<main>
  <h2>Auto Placement of Grid Items with <code>grid-auto-flow</code></h2>
  <p></p>
  <div class="btns">
    <label for="row">row <input type="radio" name="autoflow" value="row" id="row" checked></label>
    <label for="column">column <input type="radio" name="autoflow"  value="column" id="column"></label>
    <label for="dense">dense <input type="radio" name="autoflow"  value="dense" id="dense"></label>
    <label for="rowdense">row dense <input type="radio" name="autoflow"  value="rowdense" id="rowdense"></label>
    <label for="columndense">column dense <input type="radio" name="autoflow"  value="columndense" id="columndense"></label>
  </div>
  <div class="container">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
    <div class="item">4</div>
    <div class="item">5</div>
  </div>
  <div class="dir">
    <label for="ltr">left-to-right <input type="radio" value="ltr" name="direction" id="ltr" checked></label>
    <label for="rtl">right-to-left <input type="radio" value="rtl" name="direction" id="rtl"></label>
  </div>
</main>

body {
  font-family: Arial, sans-serif;
  font-size: 20px;
  padding: 0 20px 30px;
  line-height: 1.4;
}

main {
  max-width: 800px;
  margin: 0 auto;
}

h2 {
  text-align: center;
}

code {
  font-family: Consolas, monospace;
  background: #ccc;
  padding: 1px 3px;
}

.btns {
  text-align: center;
  margin-bottom: 1em;
}

button {
  margin: 0 auto 1em;
}

.container {
  border: solid 2px;
  background: #f8ddf0;
  margin-bottom: 1em;
  padding: 4px;
  display: grid;
  grid-template-areas: "head head ...."
                       "nav  main main"
                       "nav  main main"
                       "...  foot foot";
  grid-auto-flow: dense;
}

.row {
  grid-auto-flow: row;
}

.column {
  grid-auto-flow: column;
}

.dense {
  grid-auto-flow: dense;
}

.rowdense {
  grid-auto-flow: row dense;
}

.columndense {
  grid-auto-flow: column dense;
}

.item:nth-child(1) {
  grid-column: auto/span 2;
}

.item:nth-child(2) {
  grid-column: auto/span 2;
}

.ltr {
  direction: ltr;
}

.rtl {
  direction: rtl;
}

.dir {
  text-align: center;
}

label {
  margin-right: 1.3em;
  padding: 8px;
}

.item {
  background: #c565a8;
  border: solid 2px #305077;
  text-align: center;
  font-size: 2em;
  margin: 4px;
  color: white;
  font-weight: bold;
}

let btns = document.querySelector('.btns').querySelectorAll('input'),
    dirs = document.querySelector('.dir').querySelectorAll('input'),
    container = document.querySelector('.container');

for (i of btns) {
  (function(i) {
    i.addEventListener('click', function() {
      clearClasses(container, btns);
      container.classList.add(i.value);
    }, false);
  })(i);
}

for (i of dirs) {
  (function(i) {
    i.addEventListener('click', function() {
      clearClasses(container, dirs);
      container.classList.add(i.value);
    }, false);
  })(i);
}

function clearClasses (el, cl) {
  for (i of cl) {
    el.classList.remove(i.value);
  }
}