Ewa Gasperowicz [X](https://twitter.com/devnook) ![Rob Dodson](https://web.dev/images/authors/robdodson.jpg) Rob Dodson [X](https://twitter.com/rob_dodson) [GitHub](https://github.com/robdodson) [Homepage](https://robdodson.me) ![Surma](https://web.dev/images/authors/surma.jpg) Surma [X](https://twitter.com/dassurma) [GitHub](https://github.com/surma) [Homepage](https://surma.dev/)

<br />

## Summary

A `<howto-tooltip>` is a popup that displays information related to an element
when the element receives keyboard focus or the mouse hovers over it.
The element that triggers the tooltip references the tooltip element with
`aria-describedby`.

The element self-applies the role `tooltip` and sets `tabindex` to -1, as the
tooltip itself can never be focused.

## Reference

- [HowTo: Components on GitHub](https://github.com/GoogleChromeLabs/howto-components)
- [Tooltip pattern in ARIA Authoring Practices 1.1](https://www.w3.org/TR/wai-aria-practices-1.1/#tooltip)

## Demo

[View live demo on GitHub](https://googlechromelabs.github.io/howto-components/howto-tooltip/#demo)

## Example usage

    <div class="text">
    <label for="name">Your name:</label>
    <input id="name" aria-describedby="tp1"/>
    <howto-tooltip id="tp1">Ideally your name is Batman</howto-tooltip>
    <br>
    <label for="cheese">Favourite type of cheese: </label>
    <input id="cheese" aria-describedby="tp2"/>
    <howto-tooltip id="tp2">Help I am trapped inside a tooltip message</howto-tooltip>

## Code

    class HowtoTooltip extends HTMLElement {

The constructor does work that needs to be executed exactly once.

      constructor() {
        super();

These functions are used in a bunch of places, and always need to bind the correct this reference, so do it once.

        this._show = this._show.bind(this);
        this._hide = this._hide.bind(this);
    }

`connectedCallback()` fires when the element is inserted into the DOM. It's a good place to set the initial role, tabindex, internal state, and install event listeners.

      connectedCallback() {
        if (!this.hasAttribute('role'))
          this.setAttribute('role', 'tooltip');

        if (!this.hasAttribute('tabindex'))
          this.setAttribute('tabindex', -1);

        this._hide();

The element that triggers the tooltip references the tooltip element with `aria-describedby`.

        this._target = document.querySelector('[aria-describedby=' + this.id + ']');
        if (!this._target)
          return;

The tooltip needs to listen to focus/blur events from the target, as well as hover events over the target.

        this._target.addEventListener('focus', this._show);
        this._target.addEventListener('blur', this._hide);
        this._target.addEventListener('mouseenter', this._show);
        this._target.addEventListener('mouseleave', this._hide);
      }

`disconnectedCallback()` unregisters the event listeners that were set up in `connectedCallback()`.

      disconnectedCallback() {
        if (!this._target)
          return;

Remove the existing listeners, so that they don't trigger even though there's no tooltip to show.

        this._target.removeEventListener('focus', this._show);
        this._target.removeEventListener('blur', this._hide);
        this._target.removeEventListener('mouseenter', this._show);
        this._target.removeEventListener('mouseleave', this._hide);
        this._target = null;
      }

      _show() {
        this.hidden = false;
      }

      _hide() {
        this.hidden = true;
      }
    }

    customElements.define('howto-tooltip', HowtoTooltip);