---

name: build-primitive description: > Guide for building foundational Prototyper UI components from scratch when no Base UI primitive exists. Use when creating components that need custom ARIA contracts, keyboard navigation, focus management, controlled/uncontrolled state, form integration, or animation lifecycle — all without an existing Base UI wrapper. tools: [Read, Glob, Grep, Write, Edit, Bash]

Prototyper UI — Building Primitive Components

How to build a 10/10 component from scratch when no Base UI primitive exists. ARIA contracts first, then keyboard navigation, then state management, then styling hooks, then form integration.

When does this apply? When you need a component that Base UI does not provide — Breadcrumb, Avatar, Badge, Card, Skeleton, Rating, Pagination, Stepper, Tag Input, File Upload, Calendar, Data Table, Carousel, Command Palette, Resizable Panels, Tree View, etc.

Philosophy: A primitive-quality component is invisible infrastructure. It handles every edge case so consumers never have to. Every interactive element must be keyboard-operable, every state must be announced to assistive technology, every animation must respect prefers-reduced-motion, and every form control must participate in native form submission.

---

1. Decision Gate

Before building from scratch, verify that no existing solution handles the hard parts.

Does a Base UI primitive exist for this component?
│
├── Yes → Use /create-component skill (wrap the primitive)
│
├── Partially (e.g., Collapsible exists but not Accordion)
│   └── Compose from existing primitives + custom logic
│       (Accordion = Collapsible + custom keyboard nav + ARIA)
│
└── No primitive exists
    │
    ├── Is it purely presentational? (Avatar, Badge, Card, Skeleton)
    │   └── Section 2 — Presentational Template (simple, no hooks)
    │
    ├── Is it a simple interactive? (Breadcrumb, Pagination, Rating)
    │   └── Section 3 — Interactive Template (ARIA + keyboard)
    │
    ├── Is it a form control? (Tag Input, File Upload, Color Picker)
    │   └── Section 4 — Form Control Template (ARIA + keyboard + form)
    │
    └── Is it a complex composite? (Calendar, Data Table, Tree View)
        └── Section 5 — Composite Template (all patterns combined)

Can a third-party library handle the hard parts?

Before building complex interactivity from scratch, check if a headless library provides the state machine:

Component	Consider	Why
Calendar / Date Picker	react-aria (date primitives)	Date math, locale, time zones
Data Table	@tanstack/react-table	Sorting, filtering, pagination, virtualization
Command Palette	cmdk	Fuzzy search, keyboard nav, scoring
Carousel	embla-carousel-react	Snap points, drag physics, loop
Resizable Panels	react-resizable-panels	Drag, constraints, persistence
Virtual Lists	@tanstack/react-virtual	Windowing, dynamic heights
DnD / Sortable	@dnd-kit/core	Drag physics, collision, accessibility

If a headless library exists, wrap it with Prototyper UI styling patterns (data-slot, cn(), tokens, animation) rather than reimplementing the state machine.

---

2. Presentational Template

For components with no interactive behavior — just semantic HTML, styling, and slots.

Used by: Avatar, Badge, Card, Skeleton, Separator, AspectRatio

"use client";

import { cva, type VariantProps } from "class-variance-authority";

import { cn } from "@/lib/utils";

// --- Variants ---

const avatarVariants = cva(
  [
    "relative inline-flex items-center justify-center overflow-hidden",
    "rounded-full bg-muted text-muted-foreground",
    "select-none",
  ],
  {
    variants: {
      size: {
        sm: "size-8 text-xs",
        default: "size-10 text-sm",
        lg: "size-12 text-base",
      },
    },
    defaultVariants: {
      size: "default",
    },
  },
);

// --- Root ---

function Avatar({
  className,
  size,
  ...props
}: React.ComponentProps<"span"> & VariantProps<typeof avatarVariants>) {
  return (
    <span
      data-slot="avatar"
      className={cn(avatarVariants({ size }), className)}
      {...props}
    />
  );
}

// --- Image (with fallback handling) ---

function AvatarImage({
  className,
  onError,
  ...props
}: React.ComponentProps<"img">) {
  return (
    <img
      data-slot="avatar-image"
      className={cn("aspect-square size-full object-cover", className)}
      onError={onError}
      {...props}
    />
  );
}

// --- Fallback ---

function AvatarFallback({ className, ...props }: React.ComponentProps<"span">) {
  return (
    <span
      data-slot="avatar-fallback"
      className={cn(
        "flex size-full items-center justify-center bg-muted font-medium",
        className,
      )}
      {...props}
    />
  );
}

export { Avatar, AvatarImage, AvatarFallback, avatarVariants };

Presentational Rules

• Use semantic HTML elements (<nav>, <ol>, <figure>, <time>, <hr>) — never a <div> when a semantic element exists
• No ARIA roles needed when HTML semantics are sufficient
• No keyboard handling needed (unless it contains interactive children)
• Props type: React.ComponentProps<"element"> — not Base UI types
• Still needs: data-slot, cn() merge, CVA if variants exist
• Still needs: "use client", named exports, no forwardRef

---

3. Interactive Template

For components with keyboard interaction and ARIA state — but not form submission.

Used by: Breadcrumb, Pagination, Rating, Stepper, Disclosure, Tree View

"use client";

import * as React from "react";
import { cva, type VariantProps } from "class-variance-authority";

import { cn } from "@/lib/utils";

// --- Controlled/Uncontrolled State Hook ---

function useControllableState<T>(
  controlledValue: T | undefined,
  defaultValue: T,
  onChange?: (value: T) => void,
): [T, (value: T | ((prev: T) => T)) => void] {
  const [internal, setInternal] = React.useState(defaultValue);
  const isControlled = controlledValue !== undefined;
  const value = isControlled ? controlledValue : internal;

  const setValue = React.useCallback(
    (next: T | ((prev: T) => T)) => {
      const nextValue =
        typeof next === "function"
          ? (next as (prev: T) => T)(isControlled ? controlledValue! : internal)
          : next;
      if (!isControlled) setInternal(nextValue);
      onChange?.(nextValue);
    },
    [isControlled, controlledValue, internal, onChange],
  );

  return [value, setValue];
}

// --- Root ---

function Rating({
  className,
  value: controlledValue,
  defaultValue = 0,
  max = 5,
  onValueChange,
  disabled = false,
  readOnly = false,
  ...props
}: Omit<React.ComponentProps<"div">, "defaultValue" | "onChange"> & {
  value?: number;
  defaultValue?: number;
  max?: number;
  onValueChange?: (value: number) => void;
  disabled?: boolean;
  readOnly?: boolean;
}) {
  const [value, setValue] = useControllableState(
    controlledValue,
    defaultValue,
    onValueChange,
  );
  const [hovered, setHovered] = React.useState<number | null>(null);

  const handleKeyDown = React.useCallback(
    (event: React.KeyboardEvent) => {
      if (disabled || readOnly) return;

      let next = value;
      switch (event.key) {
        case "ArrowRight":
        case "ArrowUp":
          next = Math.min(value + 1, max);
          break;
        case "ArrowLeft":
        case "ArrowDown":
          next = Math.max(value - 1, 0);
          break;
        case "Home":
          next = 0;
          break;
        case "End":
          next = max;
          break;
        default:
          return;
      }

      event.preventDefault();
      setValue(next);
    },
    [disabled, readOnly, value, max, setValue],
  );

  return (
    <div
      data-slot="rating"
      role="slider"
      aria-valuenow={value}
      aria-valuemin={0}
      aria-valuemax={max}
      aria-disabled={disabled || undefined}
      aria-readonly={readOnly || undefined}
      tabIndex={disabled ? -1 : 0}
      className={cn(
        "inline-flex gap-0.5",
        "focus-visible:focus-ring rounded-sm",
        disabled && "status-disabled",
        className,
      )}
      onKeyDown={handleKeyDown}
      onMouseLeave={() => setHovered(null)}
      {...props}
    >
      {Array.from({ length: max }, (_, i) => (
        <RatingItem
          key={i}
          index={i + 1}
          filled={(hovered ?? value) >= i + 1}
          onHover={() => !disabled && !readOnly && setHovered(i + 1)}
          onSelect={() => !disabled && !readOnly && setValue(i + 1)}
        />
      ))}
    </div>
  );
}

// --- Item ---

function RatingItem({
  filled,
  onHover,
  onSelect,
  index,
}: {
  filled: boolean;
  index: number;
  onHover: () => void;
  onSelect: () => void;
}) {
  return (
    <span
      data-slot="rating-item"
      data-filled={filled || undefined}
      aria-hidden="true"
      className={cn(
        "cursor-pointer text-muted transition-colors duration-150 ease-smooth",
        "motion-reduce:transition-none",
        "data-filled:text-primary",
        "hover-only:hover:scale-110",
      )}
      onMouseEnter={onHover}
      onClick={onSelect}
    >
      ★
    </span>
  );
}

export { Rating, RatingItem };

Interactive Patterns Explained

This template demonstrates all the core patterns you need for interactive components:

1. Controlled/Uncontrolled dual-mode (the useControllableState hook) 2. ARIA role + states (role="slider", aria-valuenow, aria-valuemin, aria-valuemax) 3. Keyboard navigation (onKeyDown with arrow keys, Home/End) 4. Focus management (tabIndex, focus-visible:focus-ring) 5. Data attributes for styling (data-filled on items) 6. Disabled/ReadOnly states (aria-disabled, conditional keyboard handling)

---

4. Form Control Template

For components that participate in form submission — need hidden inputs and Field compatibility.

Used by: Tag Input, File Upload, Color Picker, Custom Select

"use client";

import * as React from "react";

import { cn } from "@/lib/utils";

// --- useControllableState (same as Section 3, extract to shared hook) ---

function useControllableState<T>(
  controlledValue: T | undefined,
  defaultValue: T,
  onChange?: (value: T) => void,
): [T, (value: T | ((prev: T) => T)) => void] {
  const [internal, setInternal] = React.useState(defaultValue);
  const isControlled = controlledValue !== undefined;
  const value = isControlled ? controlledValue : internal;

  const setValue = React.useCallback(
    (next: T | ((prev: T) => T)) => {
      const nextValue =
        typeof next === "function"
          ? (next as (prev: T) => T)(isControlled ? controlledValue! : internal)
          : next;
      if (!isControlled) setInternal(nextValue);
      onChange?.(nextValue);
    },
    [isControlled, controlledValue, internal, onChange],
  );

  return [value, setValue];
}

// --- TagInput ---

function TagInput({
  className,
  name,
  value: controlledValue,
  defaultValue = [],
  onValueChange,
  disabled = false,
  required = false,
  maxTags,
  ...props
}: Omit<React.ComponentProps<"div">, "defaultValue" | "onChange"> & {
  name?: string;
  value?: string[];
  defaultValue?: string[];
  onValueChange?: (value: string[]) => void;
  disabled?: boolean;
  required?: boolean;
  maxTags?: number;
}) {
  const [tags, setTags] = useControllableState(
    controlledValue,
    defaultValue,
    onValueChange,
  );
  const [inputValue, setInputValue] = React.useState("");
  const inputRef = React.useRef<HTMLInputElement>(null);

  const addTag = React.useCallback(
    (tag: string) => {
      const trimmed = tag.trim();
      if (!trimmed || tags.includes(trimmed)) return;
      if (maxTags && tags.length >= maxTags) return;
      setTags([...tags, trimmed]);
    },
    [tags, maxTags, setTags],
  );

  const removeTag = React.useCallback(
    (index: number) => {
      setTags(tags.filter((_, i) => i !== index));
    },
    [tags, setTags],
  );

  const handleKeyDown = React.useCallback(
    (event: React.KeyboardEvent<HTMLInputElement>) => {
      if (disabled) return;

      switch (event.key) {
        case "Enter":
          event.preventDefault();
          addTag(inputValue);
          setInputValue("");
          break;
        case "Backspace":
          if (inputValue === "" && tags.length > 0) {
            removeTag(tags.length - 1);
          }
          break;
      }
    },
    [disabled, inputValue, tags, addTag, removeTag],
  );

  return (
    <div
      data-slot="tag-input"
      data-disabled={disabled || undefined}
      className={cn(
        "flex min-h-9 flex-wrap items-center gap-1.5 rounded-lg border border-field-border",
        "bg-field-background px-3 py-1.5 shadow-field",
        "transition-[color,background-color,border-color,box-shadow,opacity] duration-150 ease-smooth",
        "motion-reduce:transition-none",
        "focus-within:focus-field-ring",
        "data-disabled:status-disabled",
        className,
      )}
      onClick={() => inputRef.current?.focus()}
      {...props}
    >
      {tags.map((tag, index) => (
        <TagInputTag
          key={tag}
          onRemove={() => removeTag(index)}
          disabled={disabled}
        >
          {tag}
        </TagInputTag>
      ))}

      <input
        ref={inputRef}
        data-slot="tag-input-field"
        type="text"
        value={inputValue}
        onChange={(e) => setInputValue(e.target.value)}
        onKeyDown={handleKeyDown}
        disabled={disabled}
        className={cn(
          "min-w-[60px] flex-1 bg-transparent text-sm outline-none",
          "placeholder:text-muted-foreground",
        )}
        aria-label="Add tag"
      />

      {/* Hidden inputs for form submission */}
      {name &&
        tags.map((tag) => (
          <input key={tag} type="hidden" name={name} value={tag} />
        ))}

      {/* Validation: required + empty = invalid */}
      {name && required && tags.length === 0 && (
        <input
          type="text"
          name={`${name}_validation`}
          required
          value=""
          onChange={() => {}}
          className="sr-only"
          tabIndex={-1}
          aria-hidden="true"
        />
      )}
    </div>
  );
}

// --- Tag ---

function TagInputTag({
  children,
  onRemove,
  disabled,
  className,
  ...props
}: React.ComponentProps<"span"> & {
  onRemove: () => void;
  disabled?: boolean;
}) {
  return (
    <span
      data-slot="tag-input-tag"
      className={cn(
        "inline-flex items-center gap-1 rounded-md bg-primary-soft px-2 py-0.5 text-xs font-medium",
        "transition-[color,background-color,border-color,box-shadow,opacity] duration-150 ease-smooth",
        "motion-reduce:transition-none",
        className,
      )}
      {...props}
    >
      {children}
      {!disabled && (
        <button
          data-slot="tag-input-remove"
          type="button"
          onClick={onRemove}
          aria-label={`Remove ${children}`}
          className={cn(
            "inline-flex size-3.5 items-center justify-center rounded-sm",
            "hover-only:hover:bg-primary-soft-hover",
            "focus-visible:focus-ring",
            "no-highlight",
          )}
        >
          ×
        </button>
      )}
    </span>
  );
}

export { TagInput, TagInputTag };

Form Integration Patterns

Hidden inputs for form submission:

// Single value — one hidden input
<input type="hidden" name={name} value={value} />;

// Multiple values — one hidden input per value (same name)
{
  values.map((v) => <input key={v} type="hidden" name={name} value={v} />);
}

Validation with hidden inputs:

// Required field with custom control
{
  required && isEmpty && (
    <input
      type="text"
      required
      value=""
      onChange={() => {}}
      className="sr-only"
      tabIndex={-1}
      aria-hidden="true"
    />
  );
}

Field wrapper compatibility: The component must work when wrapped by <Field>:

<Field>
  <FieldLabel>Tags</FieldLabel>
  <TagInput name="tags" required />
  <FieldError>Please add at least one tag.</FieldError>
</Field>

For this to work:

• The visible input must connect to Field via id (Field auto-generates this)
• Error states should respond to data-invalid from the parent Field
• Add data-invalid:invalid-field-ring to the container if it has a border

---

5. Composite Template

For complex components with multiple interactive regions, roving focus, and ARIA composite patterns.

Used by: Calendar, Data Table, Tree View, Command Palette

This template shows the roving tabindex pattern — the foundation for any composite widget with arrow-key navigation.

"use client";

import * as React from "react";

import { cn } from "@/lib/utils";

// --- Roving Focus Hook ---

function useRovingFocus(
  containerRef: React.RefObject<HTMLElement | null>,
  options: {
    orientation?: "horizontal" | "vertical" | "both";
    loop?: boolean;
    selector?: string;
  } = {},
) {
  const {
    orientation = "both",
    loop = true,
    selector = "[data-roving-item]",
  } = options;

  const handleKeyDown = React.useCallback(
    (event: React.KeyboardEvent) => {
      const container = containerRef.current;
      if (!container) return;

      const items = Array.from(
        container.querySelectorAll<HTMLElement>(selector),
      ).filter((el) => !el.hasAttribute("data-disabled"));

      const currentIndex = items.indexOf(document.activeElement as HTMLElement);
      if (currentIndex === -1) return;

      let nextIndex: number | null = null;

      const isHorizontal =
        orientation === "horizontal" || orientation === "both";
      const isVertical = orientation === "vertical" || orientation === "both";

      switch (event.key) {
        case "ArrowRight":
          if (isHorizontal) nextIndex = currentIndex + 1;
          break;
        case "ArrowLeft":
          if (isHorizontal) nextIndex = currentIndex - 1;
          break;
        case "ArrowDown":
          if (isVertical) nextIndex = currentIndex + 1;
          break;
        case "ArrowUp":
          if (isVertical) nextIndex = currentIndex - 1;
          break;
        case "Home":
          nextIndex = 0;
          break;
        case "End":
          nextIndex = items.length - 1;
          break;
        default:
          return;
      }

      if (nextIndex === null) return;
      event.preventDefault();

      // Loop or clamp
      if (loop) {
        nextIndex = ((nextIndex % items.length) + items.length) % items.length;
      } else {
        nextIndex = Math.max(0, Math.min(nextIndex, items.length - 1));
      }

      // Roving tabindex: deactivate current, activate next
      items[currentIndex].setAttribute("tabindex", "-1");
      items[nextIndex].setAttribute("tabindex", "0");
      items[nextIndex].focus();
    },
    [containerRef, orientation, loop, selector],
  );

  return { handleKeyDown };
}

// --- Example: Pagination ---

function Pagination({
  className,
  page: controlledPage,
  defaultPage = 1,
  total,
  onPageChange,
  ...props
}: Omit<React.ComponentProps<"nav">, "onChange"> & {
  page?: number;
  defaultPage?: number;
  total: number;
  onPageChange?: (page: number) => void;
}) {
  const [page, setPage] = useControllableState(
    controlledPage,
    defaultPage,
    onPageChange,
  );
  const containerRef = React.useRef<HTMLElement>(null);
  const { handleKeyDown } = useRovingFocus(containerRef, {
    orientation: "horizontal",
  });

  return (
    <nav
      ref={containerRef}
      data-slot="pagination"
      role="navigation"
      aria-label="Pagination"
      className={cn("flex items-center gap-1", className)}
      onKeyDown={handleKeyDown}
      {...props}
    >
      <PaginationItem
        disabled={page <= 1}
        onClick={() => setPage(page - 1)}
        aria-label="Previous page"
        isFirst
      >
        ‹
      </PaginationItem>

      {Array.from({ length: total }, (_, i) => (
        <PaginationItem
          key={i + 1}
          active={page === i + 1}
          onClick={() => setPage(i + 1)}
          aria-label={`Page ${i + 1}`}
          aria-current={page === i + 1 ? "page" : undefined}
        >
          {i + 1}
        </PaginationItem>
      ))}

      <PaginationItem
        disabled={page >= total}
        onClick={() => setPage(page + 1)}
        aria-label="Next page"
        isLast
      >
        ›
      </PaginationItem>
    </nav>
  );
}

function PaginationItem({
  active,
  disabled,
  isFirst,
  isLast,
  className,
  ...props
}: React.ComponentProps<"button"> & {
  active?: boolean;
  isFirst?: boolean;
  isLast?: boolean;
}) {
  return (
    <button
      data-slot="pagination-item"
      data-roving-item
      data-active={active || undefined}
      data-disabled={disabled || undefined}
      type="button"
      disabled={disabled}
      tabIndex={active || (isFirst && !disabled) ? 0 : -1}
      className={cn(
        "inline-flex size-9 items-center justify-center rounded-md text-sm",
        "transition-[color,background-color,border-color,box-shadow,opacity] duration-150 ease-smooth",
        "motion-reduce:transition-none",
        "hover-only:hover:bg-accent hover-only:hover:text-accent-foreground",
        "focus-visible:focus-ring",
        "disabled:status-disabled",
        "no-highlight",
        "data-active:bg-primary data-active:text-primary-foreground",
        className,
      )}
      {...props}
    />
  );
}

export { Pagination, PaginationItem };

---

6. ARIA Role Selection

Which role for which component?

What does the component represent?
│
├── A clickable action?
│   ├── Navigates to URL → role not needed (<a> has implicit link role)
│   ├── Triggers action → role not needed (<button> has implicit button role)
│   └── Toggles state → role="switch" or aria-pressed="true/false"
│
├── A value in a range?
│   ├── User can change it → role="slider" (+ aria-valuenow/min/max)
│   ├── Display only, known range → role="meter" (+ aria-valuenow/min/max)
│   └── Display only, indeterminate → role="progressbar" (+ aria-valuenow or none)
│
├── A selection from options?
│   ├── Single select, dropdown → role="listbox" (items: role="option")
│   ├── Single select, inline → role="radiogroup" (items: role="radio")
│   ├── Multi-select → role="listbox" + aria-multiselectable="true"
│   └── Searchable select → role="combobox" (input) + role="listbox" (popup)
│
├── A group of related controls?
│   ├── Actions/buttons → role="toolbar"
│   ├── Navigation links → role not needed (<nav> has implicit navigation role)
│   ├── Tabs → role="tablist" (items: role="tab", panels: role="tabpanel")
│   └── Generic group → role="group" (+ aria-label)
│
├── A hierarchical structure?
│   ├── Tree of items → role="tree" (items: role="treeitem")
│   ├── Tabular data → role not needed (<table> has implicit table role)
│   └── Interactive grid → role="grid" (cells: role="gridcell")
│
├── An overlay?
│   ├── Blocks interaction → role="dialog" + aria-modal="true"
│   ├── Important message → role="alertdialog" + aria-modal="true"
│   └── Tooltip-like → role="tooltip"
│
└── A notification?
    ├── Urgent → role="alert" (implies aria-live="assertive")
    ├── Status update → role="status" (implies aria-live="polite")
    └── Running log → role="log" (implies aria-live="polite")

Essential ARIA State Attributes

Attribute	Values	Use on
aria-expanded	"true" / "false"	Disclosure triggers, accordion headers, tree items
aria-selected	"true" / "false"	Tabs, listbox options, tree items
aria-checked	"true" / "false" / "mixed"	Checkboxes, radio buttons, switches
aria-pressed	"true" / "false"	Toggle buttons
aria-current	"page" / "step" / "true"	Breadcrumb current item, stepper current step
aria-disabled	"true"	Disabled interactive elements
aria-hidden	"true"	Decorative elements, icons alongside text labels
aria-valuenow	number	Sliders, meters, progress bars
aria-valuemin	number	Sliders, meters, progress bars
aria-valuemax	number	Sliders, meters, progress bars
aria-valuetext	string	Sliders when number alone is insufficient (e.g., "50%")
aria-live	"polite" / "assertive"	Regions with dynamic content updates
aria-modal	"true"	Dialogs that trap focus

Naming Rules

Does the element have visible text content?
│
├── Yes, and it's a button/link/heading → Name from content (automatic)
│
├── Yes, but text is ambiguous → aria-labelledby pointing to specific text
│   (e.g., "Read more..." → aria-labelledby="read-more heading-id")
│
├── No visible text (icon-only button) → aria-label="Close"
│
├── Form control → <label> element (explicit for= or implicit wrapping)
│
├── Group of controls → <fieldset> + <legend>
│   Or role="group" + aria-label="Group name"
│
├── Navigation region → <nav aria-label="Main navigation">
│
└── Decorative → aria-hidden="true" (remove from accessibility tree)

Never include the role name in the label. "Close" not "Close button." "Navigation" not "Navigation nav."

---

7. Keyboard Navigation Patterns

Pattern A: Single Focusable Element

For components with one interactive surface (Button, Toggle, Rating, Disclosure).

Tab → Focus the element
Enter/Space → Activate
Arrow keys → Adjust value (if applicable)

function handleKeyDown(event: React.KeyboardEvent) {
  switch (event.key) {
    case "Enter":
    case " ":
      event.preventDefault();
      activate();
      break;
    case "ArrowRight":
    case "ArrowUp":
      event.preventDefault();
      increment();
      break;
    case "ArrowLeft":
    case "ArrowDown":
      event.preventDefault();
      decrement();
      break;
    case "Home":
      event.preventDefault();
      setToMinimum();
      break;
    case "End":
      event.preventDefault();
      setToMaximum();
      break;
  }
}

Pattern B: Roving Tabindex (Composite Widget)

For components with multiple focusable items where only ONE is in the tab order at a time (Toolbar, Tabs, Listbox, Radio Group, Pagination).

Tab → Focus the active item (only one in tab order)
Arrow keys → Move focus between items
Home → Focus first item
End → Focus last item
Enter/Space → Select/activate focused item

Implementation: See the useRovingFocus hook in Section 5.

Rules:

• Active item gets tabIndex={0}, all others get tabIndex={-1}
• Arrow keys move focus AND update tabIndex values
• On initial focus, land on the selected item (or first item if none selected)
• Loop: wrap around when reaching the end (optional, configurable)

Pattern C: Active Descendant (Combobox-style)

For components where focus stays on an input while a separate list shows a visual highlight (Combobox, Command Palette).

Tab → Focus the input
Arrow keys → Move highlight in the list (focus stays on input)
Enter → Select highlighted item
Escape → Close popup / clear

<input
  role="combobox"
  aria-expanded={open}
  aria-activedescendant={highlightedId}
  aria-controls={listId}
  aria-autocomplete="list"
  onKeyDown={handleKeyDown}
/>

<ul id={listId} role="listbox">
  {items.map((item) => (
    <li
      key={item.id}
      id={item.id}
      role="option"
      aria-selected={item.id === highlightedId}
      data-highlighted={item.id === highlightedId || undefined}
    >
      {item.label}
    </li>
  ))}
</ul>

Use aria-activedescendant instead of roving tabindex when:

• Focus must stay on an input (combobox, searchbox)
• The list is virtualized (not all items are in DOM)
• You need typeahead search

Pattern D: Grid Navigation

For two-dimensional navigation (Calendar, Data Table).

ArrowRight → Next cell
ArrowLeft → Previous cell
ArrowDown → Same column, next row
ArrowUp → Same column, previous row
Home → First cell in row
End → Last cell in row
Ctrl+Home → First cell in grid
Ctrl+End → Last cell in grid
PageDown → Same cell, next page/month
PageUp → Same cell, previous page/month

function handleGridKeyDown(
  event: React.KeyboardEvent,
  row: number,
  col: number,
  rows: number,
  cols: number,
) {
  let nextRow = row;
  let nextCol = col;

  switch (event.key) {
    case "ArrowRight":
      nextCol = col + 1;
      break;
    case "ArrowLeft":
      nextCol = col - 1;
      break;
    case "ArrowDown":
      nextRow = row + 1;
      break;
    case "ArrowUp":
      nextRow = row - 1;
      break;
    case "Home":
      nextCol = event.ctrlKey ? 0 : 0;
      if (event.ctrlKey) nextRow = 0;
      break;
    case "End":
      nextCol = event.ctrlKey ? cols - 1 : cols - 1;
      if (event.ctrlKey) nextRow = rows - 1;
      break;
    default:
      return;
  }

  event.preventDefault();
  // Clamp or wrap
  nextRow = Math.max(0, Math.min(nextRow, rows - 1));
  nextCol = Math.max(0, Math.min(nextCol, cols - 1));
  focusCell(nextRow, nextCol);
}

Typeahead Search

For listbox/tree/menu components — typing characters jumps to the matching item:

function useTypeahead(
  items: { label: string; element: HTMLElement }[],
  options: { timeout?: number } = {},
) {
  const { timeout = 500 } = options;
  const searchRef = React.useRef("");
  const timerRef = React.useRef<ReturnType<typeof setTimeout>>();

  const handleKeyDown = React.useCallback(
    (event: React.KeyboardEvent) => {
      // Ignore control keys
      if (
        event.key.length !== 1 ||
        event.ctrlKey ||
        event.metaKey ||
        event.altKey
      )
        return;

      clearTimeout(timerRef.current);
      searchRef.current += event.key.toLowerCase();

      const match = items.find((item) =>
        item.label.toLowerCase().startsWith(searchRef.current),
      );
      if (match) match.element.focus();

      timerRef.current = setTimeout(() => {
        searchRef.current = "";
      }, timeout);
    },
    [items, timeout],
  );

  return { handleKeyDown };
}

---

8. Focus Management

Focus Trapping (Modals)

When building a modal from scratch (no Base UI Dialog), you must trap focus:

function useFocusTrap(containerRef: React.RefObject<HTMLElement | null>) {
  React.useEffect(() => {
    const container = containerRef.current;
    if (!container) return;

    const focusableSelector = [
      "a[href]",
      "button:not(:disabled)",
      "input:not(:disabled)",
      "select:not(:disabled)",
      "textarea:not(:disabled)",
      '[tabindex]:not([tabindex="-1"])',
    ].join(", ");

    function handleKeyDown(event: KeyboardEvent) {
      if (event.key !== "Tab") return;

      const focusable = Array.from(
        container!.querySelectorAll<HTMLElement>(focusableSelector),
      );
      if (focusable.length === 0) return;

      const first = focusable[0];
      const last = focusable[focusable.length - 1];

      if (event.shiftKey && document.activeElement === first) {
        event.preventDefault();
        last.focus();
      } else if (!event.shiftKey && document.activeElement === last) {
        event.preventDefault();
        first.focus();
      }
    }

    container.addEventListener("keydown", handleKeyDown);
    return () => container.removeEventListener("keydown", handleKeyDown);
  }, [containerRef]);
}

Focus Restoration

When an overlay closes, return focus to the element that opened it:

function useFocusRestoration(isOpen: boolean) {
  const triggerRef = React.useRef<HTMLElement | null>(null);

  React.useEffect(() => {
    if (isOpen) {
      // Capture the currently focused element when opening
      triggerRef.current = document.activeElement as HTMLElement;
    } else if (triggerRef.current) {
      // Restore focus when closing
      triggerRef.current.focus();
      triggerRef.current = null;
    }
  }, [isOpen]);
}

Initial Focus

When an overlay opens, move focus to the first focusable element inside:

React.useEffect(() => {
  if (!isOpen) return;
  // Delay to ensure DOM is painted
  requestAnimationFrame(() => {
    const container = containerRef.current;
    if (!container) return;
    const autofocus = container.querySelector<HTMLElement>(
      "[autofocus], [data-autofocus]",
    );
    const firstFocusable =
      container.querySelector<HTMLElement>(focusableSelector);
    (autofocus ?? firstFocusable ?? container).focus();
  });
}, [isOpen]);

---

9. Controlled / Uncontrolled State

The useControllableState hook (shown in Sections 3 and 4) is the foundation for every interactive primitive. It supports three usage modes:

// Uncontrolled — component manages its own state
<Rating defaultValue={3} onValueChange={(v) => console.log(v)} />

// Controlled — parent manages state
<Rating value={rating} onValueChange={setRating} />

// Fully uncontrolled — no callbacks, just internal state
<Rating defaultValue={3} />

Implementation Pattern

function useControllableState<T>(
  controlledValue: T | undefined,
  defaultValue: T,
  onChange?: (value: T) => void,
): [T, (value: T | ((prev: T) => T)) => void] {
  const [internal, setInternal] = React.useState(defaultValue);
  const isControlled = controlledValue !== undefined;
  const value = isControlled ? controlledValue : internal;

  const setValue = React.useCallback(
    (next: T | ((prev: T) => T)) => {
      const nextValue =
        typeof next === "function"
          ? (next as (prev: T) => T)(isControlled ? controlledValue! : internal)
          : next;
      if (!isControlled) setInternal(nextValue);
      onChange?.(nextValue);
    },
    [isControlled, controlledValue, internal, onChange],
  );

  return [value, setValue];
}

Props Convention

Always accept three props for controllable values:

Prop	Purpose	Example
value	Controlled value (optional)	value={3}
defaultValue	Initial uncontrolled value	defaultValue={0}
onValueChange	Callback when value changes	onValueChange={(v) => setV(v)}

Name the callback on{ValueName}Change — e.g., onValueChange, onPageChange, onOpenChange, onCheckedChange.

---

10. Data Attributes as Styling Hooks

Creating Custom Data Attributes

Expose component state via data-* attributes so consumers can style with CSS:

<div
  data-slot="rating"
  data-disabled={disabled || undefined}
  data-readonly={readOnly || undefined}
  data-value={value}
>
  <span
    data-slot="rating-item"
    data-filled={filled || undefined}
    data-highlighted={highlighted || undefined}
  />
</div>

Rules

1. Boolean attributes: Set to true/presence or undefined (never "false")

   data-active={isActive || undefined}      // Correct: present or absent
   data-active={isActive ? "true" : "false"} // Wrong: always present
   

2. Enumerated attributes: Use string values

   data-state={state}           // "idle" | "loading" | "error"
   data-orientation={orientation} // "horizontal" | "vertical"
   data-side={side}             // "top" | "bottom" | "left" | "right"
   

3. Numeric attributes: Set as string (CSS can't read attribute values, but JS can)

   data-value={String(value)}
   

Tailwind Targeting

// Boolean (presence)
"data-filled:text-primary";
"data-disabled:status-disabled";
"data-highlighted:bg-accent";

// Value-based
"data-[state=loading]:animate-pulse";
"data-[orientation=vertical]:flex-col";
"data-[side=top]:slide-in-from-bottom-2";

// Group data (from parent with group class)
"group-data-disabled:opacity-50";
"group-data-checked:bg-primary";

---

11. CSS Variables for Dynamic Values

Expose computed values as CSS custom properties for consumer-side styling:

<div
  data-slot="progress"
  style={
    {
      "--progress-value": `${(value / max) * 100}%`,
      "--progress-max": String(max),
    } as React.CSSProperties
  }
>
  <div
    data-slot="progress-indicator"
    className="h-full bg-primary transition-[width] duration-300"
    style={{ width: "var(--progress-value)" }}
  />
</div>

Common CSS Variables to Expose

Variable	Component Type	Example
--{component}-value	Progress, Meter, Slider	Percentage or raw value
--{component}-max	Progress, Meter	Maximum value
--{component}-count	Tag Input, Pagination	Number of items
--available-height	Positioned overlays	Max height before overflow
--anchor-width	Positioned overlays	Trigger element width
--transform-origin	Positioned overlays	Animation origin point

Typing CSS Variables

React's CSSProperties type doesn't include custom properties. Use a type assertion:

style={{ "--progress-value": `${pct}%` } as React.CSSProperties}

---

12. Click Outside & Escape Dismissal

For overlay-like components built without Base UI's Dialog/Popover.

function useClickOutside(
  ref: React.RefObject<HTMLElement | null>,
  handler: () => void,
  enabled: boolean = true,
) {
  React.useEffect(() => {
    if (!enabled) return;

    function handlePointerDown(event: PointerEvent) {
      if (ref.current && !ref.current.contains(event.target as Node)) {
        handler();
      }
    }

    // Use pointerdown, not click — click fires after mouseup,
    // which can be on a different element than mousedown
    document.addEventListener("pointerdown", handlePointerDown);
    return () => document.removeEventListener("pointerdown", handlePointerDown);
  }, [ref, handler, enabled]);
}

function useEscapeKey(handler: () => void, enabled: boolean = true) {
  React.useEffect(() => {
    if (!enabled) return;

    function handleKeyDown(event: KeyboardEvent) {
      if (event.key === "Escape") {
        event.stopPropagation(); // Prevent parent overlays from also closing
        handler();
      }
    }

    document.addEventListener("keydown", handleKeyDown);
    return () => document.removeEventListener("keydown", handleKeyDown);
  }, [handler, enabled]);
}

Combining Both

function Disclosure({ ... }) {
  const [open, setOpen] = useControllableState(...)
  const contentRef = React.useRef<HTMLDivElement>(null)

  useClickOutside(contentRef, () => setOpen(false), open)
  useEscapeKey(() => setOpen(false), open)

  return (
    <>
      <button
        aria-expanded={open}
        aria-controls={contentId}
        onClick={() => setOpen(!open)}
      >
        {trigger}
      </button>
      {open && (
        <div ref={contentRef} id={contentId}>
          {children}
        </div>
      )}
    </>
  )
}

---

13. Animation Lifecycle (Without Base UI)

When building overlays from scratch, you need to manage the mount/unmount animation cycle yourself. Base UI provides data-starting-style / data-ending-style and getAnimations() detection — when building from scratch, use this pattern:

CSS Transition Approach (Preferred — Interruptible)

function useAnimatedPresence(isOpen: boolean) {
  const [mounted, setMounted] = React.useState(isOpen);
  const [visible, setVisible] = React.useState(isOpen);
  const ref = React.useRef<HTMLElement>(null);

  React.useEffect(() => {
    if (isOpen) {
      setMounted(true);
      // Force a layout read before setting visible (triggers transition)
      requestAnimationFrame(() => {
        requestAnimationFrame(() => setVisible(true));
      });
    } else {
      setVisible(false);
      // Wait for transition to finish before unmounting
      const el = ref.current;
      if (!el) return setMounted(false);

      function handleTransitionEnd() {
        setMounted(false);
        el?.removeEventListener("transitionend", handleTransitionEnd);
      }
      el.addEventListener("transitionend", handleTransitionEnd);
      return () =>
        el?.removeEventListener("transitionend", handleTransitionEnd);
    }
  }, [isOpen]);

  return { ref, mounted, visible };
}

Usage:

function MyOverlay({ open }: { open: boolean }) {
  const { ref, mounted, visible } = useAnimatedPresence(open);

  if (!mounted) return null;

  return (
    <div
      ref={ref}
      data-slot="my-overlay"
      className={cn(
        "transition-[opacity,transform] duration-150 ease-out-fluid",
        "motion-reduce:transition-none",
        visible ? "opacity-100 scale-100" : "opacity-0 scale-95",
      )}
    />
  );
}

CSS Keyframe Approach (For Non-Interruptible Enter/Exit)

If you prefer keyframe animations (like all current Prototyper UI overlays use), apply the animation conditionally:

<div
  className={cn(
    open && "animate-in fade-in-0 zoom-in-95 duration-150",
    !open && "animate-out fade-out-0 zoom-out-95 duration-100",
    "motion-reduce:animate-none",
  )}
  onAnimationEnd={() => {
    if (!open) setMounted(false);
  }}
/>

---

14. Ref Merging

When your component needs an internal ref AND must accept a user ref:

function useMergedRef<T>(...refs: (React.Ref<T> | undefined)[]) {
  return React.useCallback(
    (node: T | null) => {
      refs.forEach((ref) => {
        if (!ref) return;
        if (typeof ref === "function") ref(node);
        else (ref as React.MutableRefObject<T | null>).current = node;
      });
    },
    // eslint-disable-next-line react-hooks/exhaustive-deps
    refs,
  );
}

// Usage
function MyComponent({
  ref: userRef,
  ...props
}: {
  ref?: React.Ref<HTMLDivElement>;
}) {
  const internalRef = React.useRef<HTMLDivElement>(null);
  const mergedRef = useMergedRef(internalRef, userRef);

  return <div ref={mergedRef} {...props} />;
}

---

15. Putting It All Together — Checklist

Before You Start

• Confirmed no Base UI primitive exists for this component
• Checked for headless libraries that handle the state machine
• Identified the WAI-ARIA pattern (https://www.w3.org/WAI/ARIA/apg/patterns/)
• Identified the correct ARIA role from Section 6

HTML & ARIA

• Using semantic HTML elements (<nav>, <button>, <input>, <ol>) — never <div> when semantic exists
• Correct ARIA role on the root or primary interactive element
• All required ARIA states set (aria-expanded, aria-selected, aria-checked, etc.)
• Interactive elements have accessible names (visible text, aria-label, or <label>)
• Decorative elements have aria-hidden="true"
• Live regions (aria-live) for dynamic content updates
• aria-disabled (not just HTML disabled) for custom controls

Keyboard

• All functionality reachable via keyboard
• Correct keyboard pattern (single focus, roving tabindex, or active descendant)
• Arrow keys for navigation within composites
• Home/End for first/last item
• Enter/Space for activation
• Escape for dismissal (overlays)
• Tab order makes sense (only one tab stop per composite widget)
• event.preventDefault() on handled keys (prevent scroll on arrows)

Focus

• focus-visible:focus-ring on focusable elements
• Focus trapped in modals (useFocusTrap)
• Focus restored when overlays close (useFocusRestoration)
• Initial focus set when overlays open
• Disabled items are either unfocusable or focusable-but-inert (depends on pattern)

State

• Controlled/uncontrolled via useControllableState
• Props follow the value / defaultValue / onValueChange convention
• State exposed via data attributes for CSS targeting
• CSS variables for dynamic computed values

Styling

• data-slot on every rendered element
• cn() merging with user className last
• Semantic tokens (no raw colors, no dark: utilities)
• hover-only:hover: prefix on all hover states
• motion-safe:active:scale-[0.97] for press feedback
• motion-reduce:transition-none on all transitions
• motion-reduce:animate-none on all animations
• no-highlight on interactive elements
• disabled:status-disabled on disableable elements

Form (if applicable)

• Hidden <input> for form submission with name prop
• required validation via hidden required input or constraint API
• Works when wrapped by <Field> component
• Error state responds to data-invalid from Field

Structure

• "use client" as line 1
• Import order: react → cva → icons → cn → local
• Named exports only, single export { } block
• No React.forwardRef (handle refs via prop or useMergedRef)
• No export default

---

16. Cross-References

• Wrapping an existing Base UI primitive: Invoke /create-component skill
• Animation patterns and exact class strings: Invoke /animate-ui skill
• Design tokens, component selection, and usage patterns: Invoke /prototyper-ui skill
• WAI-ARIA Authoring Practices: https://www.w3.org/WAI/ARIA/apg/patterns/
• Base UI primitives: https://base-ui.com/react/components/{name}