #6c63ff;
  border-color: #6c63ff;
  transform: rotate(0.1deg) scale(0.9);
  transform-origin: 50% 100%;
}

.box {
  height: 50px;
  width: 50px;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: #09f;
  border-radius: 4px;
  gap: 5px;
  flex-shrink: 0;
}

.box::before {
  counter-increment: item;
  content: counter(item);
  color: #fff;
}

.size {
  contain: size;
}

const containEle = document.querySelector(".container");
const buttonEle = document.querySelector(".button");
const checkboxEle = document.querySelector("#contain");

buttonEle.addEventListener("click", () => {
  const box = document.createElement("div");
  box.classList.add("box");

  containEle.append(box);
});

checkboxEle.addEventListener("change", (etv) => {
  containEle.classList.toggle("size", etv.target.checked);
});