<div class="container">
  <div class="btn1_container">
    <div class="flipBtn">
      <div class="btn"></div>
    </div>
    <span class="circle"></span>
  </div>
</div>

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

.container {
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  background: linear-gradient(to right, #e9e4f0, #d3cce3);
  display: flex;
  justify-content: center;
  align-items: center;
}

.btn1_container {
  width: 400px;
  height: 400px;
  background-color: black;
  display: flex;
  justify-content: center;
  align-items: center;
  position: relative;
  box-shadow: -1px 3px 22px 0px rgba(0, 0, 0, 0.75);
  background-image: linear-gradient(315deg, #bf3a30 0%, #864ba2 74%);
}

.flipBtn {
  width: 150px;
  height: 54px;
  background-color: #fff;
  border-radius: 50px;
  position: relative;
  border: 2px solid #111;
  position: relative;
  z-index: 10;
  transform-origin: left;
  transition: all 500ms cubic-bezier(0.77, 0, 0.175, 1);
}

.btn {
  width: 50px;
  height: 50px;
  border-radius: 50px;
  background-color: #3bb78f;
  position: absolute;
  left: 0px;
  transition: all 300ms 500ms cubic-bezier(1, 0, 1, 0.14);
  z-index: 10;
  overflow: hidden;
  border: 2px solid #fff;
}

.move {
  left: 97px;
  background-image: linear-gradient(315deg, #3bb78f 0%, #0bab64 74%);
}

.drop {
  transform: rotate(90deg);
}

.circle {
  width: 0px;
  height: 0px;
  background-image: linear-gradient(315deg, #3bb78f 0%, #0bab64 74%);
  position: absolute;
  z-index: 2;
  transition: all 200ms cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

.expand {
  width: 100%;
  height: 100%;
}

const flipBtn = document.querySelector(".flipBtn");
const btn = document.querySelector(".btn");
const circle = document.querySelector(".circle");
flipBtn.addEventListener("click", () => {
  btn.classList.toggle("move");
  flipBtn.classList.toggle("drop");
  circle.classList.toggle("expand");
});