<div class="parent">
  <div class="child">
    <div class="box"></div>
  </div>
</div>

.parent {
  height: 100px;
  width: 100px;
  background: red;
  overflow: hidden;
  position: relative;
}
.child {
  background: blue;
  height: 100%;
  width: 100%;
  transform: scale(0.1);
}
.box {
  height: 3500px;
  width: 100px;
  background: green;
}

console.log(document.querySelector(".parent").scrollHeight);