如何将一个div附加到另一个div中？

var parentDiv = document.getElementById("parentDiv");
var childDiv = document.createElement("div");
parentDiv.appendChild(childDiv);

$("#parentDiv").append("<div></div>");

<div class="parentDiv">
  <div class="childDiv"></div>
</div>

<style>
.parentDiv {
  display: grid;
  grid-template-areas: "header header"
                       "sidebar content";
}

.childDiv {
  grid-area: content;
}
</style>