From 4097e7364e3787740d038967d99c25079f65b9ad Mon Sep 17 00:00:00 2001
From: Toby Zerner <toby.zerner@gmail.com>
Date: Sat, 15 Apr 2023 17:49:43 +0930
Subject: [PATCH] feat(tabs): new `tabs` element

---
 src/index.ts       |   1 +
 src/tabs/README.md |  38 +++++++++++++++++
 src/tabs/tabs.ts   | 103 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 142 insertions(+)
 create mode 100644 src/tabs/README.md
 create mode 100644 src/tabs/tabs.ts

diff --git a/src/index.ts b/src/index.ts
index ea64c01..31dc600 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -4,5 +4,6 @@ export { default as DisclosureElement } from './disclosure/disclosure';
 export { default as MenuElement } from './menu/menu';
 export { default as ModalElement } from './modal/modal';
 export { default as PopupElement } from './popup/popup';
+export { default as TabsElement } from './tabs/tabs';
 export { default as ToolbarElement } from './toolbar/toolbar';
 export { default as TooltipElement } from './tooltip/tooltip';
diff --git a/src/tabs/README.md b/src/tabs/README.md
new file mode 100644
index 0000000..5435dec
--- /dev/null
+++ b/src/tabs/README.md
@@ -0,0 +1,38 @@
+# Tabs
+
+**A custom element for building accessible tabbed interfaces.**
+
+## Example
+
+```js
+import { TabsElement } from 'inclusive-elements';
+
+window.customElements.define('ui-tabs', TabsElement);
+```
+
+```html
+<ui-tabs>
+    <div role="tablist" aria-label="Tabs">
+        <button type="button" role="tab">Tab 1</button>
+        <button type="button" role="tab">Tab 2</button>
+        <button type="button" role="tab">Tab 3</button>
+    </div>
+    <div role="tabpanel">Tab Panel 1</div>
+    <div role="tabpanel" hidden>Tab Panel 2</div>
+    <div role="tabpanel" hidden>Tab Panel 3</div>
+</ui-tabs>
+```
+
+## Behavior
+
+-   Descendants with `role="tab"` and `role="tabpanel"` will have appropriate `id`, `aria-controls`, and `aria-labelledby` attributes generated if they are not already set.
+
+-   The active `tab` will have the `aria-selected="true"` attribute set. Inactive tabs will have their `tabindex` set to `-1` so that focus remains on the active tab.
+
+-   When focus is on the active `tab`, pressing the `Left Arrow`, `Right Arrow`, `Home`, and `End` keys can be used for navigation. If the `tablist` has `aria-orientation="vertical"`, `Down Arrow` and `Up Arrow` are used instead.
+
+-   The `tab` with focus is automatically activated, and its corresponding `tabpanel` will become visible.
+
+## Further Reading
+
+-   [ARIA Authoring Practices Guide: Tabs Pattern](https://www.w3.org/WAI/ARIA/apg/patterns/tabs/)
diff --git a/src/tabs/tabs.ts b/src/tabs/tabs.ts
new file mode 100644
index 0000000..1443504
--- /dev/null
+++ b/src/tabs/tabs.ts
@@ -0,0 +1,103 @@
+let idCounter = 0;
+
+export default class TabsElement extends HTMLElement {
+    private idPrefix = 'tabs' + ++idCounter;
+
+    public connectedCallback(): void {
+        this.tablist.addEventListener('keydown', this.onKeyDown);
+        this.tablist.addEventListener('click', this.onClick);
+
+        this.selectTab(0, false);
+
+        this.tabs.forEach((tab, i) => {
+            const panel = this.tabpanels[i];
+            const tabId = tab.getAttribute('id') || this.idPrefix + '_tab_' + i;
+            const panelId =
+                panel.getAttribute('id') || this.idPrefix + '_panel_' + i;
+
+            tab.setAttribute('id', tabId);
+            tab.setAttribute('aria-controls', panelId);
+
+            panel.setAttribute('id', panelId);
+            panel.setAttribute('aria-labelledby', tabId);
+            panel.setAttribute('tabindex', '0');
+        });
+    }
+
+    public disconnectedCallback(): void {
+        this.tablist.removeEventListener('keydown', this.onKeyDown);
+        this.tablist.removeEventListener('click', this.onClick);
+    }
+
+    private selectTab(index: number, focus = true) {
+        if (index < 0) index += this.tabs.length;
+        else if (index >= this.tabs.length) index -= this.tabs.length;
+
+        this.tabs.forEach((tab, i) => {
+            if (i === index) {
+                tab.setAttribute('aria-selected', 'true');
+                tab.removeAttribute('tabindex');
+                this.tabpanels[i].hidden = false;
+                if (focus) tab.focus();
+            } else {
+                tab.setAttribute('aria-selected', 'false');
+                tab.setAttribute('tabindex', '-1');
+                this.tabpanels[i].hidden = true;
+            }
+        });
+    }
+
+    private onKeyDown = (e: KeyboardEvent) => {
+        const target = e.target as HTMLElement;
+        const index = this.tabs.indexOf(target);
+        const vertical =
+            this.tablist.getAttribute('aria-orientation') === 'vertical';
+        let captured = false;
+
+        switch (e.key) {
+            case vertical ? 'ArrowUp' : 'ArrowLeft':
+                this.selectTab(index - 1);
+                captured = true;
+                break;
+
+            case vertical ? 'ArrowDown' : 'ArrowRight':
+                this.selectTab(index + 1);
+                captured = true;
+                break;
+
+            case 'Home':
+                this.selectTab(0);
+                captured = true;
+                break;
+
+            case 'End':
+                this.selectTab(this.tabs.length - 1);
+                captured = true;
+        }
+
+        if (captured) {
+            e.stopPropagation();
+            e.preventDefault();
+        }
+    };
+
+    private onClick = (e: MouseEvent) => {
+        const target = e.target as HTMLElement;
+        const tab = target.closest<HTMLElement>('[role=tab]');
+        if (tab) {
+            this.selectTab(this.tabs.indexOf(tab));
+        }
+    };
+
+    private get tablist(): HTMLElement {
+        return this.querySelector('[role=tablist]')!;
+    }
+
+    private get tabs(): HTMLElement[] {
+        return Array.from(this.querySelectorAll('[role=tab]'));
+    }
+
+    private get tabpanels(): HTMLElement[] {
+        return Array.from(this.querySelectorAll('[role=tabpanel]'));
+    }
+}