How would you build an accessible accordion component?

An accordion is a list of collapsible disclosure sections. The interview test: did you use a real <button> (so keyboard works for free), wire aria-expanded, animate height correctly, and decide single vs multi-open mode?

Component shape.

<Accordion mode="single">          // or "multi"
  <AccordionItem id="overview">
    <AccordionHeader>Overview</AccordionHeader>
    <AccordionPanel>...</AccordionPanel>
  </AccordionItem>
  <AccordionItem id="reviews">
    <AccordionHeader>Reviews</AccordionHeader>
    <AccordionPanel>...</AccordionPanel>
  </AccordionItem>
</Accordion>

Open-state model.

• Single: open: string | null. Click toggles or replaces.
• Multi: open: Set<string>. Click toggles independently.

function useAccordion(mode: "single" | "multi") {
  const [open, setOpen] = useState<Set<string>>(new Set());
  const isOpen = (id: string) => open.has(id);
  const toggle = (id: string) => setOpen(prev => {
    const next = new Set(prev);
    if (next.has(id)) { next.delete(id); }
    else {
      if (mode === "single") next.clear();
      next.add(id);
    }
    return next;
  });
  return { isOpen, toggle };
}

Markup — semantic + accessible.

<div>
  {items.map(item => (
    <div key={item.id}>
      <h3>
        <button
          aria-expanded={isOpen(item.id)}
          aria-controls={`panel-${item.id}`}
          id={`header-${item.id}`}
          onClick={() => toggle(item.id)}
        >
          {item.title}
        </button>
      </h3>
      <div
        id={`panel-${item.id}`}
        role="region"
        aria-labelledby={`header-${item.id}`}
        hidden={!isOpen(item.id)}
      >
        {item.body}
      </div>
    </div>
  ))}
</div>

The <button> inside an <h3> is the WAI-ARIA accordion pattern. The heading level depends on context (h2/h3/h4). The hidden attribute removes the panel from the a11y tree when collapsed.

Animating height — the modern trick. Animating height from auto is famously hard because the browser can't transition to/from auto. Three approaches:

1. grid-template-rows: 0fr → 1fr (modern, ~95% browser support):

.panel-wrap {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 200ms ease;
}
.panel-wrap[data-open="true"] {
  grid-template-rows: 1fr;
}
.panel-wrap > .panel-inner {
  overflow: hidden; /* clip during animation */
}

No JS measurements — the browser handles it. Cleanest solution today.

1. Measure with JS: requestAnimationFrame(() => panel.style.height = panel.scrollHeight + "px"). Robust but JS-heavy.

1. max-height trick: max-height: 1000px — works only when you know an upper bound; animation feels off because it's a transition between max values.

Keyboard model. WAI-ARIA accordion expects:

• Tab moves focus through the headers (since they're <button>, this works automatically).
• Enter / Space on a focused header toggles.
• Arrow keys to move between headers are optional per the pattern (different from Tabs, where arrow keys are required).

Common patterns to support.

• Default open: pass defaultOpen={["overview"]} (multi) or defaultOpen="overview" (single).
• Controlled mode: optional open + onOpenChange props.
• Disabled item: button disabled, panel stays closed.
• Lazy panel content: {isOpen && <Heavy/>} — saves work when panels have charts/data fetches. Trade-off: state inside panel resets on collapse.

Common mistakes.

• Using <div onClick> for the header — no keyboard support, no focus ring, no role.
• Animating height: auto → 0 directly — doesn't work.
• Forgetting aria-expanded — screen reader users don't know it's a disclosure.
• display: none panel when expanded but with CSS transition — no animation possible (display is non-animatable).

When to use <details> / <summary> instead. Native disclosure widget — keyboard, ARIA, animation (with open attribute) come for free. Use it when single-section disclosure suffices and you don't need fine UI control. Limitations: harder to animate, harder to coordinate single-mode across multiple, browser styles are inconsistent.