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<!DOCTYPE html>
<html>
<head>
<style>
.item3 { grid-column: auto / span 2; }
.grid-container {
  display: grid;
  grid-template-columns: auto auto auto;
  grid-template-rows: auto auto;
  grid-auto-flow: row;
  grid-gap: 10px;
  background-color: #2196F3;
  padding: 10px;
}
.grid-container > div {
  background-color: rgba(255, 255, 255, 0.8);
  text-align: center;
  padding: 20px 0;
  font-size: 30px;
}
</style>
</head>
<body>
<h1>Change the grid-auto-flow Property with JavaScript</h1>
<p>Click the button to set the grid-auto-flow to "row dense":</p>
<p><button onclick="myFunction()">Fill the hole</button></p>
<div class="grid-container" id="myDIV">
  <div class="item1">1</div>
  <div class="item2">2</div>
  <div class="item3">3</div>
  <div class="item4">4</div>
</div>
<script>
function myFunction() {
  document.getElementById("myDIV").style.gridAutoFlow = "row dense";