feat: Allow a specific Tab to be made active

packages/react/src/Tabs/Tabs.test.tsx

@@ -97,7 +97,111 @@ describe('Tabs', () => {
     // This feature has not been implemented yet
   })
 
-  it.skip('should be able to set the active initial tab', () => {
-    // This feature has not been implemented yet
+  it('should be able to set the initially active tab', () => {
+    render(
+      <Tabs activeTab={2}>
+        <Tabs.List>
+          <Tabs.Button tab={0}>Tab 1</Tabs.Button>
+          <Tabs.Button tab={1}>Tab 2</Tabs.Button>
+          <Tabs.Button tab={2}>Tab 3</Tabs.Button>
+          <Tabs.Button tab={3}>Tab 4</Tabs.Button>
+        </Tabs.List>
+        <Tabs.Panel tab={0}>Content 1</Tabs.Panel>
+        <Tabs.Panel tab={1}>Content 2</Tabs.Panel>
+        <Tabs.Panel tab={2}>Content 3</Tabs.Panel>
+        <Tabs.Panel tab={3}>Content 4</Tabs.Panel>
+      </Tabs>,
+    )
+
+    const tabOne = screen.getByRole('tab', { name: 'Tab 1' })
+    const tabThree = screen.getByRole('tab', { name: 'Tab 3' })
+
+    expect(tabOne).toHaveAttribute('aria-selected', 'false')
+    expect(tabOne).toHaveAttribute('tabindex', '-1')
+
+    expect(tabThree).toHaveAttribute('aria-selected', 'true')
+    expect(tabThree).toHaveAttribute('tabindex', '0')
+
+    expect(screen.getByRole('tabpanel')).toHaveTextContent('Content 3')
+  })
+
+  it('should set the first tab as the initially active tab with a too small argument', async () => {
+    render(
+      <Tabs activeTab={-3}>
+        <Tabs.List>
+          <Tabs.Button tab={0}>Tab 1</Tabs.Button>
+          <Tabs.Button tab={1}>Tab 2</Tabs.Button>
+          <Tabs.Button tab={2}>Tab 3</Tabs.Button>
+          <Tabs.Button tab={3}>Tab 4</Tabs.Button>
+        </Tabs.List>
+        <Tabs.Panel tab={0}>Content 1</Tabs.Panel>
+        <Tabs.Panel tab={1}>Content 2</Tabs.Panel>
+        <Tabs.Panel tab={2}>Content 3</Tabs.Panel>
+        <Tabs.Panel tab={3}>Content 4</Tabs.Panel>
+      </Tabs>,
+    )
+
+    const firstTab = screen.getByRole('tab', { name: 'Tab 1' })
+    const lastTab = screen.getByRole('tab', { name: 'Tab 4' })
+
+    expect(firstTab).toHaveAttribute('aria-selected', 'true')
+    expect(firstTab).toHaveAttribute('tabindex', '0')
+
+    expect(lastTab).toHaveAttribute('aria-selected', 'false')
+    expect(lastTab).toHaveAttribute('tabindex', '-1')
+
+    expect(screen.getByRole('tabpanel')).toHaveTextContent('Content 1')
+  })
+
+  it('should set the last tab as the initially active tab with a too large argument', async () => {
+    render(
+      <Tabs activeTab={Infinity}>
+        <Tabs.List>
+          <Tabs.Button tab={0}>Tab 1</Tabs.Button>
+          <Tabs.Button tab={1}>Tab 2</Tabs.Button>
+          <Tabs.Button tab={2}>Tab 3</Tabs.Button>
+          <Tabs.Button tab={3}>Tab 4</Tabs.Button>
+        </Tabs.List>
+        <Tabs.Panel tab={0}>Content 1</Tabs.Panel>
+        <Tabs.Panel tab={1}>Content 2</Tabs.Panel>
+        <Tabs.Panel tab={2}>Content 3</Tabs.Panel>
+        <Tabs.Panel tab={3}>Content 4</Tabs.Panel>
+      </Tabs>,
+    )
+
+    const firstTab = screen.getByRole('tab', { name: 'Tab 1' })
+    const lastTab = screen.getByRole('tab', { name: 'Tab 4' })
+
+    expect(firstTab).toHaveAttribute('aria-selected', 'true')
+    expect(firstTab).toHaveAttribute('tabindex', '0')
+
+    expect(lastTab).toHaveAttribute('aria-selected', 'false')
+    expect(lastTab).toHaveAttribute('tabindex', '-1')
+
+    expect(screen.getByRole('tabpanel')).toHaveTextContent('Content 1')
+  })
+
+  it('should set the first tab as the initially active tab with an invalid number', async () => {
+    render(
+      <Tabs activeTab={NaN}>
+        <Tabs.List>
+          <Tabs.Button tab={0}>Tab 1</Tabs.Button>
+          <Tabs.Button tab={1}>Tab 2</Tabs.Button>
+          <Tabs.Button tab={2}>Tab 3</Tabs.Button>
+          <Tabs.Button tab={3}>Tab 4</Tabs.Button>
+        </Tabs.List>
+        <Tabs.Panel tab={0}>Content 1</Tabs.Panel>
+        <Tabs.Panel tab={1}>Content 2</Tabs.Panel>
+        <Tabs.Panel tab={2}>Content 3</Tabs.Panel>
+        <Tabs.Panel tab={3}>Content 4</Tabs.Panel>
+      </Tabs>,
+    )
+
+    const firstTab = screen.getByRole('tab', { name: 'Tab 1' })
+
+    expect(firstTab).toHaveAttribute('aria-selected', 'true')
+    expect(firstTab).toHaveAttribute('tabindex', '0')
+
+    expect(screen.getByRole('tabpanel')).toHaveTextContent('Content 1')
   })
 })

packages/react/src/Tabs/Tabs.tsx

@@ -4,41 +4,64 @@
  */
 
 import clsx from 'clsx'
-import { forwardRef, useId, useImperativeHandle, useRef, useState } from 'react'
-import type { ForwardedRef, HTMLAttributes, PropsWithChildren } from 'react'
+import { forwardRef, useEffect, useId, useImperativeHandle, useMemo, useRef, useState } from 'react'
+import type { ForwardedRef, HTMLAttributes, PropsWithChildren, ReactNode } from 'react'
 import { TabsButton } from './TabsButton'
 import { TabsContext } from './TabsContext'
 import { TabsList } from './TabsList'
 import { TabsPanel } from './TabsPanel'
 import { useKeyboardFocus } from '../common/useKeyboardFocus'
 
-export type TabsProps = PropsWithChildren<HTMLAttributes<HTMLDivElement>>
+export type TabsProps = {
+  /** The number of the active tab. Corresponds to its `tab` value. */
+  activeTab?: number
+} & PropsWithChildren<HTMLAttributes<HTMLDivElement>>
 
-const TabsRoot = forwardRef(({ children, className, ...restProps }: TabsProps, ref: ForwardedRef<HTMLDivElement>) => {
-  const tabsId = useId()
-  const [activeTab, setActiveTab] = useState(0)
-  const innerRef = useRef<HTMLDivElement>(null)
+const TabsRoot = forwardRef(
+  ({ activeTab, children, className, ...restProps }: TabsProps, ref: ForwardedRef<HTMLDivElement>) => {
+    const tabsId = useId()
+    const innerRef = useRef<HTMLDivElement>(null)
+    const [activeTabId, setActiveTabId] = useState(0)
 
-  const updateTab = (tab: number) => {
-    setActiveTab(tab)
-  }
+    const allTabs = useMemo(() => {
+      if (!Array.isArray(children)) return []
+      return (children[0].props.children as ReactNode[]).map((child) => child)
+    }, [children])
 
-  // use a passed ref if it's there, otherwise use innerRef
-  useImperativeHandle(ref, () => innerRef.current as HTMLDivElement)
+    useEffect(() => {
+      if (typeof activeTab !== 'number') return
+      if (!Number.isInteger(activeTab)) return
 
-  const { keyDown } = useKeyboardFocus(innerRef, {
-    rotating: true,
-    horizontally: true,
-  })
+      if (activeTab < 0) {
+        setActiveTabId(0)
+      } else if (activeTab > allTabs.length - 1) {
+        setActiveTabId(allTabs.length - 1)
+      } else {
+        setActiveTabId(activeTab)
+      }
+    }, [activeTab, allTabs])
 
-  return (
-    <TabsContext.Provider value={{ activeTab, updateTab, tabsId }}>
-      <div {...restProps} role="tabs" ref={innerRef} onKeyDown={keyDown} className={clsx('ams-tabs', className)}>
-        {children}
-      </div>
-    </TabsContext.Provider>
-  )
-})
+    const updateTab = (tab: number) => {
+      setActiveTabId(tab)
+    }
+
+    // Use a passed ref if it's there, otherwise use innerRef
+    useImperativeHandle(ref, () => innerRef.current as HTMLDivElement)
+
+    const { keyDown } = useKeyboardFocus(innerRef, {
+      rotating: true,
+      horizontally: true,
+    })
+
+    return (
+      <TabsContext.Provider value={{ activeTab: activeTabId, updateTab, tabsId }}>
+        <div {...restProps} role="tabs" ref={innerRef} onKeyDown={keyDown} className={clsx('ams-tabs', className)}>
+          {children}
+        </div>
+      </TabsContext.Provider>
+    )
+  },
+)
 
 TabsRoot.displayName = 'Tabs'
 

packages/react/src/Tabs/TabsButton.tsx

@@ -15,23 +15,23 @@ export type TabsButtonProps = {
 
 export const TabsButton = forwardRef(
   ({ children, className, tab = 0, ...restProps }: TabsButtonProps, ref: ForwardedRef<HTMLButtonElement>) => {
-    const { activeTab, updateTab, tabsId } = useContext(TabsContext)
+    const { activeTab, tabsId, updateTab } = useContext(TabsContext)
 
     return (
       <button
         {...restProps}
-        role="tab"
-        id={`${tabsId}-tab-${tab}`}
         aria-controls={`${tabsId}-panel-${tab}`}
         aria-selected={activeTab === tab}
-        tabIndex={activeTab === tab ? 0 : -1}
-        ref={ref}
+        className={clsx('ams-tabs__button', className)}
+        id={`${tabsId}-tab-${tab}`}
         onClick={() => {
           startTransition(() => {
             updateTab(tab)
           })
         }}
-        className={clsx('ams-tabs__button', className)}
+        ref={ref}
+        role="tab"
+        tabIndex={activeTab === tab ? 0 : -1}
       >
         {children}
       </button>

storybook/src/components/Tabs/Tabs.docs.mdx

@@ -8,8 +8,18 @@ import README from "../../../../packages/css/src/components/tabs/README.md?raw";
 
 <Markdown>{README}</Markdown>
 
+## Examples
+
+### Default
+
+Each tab consists of a button and a panel.
+A `tab` prop with a corresponding value connects them.
+
 <Primary />
 
-## Tab
+### With Initial Tab
+
+The first tab is active by default.
+Another tab’s panel can be displayed initially as well.
 
-<Canvas of={TabsStories.Tab} />
+<Canvas of={TabsStories.WithInitialTab} />