Event delegation

Capturing and bubbling allow us to implement one of most powerful event handling patterns called event delegation.

The idea is that if we have a lot of elements handled in a similar way, then instead of assigning a handler to each of them – we put a single handler on their common ancestor.

In the handler we get event.target, see where the event actually happened and handle it.

Let’s see an example – the Ba-Gua diagram reflecting the ancient Chinese philosophy.

Here it is:

The HTML is like this:

<table>
  <tr>
    <th colspan="3"><em>Bagua</em> Chart: Direction, Element, Color, Meaning</th>
  </tr>
  <tr>
    <td>...<strong>Northwest</strong>...</td>
    <td>...</td>
    <td>...</td>
  </tr>
  <tr>...2 more lines of this kind...</tr>
  <tr>...2 more lines of this kind...</tr>
</table>

The table has 9 cells, but there could be 99 or 9999, doesn’t matter.

Our task is to highlight a cell <td> on click.

Instead of assign an onclick handler to each <td> (can be many) – we’ll setup the “catch-all” handler on <table> element.

It will use event.target to get the clicked element and highlight it.

The code:

let selectedTd;

table.onclick = function(event) {
  let target = event.target; // where was the click?

  if (target.tagName != 'TD') return; // not on TD? Then we're not interested

  highlight(target); // highlight it
};

function highlight(td) {
  if (selectedTd) { // remove the existing highlight if any
    selectedTd.classList.remove('highlight');
  }
  selectedTd = td;
  selectedTd.classList.add('highlight'); // highlight the new td
}

Such a code doesn’t care how many cells there are in the table. We can add/remove <td> dynamically at any time and the highlighting will still work.

Still, there’s a drawback.

The click may occur not on the <td>, but inside it.

In our case if we take a look inside the HTML, we can see nested tags inside <td>, like <strong>:

<td>
  <strong>Northwest</strong>
  ...
</td>

Naturally, if a click happens on that <strong> then it becomes the value of event.target.

In the handler table.onclick we should take such event.target and find out whether the click was inside <td> or not.

Here’s the improved code:

table.onclick = function(event) {
  let td = event.target.closest('td'); // (1)

  if (!td) return; // (2)

  if (!table.contains(td)) return; // (3)

  highlight(td); // (4)
};

Explanations:

1. The method elem.closest(selector) returns the nearest ancestor that matches the selector. In our case we look for <td> on the way up from the source element.
2. If event.target is not inside any <td>, then the call returns null, and we don’t have to do anything.
3. In case of nested tables, event.target may be a <td> lying outside of the current table. So we check if that’s actually our table’s <td>.
4. And, if it’s so, then highlight it.

Delegation example: actions in markup

The event delegation may be used to optimize event handling. We use a single handler for similar actions on many elements. Like we did it for highlighting <td>.

But we can also use a single handler as an entry point for many different things.

For instance, we want to make a menu with buttons “Save”, “Load”, “Search” and so on. And there’s an object with methods save, load, search….

The first idea may be to assign a separate handler to each button. But there’s a more elegant solution. We can add a handler for the whole menu and data-action attributes for buttons that has the method to call:

<button data-action="save">Click to Save</button>

The handler reads the attribute and executes the method. Take a look at the working example:

<div id="menu">
  <button data-action="save">Save</button>
  <button data-action="load">Load</button>
  <button data-action="search">Search</button>
</div>

<script>
  class Menu {
    constructor(elem) {
      this._elem = elem;
      elem.onclick = this.onClick.bind(this); // (*)
    }

    save() {
      alert('saving');
    }

    load() {
      alert('loading');
    }

    search() {
      alert('searching');
    }

    onClick(event) {
      let action = event.target.dataset.action;
      if (action) {
        this[action]();
      }
    };
  }

  new Menu(menu);
</script>

Please note that this.onClick is bound to this in (*). That’s important, because otherwise this inside it would reference the DOM element (elem), not the menu object, and this[action] would not be what we need.

So, what the delegation gives us here?

We could also use classes .action-save, .action-load, but an attribute data-action is better semantically. And we can use it in CSS rules too.

The “behavior” pattern

We can also use event delegation to add “behaviors” to elements declaratively, with special attributes and classes.

The pattern has two parts:

1. We add a special attribute to an element.
2. A document-wide handler tracks events, and if an event happens on an attributed element – performs the action.

Counter

For instance, here the attribute data-counter adds a behavior: “increase value on click” to buttons:

Counter: <input type="button" value="1" data-counter>
One more counter: <input type="button" value="2" data-counter>

<script>
  document.addEventListener('click', function(event) {

    if (event.target.dataset.counter != undefined) { // if the attribute exists...
      event.target.value++;
    }

  });
</script>

If we click a button – its value is increased. Not buttons, but the general approach is important here.

There can be as many attributes with data-counter as we want. We can add new ones to HTML at any moment. Using the event delegation we “extended” HTML, added an attribute that describes a new behavior.

Toggler

One more example. A click on an element with the attribute data-toggle-id will show/hide the element with the given id:

<button data-toggle-id="subscribe-mail">
  Show the subscription form
</button>

<form id="subscribe-mail" hidden>
  Your mail: <input type="email">
</form>

<script>
  document.addEventListener('click', function(event) {
    let id = event.target.dataset.toggleId;
    if (!id) return;

    let elem = document.getElementById(id);

    elem.hidden = !elem.hidden;
  });
</script>

Let’s note once again what we did. Now, to add toggling functionality to an element – there’s no need to know JavaScript, just use the attribute data-toggle-id.

That may become really convenient – no need to write JavaScript for every such element. Just use the behavior. The document-level handler makes it work for any element of the page.

We can combine multiple behaviors on a single element as well.

The “behavior” pattern can be an alternative of mini-fragments of JavaScript.

Summary

Event delegation is really cool! It’s one of the most helpful patterns for DOM events.

It’s often used to add same handling for many similar elements, but not only for that.

The algorithm:

1. Put a single handler on the container.
2. In the handler – check the source element event.target.
3. If the event happened inside an element that interests us, then handle the event.

Benefits:

The delegation has its limitations of course: