change-grid-auto-flow.md

html

<div id="grid">
  <div id="item1"></div>
  <div id="item2"></div>
  <div id="item3"></div>
  <div id="item4"></div>
  <div id="item5"></div>
</div>
<select id="direction" onchange="changeGridAutoFlow()">
  <option value="column">column</option>
  <option value="row">row</option>
</select>
<input id="dense" type="checkbox" onchange="changeGridAutoFlow()">
<label for="dense">dense</label>

css

#grid {
  height: 200px;
  width: 200px;
  display: grid;
  grid-gap: 10px;
  grid-template: repeat(4, 1fr) / repeat(2, 1fr);
  grid-auto-flow: column;  /* or 'row', 'row dense', 'column dense' */
}

#item1 {
  background-color: lime;
  grid-row-start: 3;
}

#item2 {
  background-color: yellow;
}

#item3 {
  background-color: blue;
}

#item4 {
  grid-column-start: 2;
  background-color: red;
}

#item5 {
  background-color: aqua;
}

js

window.changeGridAutoFlow = function() {
  var grid = document.getElementById("grid");
  var direction = document.getElementById("direction");
  var dense = document.getElementById("dense");
  var gridAutoFlow = direction.value === "row" ? "row" : "column";

  if (dense.checked) {
    gridAutoFlow += " dense";
  }

  grid.style.gridAutoFlow = gridAutoFlow;
}

Ref: MDN Web Docs - grid-auto-flow