Render props

Three props replace the three pieces of a row: renderItem for the body, renderCheckbox for the box, renderExpander for the folder toggle. Each one is called with a single object, not with positional arguments.

renderItem?:      (props: TreeItemRenderProps) => ReactNode;
renderCheckbox?:  (props: TreeCheckboxRenderProps) => ReactNode;
renderExpander?:  (props: TreeExpanderRenderProps) => ReactNode;

Everything else about the row — the click handling, the keyboard, the ARIA, the absolute positioning the virtualizer needs — stays with the library. You are replacing the contents of a row, not the row.

What does the row already handle?

Do not re-implement any of this inside a render prop:

Click to act. Clicking anywhere on a folder row toggles its expansion; clicking anywhere on a leaf row toggles its checkbox. Clicking the checkbox area toggles the checkbox on any row, folder included.
Keyboard. ArrowUp / ArrowDown / ArrowLeft / ArrowRight, Home, End, Space, Enter, * to expand everything, Ctrl/Cmd+A to select or clear everything, and type-ahead. See Accessibility.
ARIA. The row carries role="treeitem", aria-level, aria-posinset, aria-setsize, aria-expanded and aria-checked ("true", "false" or "mixed").
Indentation. paddingInlineStart is computed from level and the indent prop. Leaves get one extra step of indent so their labels line up with folder labels.
Positioning and height. The row is absolutely positioned and its height comes from estimateSize. Both are inline styles, so a plain stylesheet rule loses to them — change estimateSize, not height in a stylesheet.
Data attributes. data-state, data-level, data-expanded, data-leaf and data-active are on the row element, outside your render prop. See Styling.

What is a11yProps and why must I spread it?

renderCheckbox and renderExpander receive an a11yProps object, and it must be spread onto the control you return:

a11yProps: { "aria-hidden": true; tabIndex: -1 }

The row is the interactive element. It carries role="treeitem" and aria-checked, and the tree container is the single tab stop for the whole tree. Your checkbox and your expander are decoration on top of that. If you leave a11yProps off:

A screen reader announces every row twice — once as a tree item, once as your checkbox or button.
Tab walks through two extra controls per row. On a 100,000-row tree that is a keyboard trap with no bottom.
The reported checked state can disagree with aria-checked on the row.

tsx

// Wrong — this checkbox is announced separately and lands in the tab order.
renderCheckbox={({ checkedState }) => <MyBox state={checkedState} />}

// Right — the control is decorative; the row speaks for it.
renderCheckbox={({ a11yProps, checkedState }) => <MyBox {...a11yProps} state={checkedState} />}

renderItem

Replaces the row body. Defaults to item.label.

Field	Type	Meaning
`checkedState`	`CheckedState`	`"checked"`, `"unchecked"` or `"indeterminate"`.
`id`	`string`	The node ID.
`isActive`	`boolean`	Whether this is the keyboard-active row.
`isExpanded`	`boolean`	Folder open state. Always `false` for leaves.
`isFolder`	`boolean`	Whether the node has children.
`item`	`TreeItem`	The node, including your `item.data` blob.
`level`	`number`	Depth below the root. Top-level rows are `0`.

A two-line row with an icon and a secondary line. The extra line needs vertical room, so estimateSize goes up with it:

tsx

"use client";

import {
  CheckedState,
  Tree,
  type TreeDefinition,
  type TreeItemRenderProps,
} from "react-virtual-checkbox-tree";

const fileTree: TreeDefinition = {
  __root__: { id: "__root__", label: "root", children: ["docs", "src"] },
  docs: { id: "docs", label: "docs", children: ["readme", "guide"] },
  readme: { id: "readme", label: "README.md", data: { size: "4.1 kB" } },
  guide: { id: "guide", label: "guide.md", data: { size: "12.4 kB" } },
  src: { id: "src", label: "src", children: ["engine", "ui"] },
  engine: { id: "engine", label: "engine.ts", data: { size: "31.2 kB" } },
  ui: { id: "ui", label: "ui", children: ["tree", "row"] },
  tree: { id: "tree", label: "tree.tsx", data: { size: "18.7 kB" } },
  row: { id: "row", label: "row.tsx", data: { size: "3.8 kB" } },
};

// Defined outside the component: a fresh function identity on every render
// would re-render every mounted row.
function Row({ checkedState, isFolder, item }: TreeItemRenderProps) {
  const size = typeof item.data?.size === "string" ? item.data.size : null;

  return (
    <span style={{ alignItems: "center", display: "flex", gap: 8 }}>
      <span aria-hidden style={{ opacity: 0.7 }}>{isFolder ? "▸" : "▪"}</span>
      <span style={{ display: "flex", flexDirection: "column", lineHeight: 1.25 }}>
        <span
          style={{
            color: checkedState === CheckedState.Unchecked ? "#8a8a8a" : "#fff",
            fontSize: 13,
          }}
        >
          {item.label}
        </span>
        <span style={{ fontSize: 10.5, opacity: 0.6 }}>
          {isFolder ? `${item.children?.length ?? 0} items` : (size ?? "—")}
        </span>
      </span>
    </span>
  );
}

export default function App() {
  return (
    <Tree
      aria-label="Project files"
      data={fileTree}
      estimateSize={44}
      height={320}
      renderItem={Row}
    />
  );
}

How do I use my own expander?

renderExpander is called only for folders. Leaves render nothing in that slot — there is no placeholder element to style away, and the leaf's extra indent is what keeps the labels aligned.

Field	Type	Meaning
`a11yProps`	`{ "aria-hidden": true; tabIndex: -1 }`	Spread onto your control.
`id`	`string`	The node ID.
`isExpanded`	`boolean`	Folder open state.
`isFolder`	`boolean`	Always `true` here.
`item`	`TreeItem`	The node.
`level`	`number`	Depth below the root.
`onToggle`	`() => void`	Opens or closes this folder.

tsx

"use client";

import {
  Tree,
  type TreeDefinition,
  type TreeExpanderRenderProps,
} from "react-virtual-checkbox-tree";

const fileTree: TreeDefinition = {
  __root__: { id: "__root__", label: "root", children: ["docs", "src"] },
  docs: { id: "docs", label: "docs", children: ["readme", "guide"] },
  readme: { id: "readme", label: "README.md" },
  guide: { id: "guide", label: "guide.md" },
  src: { id: "src", label: "src", children: ["engine", "ui"] },
  engine: { id: "engine", label: "engine.ts" },
  ui: { id: "ui", label: "ui", children: ["tree", "row"] },
  tree: { id: "tree", label: "tree.tsx" },
  row: { id: "row", label: "row.tsx" },
};

function Chevron({ a11yProps, isExpanded, onToggle }: TreeExpanderRenderProps) {
  return (
    <button
      {...a11yProps}
      onClick={(event) => {
        event.stopPropagation(); // the row would otherwise toggle it back
        onToggle();
      }}
      style={{
        alignItems: "center",
        background: "none",
        border: 0,
        cursor: "pointer",
        display: "inline-flex",
        height: 16,
        justifyContent: "center",
        padding: 0,
        transform: isExpanded ? "rotate(90deg)" : "none",
        transition: "transform 150ms ease",
        width: 16,
      }}
      type="button"
    >
      <svg fill="none" height="12" viewBox="0 0 12 12" width="12">
        <path d="M4.5 2.5L8 6l-3.5 3.5" stroke="currentColor" strokeWidth="1.5" />
      </svg>
    </button>
  );
}

export default function App() {
  return (
    <Tree aria-label="Project files" data={fileTree} height={320} renderExpander={Chevron} />
  );
}

How do I use my own checkbox?

renderCheckbox is called for every row, folders included — a folder's box is what shows the indeterminate state.

Field	Type	Meaning
`a11yProps`	`{ "aria-hidden": true; tabIndex: -1 }`	Spread onto your control.
`checkedState`	`CheckedState`	`"checked"`, `"unchecked"` or `"indeterminate"`.
`id`	`string`	The node ID.
`isActive`	`boolean`	Whether this is the keyboard-active row.
`isExpanded`	`boolean`	Folder open state.
`isFolder`	`boolean`	Whether the node has children.
`item`	`TreeItem`	The node.
`level`	`number`	Depth below the root.
`onChange`	`(nextChecked: boolean) => void`	Writes the next checked value.

Unlike the expander, the checkbox slot is wrapped by the library in a span that already toggles the node and already calls stopPropagation(). So a purely visual checkbox — a span with a border — needs no handler at all:

tsx

"use client";

import {
  CheckedState,
  Tree,
  type TreeCheckboxRenderProps,
  type TreeDefinition,
} from "react-virtual-checkbox-tree";

const fileTree: TreeDefinition = {
  __root__: { id: "__root__", label: "root", children: ["docs", "src"] },
  docs: { id: "docs", label: "docs", children: ["readme", "guide"] },
  readme: { id: "readme", label: "README.md" },
  guide: { id: "guide", label: "guide.md" },
  src: { id: "src", label: "src", children: ["engine", "ui"] },
  engine: { id: "engine", label: "engine.ts" },
  ui: { id: "ui", label: "ui", children: ["tree", "row"] },
  tree: { id: "tree", label: "tree.tsx" },
  row: { id: "row", label: "row.tsx" },
};

function Box({ a11yProps, checkedState }: TreeCheckboxRenderProps) {
  const checked = checkedState === CheckedState.Checked;
  const mixed = checkedState === CheckedState.Indeterminate;

  return (
    <span
      {...a11yProps}
      style={{
        alignItems: "center",
        background: checked || mixed ? "#3b82f6" : "transparent",
        border: `1px solid ${checked || mixed ? "#3b82f6" : "#555"}`,
        borderRadius: 4,
        color: "#fff",
        display: "inline-flex",
        flexShrink: 0,
        fontSize: 10,
        height: 16,
        justifyContent: "center",
        width: 16,
      }}
    >
      {checked ? "✓" : mixed ? "–" : null}
    </span>
  );
}

export default function App() {
  return <Tree aria-label="Project files" data={fileTree} height={320} renderCheckbox={Box} />;
}

When do I need onChange?

Only when your control swallows the click. A component that calls stopPropagation() internally — many design-system checkboxes do — never lets the event reach the library's wrapper, so nothing toggles unless you call onChange yourself:

tsx

renderCheckbox={({ a11yProps, checkedState, onChange }) => (
  <SomeDesignSystemCheckbox
    {...a11yProps}
    checked={checkedState === CheckedState.Indeterminate ? "indeterminate" : checkedState === CheckedState.Checked}
    onCheckedChange={(next) => onChange(next === true)}
  />
)}

Wiring onChange when it was not strictly necessary is harmless: both paths compute the same next value from the same pre-click state, so the second write is a no-op rather than a second toggle. There is a full shadcn/ui recipe in Styling.

Can I put a button inside a row?

Yes, with two rules. The row's click handler will fire for any click that reaches it, so your control must stop the event; and the tree is a single tab stop, so your control must not join the tab order.

tsx

renderItem={({ id, item }) => (
  <span style={{ alignItems: "center", display: "flex", gap: 8 }}>
    <span>{item.label}</span>
    <button
      aria-hidden
      onClick={(event) => {
        event.stopPropagation(); // do not toggle the row as well
        preview(id);
      }}
      tabIndex={-1}
      type="button"
    >
      preview
    </button>
  </span>
)}

Do render props hurt performance?

Only if you define them inline. Rows are memoized on their props, and the render props are props, so a new function identity on every parent render re-renders every mounted row. That is roughly 15–30 rows, not 100,000 — survivable, but free to avoid:

tsx

// Re-renders every mounted row whenever the parent renders.
<Tree data={data} renderItem={({ item }) => <b>{item.label}</b>} />

// Stable identity: define it at module scope, or wrap it in useCallback.
const Row = ({ item }: TreeItemRenderProps) => <b>{item.label}</b>;
<Tree data={data} renderItem={Row} />

All three together

checked leavesengine

The demo above is a 44px row with a chevron expander, a custom box, an icon and a secondary line — the fileTree from every other page, rendered three ways at once.

tsx

"use client";

import { useState } from "react";
import {
  CheckedState,
  Tree,
  type TreeCheckboxRenderProps,
  type TreeDefinition,
  type TreeExpanderRenderProps,
  type TreeItemRenderProps,
} from "react-virtual-checkbox-tree";

const fileTree: TreeDefinition = {
  __root__: { id: "__root__", label: "root", children: ["docs", "src"] },
  docs: { id: "docs", label: "docs", children: ["readme", "guide"] },
  readme: { id: "readme", label: "README.md" },
  guide: { id: "guide", label: "guide.md" },
  src: { id: "src", label: "src", children: ["engine", "ui"] },
  engine: { id: "engine", label: "engine.ts" },
  ui: { id: "ui", label: "ui", children: ["tree", "row"] },
  tree: { id: "tree", label: "tree.tsx" },
  row: { id: "row", label: "row.tsx" },
};

const EXPANDED = ["docs", "src", "ui"];

function Chevron({ a11yProps, isExpanded, onToggle }: TreeExpanderRenderProps) {
  return (
    <button
      {...a11yProps}
      onClick={(event) => {
        event.stopPropagation();
        onToggle();
      }}
      style={{
        background: "none",
        border: 0,
        cursor: "pointer",
        padding: 0,
        transform: isExpanded ? "rotate(90deg)" : "none",
        width: 16,
      }}
      type="button"
    >
      ›
    </button>
  );
}

function Box({ a11yProps, checkedState }: TreeCheckboxRenderProps) {
  const checked = checkedState === CheckedState.Checked;
  const mixed = checkedState === CheckedState.Indeterminate;
  return (
    <span
      {...a11yProps}
      style={{
        alignItems: "center",
        background: checked || mixed ? "#3b82f6" : "transparent",
        border: `1px solid ${checked || mixed ? "#3b82f6" : "#555"}`,
        borderRadius: 4,
        display: "inline-flex",
        height: 16,
        justifyContent: "center",
        width: 16,
      }}
    >
      {checked ? "✓" : mixed ? "–" : null}
    </span>
  );
}

function Row({ isFolder, item }: TreeItemRenderProps) {
  return (
    <span style={{ alignItems: "center", display: "flex", gap: 8 }}>
      <span aria-hidden>{isFolder ? "📁" : "📄"}</span>
      <span>{item.label}</span>
    </span>
  );
}

export default function App() {
  const [checked, setChecked] = useState<string[]>(["engine"]);

  return (
    <Tree
      aria-label="Project files"
      checkedItems={checked}
      data={fileTree}
      estimateSize={44}
      expandedItems={EXPANDED}
      height={280}
      indent={22}
      onCheck={setChecked}
      renderCheckbox={Box}
      renderExpander={Chevron}
      renderItem={Row}
    />
  );
}

Styling — the data attributes, a Tailwind skin, and the shadcn/ui checkbox.
Imperative API — driving the tree from a toolbar.
Types — the full type definitions for all three prop objects.