How to detect CSS flex wrap event

Question

Here’s one potential solution. There might be other gotchas and edge cases you need to check for.

The basic idea is to loop through the flex items and test their top position against the previous sibling. If the top value is greater (hence further down the page) then the item has wrapped.

The function detectWrap returns an array of DOM elements that have wrapped, and could be used to style as desired.

The function could ideally be used with a ResizeObserver (while using window‘s resize event as a fallback) as a trigger to check for wrapping as the window is resized or as elements in the page change due to scripts and other user-interaction. Because the StackOverflow code window doesn’t resize it won’t work here.

Here’s a CodePen that works with a screen resize.

var detectWrap = function(className) {
  
  var wrappedItems = [];
  var prevItem = {};
  var currItem = {};
  var items = document.getElementsByClassName(className);

  for (var i = 0; i < items.length; i++) {
    currItem = items[i].getBoundingClientRect();
    if (prevItem && prevItem.top < currItem.top) {
      wrappedItems.push(items[i]);
    }
    prevItem = currItem;
  };
  
  return wrappedItems;

}

window.onload = function(event){
  var wrappedItems = detectWrap('item');
  for (var k = 0; k < wrappedItems.length; k++) {
    wrappedItems[k].className = "wrapped";
  }
};

div  {
  display: flex;
  flex-wrap: wrap;
}

div > div {
  flex-grow: 1;
  flex-shrink: 1;
  justify-content: center;
  background-color: #222222;
  padding: 20px 0px;
  color: #FFFFFF;
  font-family: Arial;
  min-width: 300px;
}

div.wrapped {
  background-color: red;
}

<div>
  <div class="item">A</div>
  <div class="item">B</div>
  <div class="item">C</div>
</div>

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