Example

by bornasepic

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/justify-content -->

<div id="container">
  <div></div>
  <div></div>
  <div></div>
</div>
<select id="justifyContent">
  <option value="start">start</option>
  <option value="end">end</option>
  <option value="flex-start">flex-start</option>
  <option value="flex-end">flex-end</option>
  <option value="center">center</option>
  <option value="left">left</option>
  <option value="right">right</option>
  <option value="baseline">baseline</option>
  <option value="first baseline">first baseline</option>
  <option value="last baseline">last baseline</option>
  <option value="space-between" selected>space-between</option>
  <option value="space-around">space-around</option>
  <option value="space-evenly">space-evenly</option>
  <option value="stretch">stretch</option>
</select>

CSS

#container {
  display: flex;
  flex-flow: column;
}

#container > div {
  width: 100px;
  height: 100px;
  background: linear-gradient(-45deg, #788cff, #b4c8ff);
}

JavaScript

var justifyContent = document.getElementById("justifyContent");
justifyContent.addEventListener("change", function (evt) {
  document.getElementById("container").style.justifyContent =
      evt.target.value;
});