Compound Components

Overview

Build accessible, composable components using the compound component pattern. This is how modern component libraries (Radix UI, Headless UI, React Aria) create flexible, accessible primitives that handle complex interactions.

When to Use

• Building a component library from scratch
• Creating accessible interactive components (Select, Dialog, Tabs, Accordion)
• Need flexibility in how components render
• Want to separate behavior from styling
• Components with complex state (open/closed, selected, focused)

Implementation Checklist

Copy this checklist when building a compound component:

Compound Component Build:
- [ ] Define component parts and responsibilities (Root, Trigger, Content, Item, etc.)
- [ ] Create Root with context provider (state, refs, generated IDs)
- [ ] Implement child components with ARIA roles and data-state attributes
- [ ] Add keyboard navigation (Arrow keys, Enter/Space, Escape, Tab)
- [ ] Handle focus management (initial focus, focus trap for modals, focus restoration)
- [ ] Test with keyboard-only navigation and screen reader

Quick Reference: Pattern Comparison

Pattern	Example	Flexibility	Complexity
Monolithic	<Select options={[...]} />	Low	Low
Compound	<Select.Root><Select.Trigger /><Select.Content /></Select.Root>	High	Medium
Headless	useSelect() hook	Highest	High

The Compound Pattern

// Usage example - caller controls structure
<Dialog.Root>
  <Dialog.Trigger>Open</Dialog.Trigger>
  <Dialog.Portal>
    <Dialog.Overlay />
    <Dialog.Content>
      <Dialog.Title>Settings</Dialog.Title>
      <Dialog.Description>Manage preferences</Dialog.Description>
      {/* Custom content */}
      <Dialog.Close>Done</Dialog.Close>
    </Dialog.Content>
  </Dialog.Portal>
</Dialog.Root>

---

Architecture

Component Parts

Part	Purpose	Required
Root	Provides context, manages state	Yes
Trigger	Opens/activates the component	Usually
Content	Main content area	Yes
Portal	Renders outside DOM hierarchy	For overlays
Overlay	Backdrop behind content	For modals
Title	Accessible heading	For dialogs
Description	Accessible description	Optional
Close	Closes the component	Usually
Item	Individual selectable items	For lists

Context Flow

┌─────────────────────────────────────────────────┐
│  Root (Context Provider)                         │
│  ┌───────────────────────────────────────────┐  │
│  │  • open/setOpen state                      │  │
│  │  • triggerRef                              │  │
│  │  • contentRef                              │  │
│  │  • generated IDs                           │  │
│  └───────────────────────────────────────────┘  │
│                      │                           │
│           ┌─────────┴─────────┐                 │
│           ▼                   ▼                 │
│       Trigger              Content              │
│    (reads context)     (reads context)          │
│                              │                  │
│                     ┌────────┴────────┐         │
│                     ▼        ▼        ▼         │
│                  Title   Items    Close         │
└─────────────────────────────────────────────────┘

---

Implementation: React

Select Component

// select.tsx
import * as React from 'react';
import { createContext, useContext, useId, useRef, useState } from 'react';

// Types
interface SelectContextValue {
  open: boolean;
  setOpen: (open: boolean) => void;
  value: string | undefined;
  setValue: (value: string) => void;
  triggerId: string;
  contentId: string;
  triggerRef: React.RefObject<HTMLButtonElement>;
}

interface SelectRootProps {
  children: React.ReactNode;
  value?: string;
  onValueChange?: (value: string) => void;
  defaultValue?: string;
}

// Context
const SelectContext = createContext<SelectContextValue | null>(null);

function useSelectContext() {
  const context = useContext(SelectContext);
  if (!context) {
    throw new Error('Select components must be used within Select.Root');
  }
  return context;
}

// Root - manages all state
function SelectRoot({
  children,
  value: controlledValue,
  onValueChange,
  defaultValue
}: SelectRootProps) {
  const [open, setOpen] = useState(false);
  const [internalValue, setInternalValue] = useState(defaultValue);
  const triggerRef = useRef<HTMLButtonElement>(null);

  const id = useId();
  const triggerId = `select-trigger-${id}`;
  const contentId = `select-content-${id}`;

  // Controlled vs uncontrolled
  const value = controlledValue ?? internalValue;
  const setValue = (newValue: string) => {
    setInternalValue(newValue);
    onValueChange?.(newValue);
    setOpen(false);
  };

  return (
    <SelectContext.Provider value={{
      open,
      setOpen,
      value,
      setValue,
      triggerId,
      contentId,
      triggerRef,
    }}>
      {children}
    </SelectContext.Provider>
  );
}

// Trigger - opens the select
interface SelectTriggerProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
  children: React.ReactNode;
}

function SelectTrigger({ children, className, ...props }: SelectTriggerProps) {
  const { open, setOpen, triggerId, contentId, triggerRef } = useSelectContext();

  return (
    <button
      ref={triggerRef}
      id={triggerId}
      type="button"
      role="combobox"
      aria-expanded={open}
      aria-haspopup="listbox"
      aria-controls={contentId}
      onClick={() => setOpen(!open)}
      className={className}
      {...props}
    >
      {children}
    </button>
  );
}

// Value - displays current selection
function SelectValue({ placeholder }: { placeholder?: string }) {
  const { value } = useSelectContext();
  return <span>{value || placeholder}</span>;
}

// Content - dropdown container
interface SelectContentProps {
  children: React.ReactNode;
  className?: string;
}

function SelectContent({ children, className }: SelectContentProps) {
  const { open, contentId, triggerId, triggerRef } = useSelectContext();
  const contentRef = useRef<HTMLDivElement>(null);

  // Handle keyboard navigation
  React.useEffect(() => {
    if (!open) return;

    function handleKeyDown(e: KeyboardEvent) {
      if (e.key === 'Escape') {
        triggerRef.current?.focus();
      }
    }

    document.addEventListener('keydown', handleKeyDown);
    return () => document.removeEventListener('keydown', handleKeyDown);
  }, [open, triggerRef]);

  // Handle click outside
  React.useEffect(() => {
    if (!open) return;

    function handleClickOutside(e: MouseEvent) {
      if (
        contentRef.current &&
        !contentRef.current.contains(e.target as Node) &&
        !triggerRef.current?.contains(e.target as Node)
      ) {
        // Close handled by Root
      }
    }

    document.addEventListener('mousedown', handleClickOutside);
    return () => document.removeEventListener('mousedown', handleClickOutside);
  }, [open, triggerRef]);

  if (!open) return null;

  return (
    <div
      ref={contentRef}
      id={contentId}
      role="listbox"
      aria-labelledby={triggerId}
      className={className}
    >
      {children}
    </div>
  );
}

// Item - individual option
interface SelectItemProps {
  value: string;
  children: React.ReactNode;
  className?: string;
  disabled?: boolean;
}

function SelectItem({ value, children, className, disabled }: SelectItemProps) {
  const { value: selectedValue, setValue } = useSelectContext();
  const isSelected = value === selectedValue;

  return (
    <div
      role="option"
      aria-selected={isSelected}
      aria-disabled={disabled}
      data-selected={isSelected ? '' : undefined}
      data-disabled={disabled ? '' : undefined}
      onClick={() => !disabled && setValue(value)}
      className={className}
    >
      {children}
    </div>
  );
}

// Export compound component
export const Select = {
  Root: SelectRoot,
  Trigger: SelectTrigger,
  Value: SelectValue,
  Content: SelectContent,
  Item: SelectItem,
};

Usage

import { Select } from './select';

function Example() {
  const [value, setValue] = useState('');

  return (
    <Select.Root value={value} onValueChange={setValue}>
      <Select.Trigger className="btn">
        <Select.Value placeholder="Select a fruit" />
        <ChevronDownIcon />
      </Select.Trigger>
      <Select.Content className="dropdown">
        <Select.Item value="apple">Apple</Select.Item>
        <Select.Item value="banana">Banana</Select.Item>
        <Select.Item value="cherry" disabled>Cherry (sold out)</Select.Item>
      </Select.Content>
    </Select.Root>
  );
}

---

Implementation: Dialog/Modal

// dialog.tsx
import * as React from 'react';
import { createPortal } from 'react-dom';

interface DialogContextValue {
  open: boolean;
  setOpen: (open: boolean) => void;
  titleId: string;
  descriptionId: string;
}

const DialogContext = React.createContext<DialogContextValue | null>(null);

function useDialogContext() {
  const context = React.useContext(DialogContext);
  if (!context) throw new Error('Dialog components must be within Dialog.Root');
  return context;
}

// Root
function DialogRoot({
  children,
  open: controlledOpen,
  onOpenChange,
  defaultOpen = false
}: {
  children: React.ReactNode;
  open?: boolean;
  onOpenChange?: (open: boolean) => void;
  defaultOpen?: boolean;
}) {
  const [internalOpen, setInternalOpen] = React.useState(defaultOpen);
  const open = controlledOpen ?? internalOpen;

  const setOpen = (value: boolean) => {
    setInternalOpen(value);
    onOpenChange?.(value);
  };

  const id = React.useId();

  return (
    <DialogContext.Provider value={{
      open,
      setOpen,
      titleId: `dialog-title-${id}`,
      descriptionId: `dialog-desc-${id}`,
    }}>
      {children}
    </DialogContext.Provider>
  );
}

// Trigger
function DialogTrigger({
  children,
  asChild,
  ...props
}: React.ButtonHTMLAttributes<HTMLButtonElement> & { asChild?: boolean }) {
  const { setOpen } = useDialogContext();

  if (asChild && React.isValidElement(children)) {
    return React.cloneElement(children as React.ReactElement<any>, {
      onClick: () => setOpen(true),
      ...props,
    });
  }

  return (
    <button type="button" onClick={() => setOpen(true)} {...props}>
      {children}
    </button>
  );
}

// Portal
function DialogPortal({ children }: { children: React.ReactNode }) {
  const { open } = useDialogContext();
  if (!open) return null;
  return createPortal(children, document.body);
}

// Overlay
function DialogOverlay({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) {
  const { setOpen } = useDialogContext();

  return (
    <div
      className={className}
      onClick={() => setOpen(false)}
      aria-hidden="true"
      {...props}
    />
  );
}

// Content
function DialogContent({
  children,
  className,
  ...props
}: React.HTMLAttributes<HTMLDivElement>) {
  const { setOpen, titleId, descriptionId } = useDialogContext();
  const contentRef = React.useRef<HTMLDivElement>(null);

  // Trap focus
  React.useEffect(() => {
    const focusableElements = contentRef.current?.querySelectorAll(
      'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])'
    );
    const firstElement = focusableElements?.[0] as HTMLElement;
    firstElement?.focus();
  }, []);

  // Handle escape
  React.useEffect(() => {
    function handleKeyDown(e: KeyboardEvent) {
      if (e.key === 'Escape') setOpen(false);
    }
    document.addEventListener('keydown', handleKeyDown);
    return () => document.removeEventListener('keydown', handleKeyDown);
  }, [setOpen]);

  // Lock body scroll
  React.useEffect(() => {
    document.body.style.overflow = 'hidden';
    return () => { document.body.style.overflow = ''; };
  }, []);

  return (
    <div
      ref={contentRef}
      role="dialog"
      aria-modal="true"
      aria-labelledby={titleId}
      aria-describedby={descriptionId}
      className={className}
      onClick={(e) => e.stopPropagation()}
      {...props}
    >
      {children}
    </div>
  );
}

// Title
function DialogTitle({
  children,
  className,
  ...props
}: React.HTMLAttributes<HTMLHeadingElement>) {
  const { titleId } = useDialogContext();
  return <h2 id={titleId} className={className} {...props}>{children}</h2>;
}

// Description
function DialogDescription({
  children,
  className,
  ...props
}: React.HTMLAttributes<HTMLParagraphElement>) {
  const { descriptionId } = useDialogContext();
  return <p id={descriptionId} className={className} {...props}>{children}</p>;
}

// Close
function DialogClose({
  children,
  asChild,
  ...props
}: React.ButtonHTMLAttributes<HTMLButtonElement> & { asChild?: boolean }) {
  const { setOpen } = useDialogContext();

  if (asChild && React.isValidElement(children)) {
    return React.cloneElement(children as React.ReactElement<any>, {
      onClick: () => setOpen(false),
    });
  }

  return (
    <button type="button" onClick={() => setOpen(false)} {...props}>
      {children}
    </button>
  );
}

export const Dialog = {
  Root: DialogRoot,
  Trigger: DialogTrigger,
  Portal: DialogPortal,
  Overlay: DialogOverlay,
  Content: DialogContent,
  Title: DialogTitle,
  Description: DialogDescription,
  Close: DialogClose,
};

---

Common Components

Tabs

// Tab compound structure
<Tabs.Root defaultValue="tab1">
  <Tabs.List>
    <Tabs.Trigger value="tab1">Account</Tabs.Trigger>
    <Tabs.Trigger value="tab2">Settings</Tabs.Trigger>
  </Tabs.List>
  <Tabs.Content value="tab1">Account settings...</Tabs.Content>
  <Tabs.Content value="tab2">General settings...</Tabs.Content>
</Tabs.Root>

// Context needs: activeTab, setActiveTab, orientation
// ARIA: tablist, tab, tabpanel, aria-selected, aria-controls

Accordion

// Accordion compound structure
<Accordion.Root type="single" collapsible>
  <Accordion.Item value="item1">
    <Accordion.Trigger>Section 1</Accordion.Trigger>
    <Accordion.Content>Content 1...</Accordion.Content>
  </Accordion.Item>
  <Accordion.Item value="item2">
    <Accordion.Trigger>Section 2</Accordion.Trigger>
    <Accordion.Content>Content 2...</Accordion.Content>
  </Accordion.Item>
</Accordion.Root>

// Context needs: openItems, setOpenItems, type (single/multiple)
// ARIA: region, button, aria-expanded, aria-controls

Dropdown Menu

// Menu compound structure
<Menu.Root>
  <Menu.Trigger>Options</Menu.Trigger>
  <Menu.Content>
    <Menu.Item onSelect={() => {}}>Edit</Menu.Item>
    <Menu.Item onSelect={() => {}}>Duplicate</Menu.Item>
    <Menu.Separator />
    <Menu.Item onSelect={() => {}} variant="destructive">Delete</Menu.Item>
  </Menu.Content>
</Menu.Root>

// Context needs: open, setOpen, highlightedIndex
// ARIA: menu, menuitem, aria-haspopup

---

Styling Patterns

Data Attributes for State

// Component outputs data attributes
<div
  data-state={open ? 'open' : 'closed'}
  data-selected={isSelected ? '' : undefined}
  data-disabled={disabled ? '' : undefined}
/>

/* Style with data attributes */
[data-state="open"] {
  opacity: 1;
}

[data-state="closed"] {
  opacity: 0;
}

[data-selected] {
  background: var(--color-bg-selected);
}

[data-disabled] {
  opacity: 0.5;
  pointer-events: none;
}

With Tailwind

<Select.Content className="
  absolute mt-1 w-full rounded-md bg-white shadow-lg
  data-[state=open]:animate-in data-[state=closed]:animate-out
  data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0
">

CSS Variables for Customization

// Component defines CSS variable contract
<Dialog.Content style={{
  '--dialog-width': '400px',
  '--dialog-padding': 'var(--spacing-lg)',
}}>

.dialog-content {
  width: var(--dialog-width, 500px);
  padding: var(--dialog-padding, 1.5rem);
}

---

The asChild Pattern

Allows rendering as a different element while keeping behavior:

interface AsChildProps {
  asChild?: boolean;
  children: React.ReactNode;
}

function SlotButton({ asChild, children, ...props }: AsChildProps & ButtonProps) {
  if (asChild && React.isValidElement(children)) {
    return React.cloneElement(children, {
      ...props,
      ...children.props,
    });
  }

  return <button {...props}>{children}</button>;
}

// Usage
<Dialog.Trigger asChild>
  <a href="#">Open as link</a>
</Dialog.Trigger>

---

Accessibility Checklist

Component	ARIA Roles	Keyboard
Dialog	dialog, aria-modal, aria-labelledby	Escape closes, focus trap
Select	combobox, listbox, option	Arrow keys, Enter, Escape
Tabs	tablist, tab, tabpanel	Arrow keys, Home/End
Accordion	region, button, aria-expanded	Enter/Space toggles
Menu	menu, menuitem	Arrow keys, Enter, Escape

---

Testing

// testing-library example
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';

test('Select opens on click and selects item', async () => {
  const onValueChange = jest.fn();

  render(
    <Select.Root onValueChange={onValueChange}>
      <Select.Trigger>Select fruit</Select.Trigger>
      <Select.Content>
        <Select.Item value="apple">Apple</Select.Item>
        <Select.Item value="banana">Banana</Select.Item>
      </Select.Content>
    </Select.Root>
  );

  // Open
  await userEvent.click(screen.getByRole('combobox'));
  expect(screen.getByRole('listbox')).toBeVisible();

  // Select
  await userEvent.click(screen.getByRole('option', { name: 'Apple' }));
  expect(onValueChange).toHaveBeenCalledWith('apple');
});

---

When NOT to Use Compound Components

Situation	Better Alternative
Simple button	Single component
Static content	Plain HTML
One-off component	Monolithic with props
No composition needed	Props-based API

Compound components add complexity. Use them when you need the flexibility they provide.