Focus Zone Behavior

The focusZone behavior and useFocusZone hook are used to designate a container where focus can be moved using keys other than Tab. This is useful for implementing many of the patterns described in Section 6 of the WAI-ARIA Authoring Practices document. The most common use case of this behavior is to allow arrow keys (up and down or left and right) to move focus between related elements, such as items in a menu.

At a high level, the focusZone behavior works by adjusting the tabindex attribute on focusable elements and setting up event listeners on the container that respond to the relevant key presses.

Settings for this behavior allow the user to customize several aspects of the focus zone. See FocusZoneSettings below for a detailed description of these settings.

Focusability: which elements participate in the Focus Zone?

When the focusZone behavior is established, it will discover all focusable elements within the given container and allow them to be focused with the bound keys. Focusable elements are those that either are normally focusable via the Tab key OR have a valid tabindex attribute (including "-1"). The easiest way to ensure an element participates in the focus zone is by applying the attribute tabindex="-1". If you need to prevent a focusable element from participating in the focus zone, you can provide a focusableElementFilter.

Focus order

The focusZone behavior uses the natural DOM ordering of elements to determine focus order. This means that a key that moves focus will either move to a "next" element or to a "previous" element. For example, the left arrow key and the up arrow key would both move focus to the previous element in the DOM order, while the right arrow key and the down arrow key would both move focus to the next element in the DOM order.

Focus cannot be moved beyond the last element of the container (other than using the Tab key). The focusOutBehavior option can be used to allow focus to wrap around from last to first element (or vice-versa).

For a more customized focus movement behavior, the consumer has the ability to supply a custom callback that identifies the next element to focus.

Entering the focus zone

By default, when focus enters a focus zone, the element that receives focus will be the most recently-focused element within that focus zone. If no element had previously been focused, or if that previously-focused element was removed, focus will revert to the first focusable element within the focus zone, regardless of the direction of focus movement.

Using the focusInStrategy option, you can change this behavior. Setting this option to "first" will simply cause the first focusable element in the container to be focused whenever focus enters the focus zone. Setting it to "closest" will cause either the first or last focusable element in the container to be focused depending on the direction of focus movement (for example, a shift+tab that brings focus to the container will cause the last focusable element to be focused, whereas a regular tab would cause the first focusable element to be focused). Otherwise, you may provide a callback to choose a custom element to receive initial focus. One scenario where this would be useful is if you wanted to focus an item that is "selected" in a list.

For more information on choosing the right focus in behavior, see 6.6 Keyboard Navigation Inside Components from the ARIA Authoring Practices document.

Supported keys

The focusZone behavior supports different sets of keys for moving focus around. The bindKeys option is used to set which of the following keys can be used to move focus.

DOM Focus vs. Active Descendant

The focusZone behavior supports two modes of operation: DOM Focus and Active Descendant.

• DOM Focus is the default mode and by far the most commonly needed. When a key is used to move focus, we call .focus() directly on the element to receive focus. This results in document.activeElement getting set to this new element, and it will receive any necessary styles via :focus and :focus-within.
• Active Descendant mode does not move DOM focus. Instead, focus remains on the control element, and its aria-activedescendant attribute is set to the ID of the relevant element. Because there are no :focus styles applied and no focus events fired, you can supply an onActiveDescendantChanged callback to handle any necessary styles or other logic as the active descendant changes. For more information on the Active Descendant focus pattern, see 6.6.2 Managing Focus in Composites Using aria-activedescendant from the ARIA Authoring Practices document.

Demo

Deploy Storybook to see live demos of focusZone.

Usage

const settings = {
  bindKeys: FocusKeys.ArrowVertical | FocusKeys.HomeAndEnd,
} as FocusZoneSettings
const focusZone = document.getElementById('focusZoneContainer')
focusZone(focusZone, settings)

API

The focusZone function takes the following arguments.

FocusZoneSettings interface

FocusZoneSettings is an object with the following interface. All properties are optional and have default behaviors.