bippy

Warning

⚠️⚠️⚠️ this project may break production apps and cause unexpected behavior ⚠️⚠️⚠️

this project uses react internals, which can change at any time. it is not recommended to depend on internals unless you really, really have to. by proceeding, you acknowledge the risk of breaking your own code or apps that use your code.

bippy

bippy is a toolkit to hack into react internals

by default, you cannot access react internals. bippy bypasses this by "pretending" to be react devtools, giving you access to the fiber tree and other internals.

works outside of react – no react code modification needed
utility functions that work across modern react (v17-19)
no prior react source code knowledge required

import { instrument, traverseFiber } from 'bippy';

instrument({
  onCommitFiberRoot(_, root) {
    traverseFiber(root.current, (fiber) => {
      // will print every fiber in the current React tree
      console.log('fiber:', fiber);
    });
  },
});

open live demo ↗

how it works & motivation

bippy allows you to access and use react fibers outside of react components.

a react fiber is a "unit of execution." this means react will do something based on the data in a fiber. each fiber either represents a composite (function/class component) or a host (dom element).

here is a live visualization of what the fiber tree looks like, and here is a deep dive article.

fibers are useful because they contain information about the react app (component props, state, contexts, etc.). a simplified version of a fiber looks roughly like this:

interface Fiber {
  // component type (function/class)
  type: any;

  child: Fiber | null;
  sibling: Fiber | null;

  // stateNode is the host fiber (e.g. DOM element)
  stateNode: Node | null;

  // parent fiber
  return: Fiber | null;

  // the previous or current version of the fiber
  alternate: Fiber | null;

  // saved props input
  memoizedProps: any;

  // state (useState, useReducer, useSES, etc.)
  memoizedState: any;

  // contexts (useContext)
  dependencies: Dependencies | null;

  // effects (useEffect, useLayoutEffect, etc.)
  updateQueue: any;
}

here, the child, sibling, and return properties are pointers to other fibers in the tree.

additionally, memoizedProps, memoizedState, and dependencies are the fiber's props, state, and contexts.

while all of the information is there, it's not super easy to work with, and changes frequently across different versions of react. bippy simplifies this by providing utility functions like:

createFiberVisitor to detect renders and traverseFiber to traverse the overall fiber tree
- (instead of child, sibling, and return pointers)
traverseProps, traverseState, and traverseContexts to traverse the fiber's props, state, and contexts
- (instead of memoizedProps, memoizedState, and dependencies)

however, fibers aren't directly accessible by the user. so, we have to hack our way around to accessing it.

luckily, react reads from a property in the window object: window.__REACT_DEVTOOLS_GLOBAL_HOOK__ and runs handlers on it when certain events happen. this property must exist before react's bundle is executed. this is intended for react devtools, but we can use it to our advantage.

here's what it roughly looks like:

interface __REACT_DEVTOOLS_GLOBAL_HOOK__ {
  // list of renderers (react-dom, react-native, etc.)
  renderers: Map<RendererID, reactRenderer>;

  // called when react has rendered everything for an update and the fiber tree is fully built and ready to
  // apply changes to the host tree (e.g. DOM mutations)
  onCommitFiberRoot: (
    rendererID: RendererID,
    root: FiberRoot,
    commitPriority?: number
  ) => void;

  // called when effects run
  onPostCommitFiberRoot: (rendererID: RendererID, root: FiberRoot) => void;

  // called when a specific fiber unmounts
  onCommitFiberUnmount: (rendererID: RendererID, fiber: Fiber) => void;
}

bippy works by monkey-patching window.__REACT_DEVTOOLS_GLOBAL_HOOK__ with our own custom handlers. bippy simplifies this by providing utility functions like:

instrument to safely patch window.__REACT_DEVTOOLS_GLOBAL_HOOK__
- (instead of directly mutating onCommitFiberRoot, ...)
secure to wrap your handlers in a try/catch and determine if handlers are safe to run
- (instead of rawdogging window.__REACT_DEVTOOLS_GLOBAL_HOOK__ handlers, which may crash your app)
createFiberVisitor to traverse the fiber tree and determine which fibers have actually rendered
- (instead of child, sibling, and return pointers)
traverseFiber to traverse the fiber tree, regardless of whether it has rendered
- (instead of child, sibling, and return pointers)
setFiberId / getFiberId to set and get a fiber's id
- (instead of anonymous fibers with no identity)

how to use

you can either install via a npm (recommended) or a script tag.

this package should be imported before a React app runs. this will add a special object to the global which is used by React for providing its internals to the tool for analysis (React Devtools does the same). as soon as React library is loaded and attached to the tool, bippy starts collecting data about what is going on in React's internals.

npm install bippy

or, use via script tag. must be added before any other scripts run:

<script src="https://unpkg.com/bippy"></script>

this will cause bippy to be accessible under a window.Bippy global.

next, you can use the api to get data about the fiber tree. below is a (useful) subset of the api. for the full api, read the source code.

instrument

patches window.__REACT_DEVTOOLS_GLOBAL_HOOK__ with your handlers. must be imported before react, and must be initialized to properly run any other methods.

use with the secure function to prevent uncaught errors from crashing your app.

import { instrument, secure } from 'bippy'; // must be imported BEFORE react
import * as React from 'react';

instrument(
  secure({
    onCommitFiberRoot(rendererID, root) {
      console.log('root ready to commit', root);
    },
    onPostCommitFiberRoot(rendererID, root) {
      console.log('root with effects committed', root);
    },
    onCommitFiberUnmount(rendererID, fiber) {
      console.log('fiber unmounted', fiber);
    },
  })
);

createFiberVisitor

not every fiber in the fiber tree renders. createFiberVisitor allows you to traverse the fiber tree and determine which fibers have actually rendered.

import { instrument, secure, createFiberVisitor } from 'bippy'; // must be imported BEFORE react
import * as React from 'react';

const visit = createFiberVisitor({
  onRender(fiber) {
    console.log('fiber rendered', fiber);
  },
});

instrument(
  secure({
    onCommitFiberRoot(rendererID, root) {
      visit(rendererID, root);
    },
  })
);

traverseFiber

calls a callback on every fiber in the fiber tree.

import { instrument, secure, traverseFiber } from 'bippy'; // must be imported BEFORE react
import * as React from 'react';

instrument(
  secure({
    onCommitFiberRoot(rendererID, root) {
      traverseFiber(root.current, (fiber) => {
        console.log(fiber);
      });
    },
  })
);

traverseProps

traverses the props of a fiber.

import { traverseProps } from 'bippy';

// ...

traverseProps(fiber, (propName, next, prev) => {
  console.log(propName, next, prev);
});

traverseState

traverses the state (useState, useReducer, etc.) and effects that set state of a fiber.

import { traverseState } from 'bippy';

// ...

traverseState(fiber, (next, prev) => {
  console.log(next, prev);
});

traverseEffects

traverses the effects (useEffect, useLayoutEffect, etc.) of a fiber.

import { traverseEffects } from 'bippy';

// ...

traverseEffects(fiber, (effect) => {
  console.log(effect);
});

traverseContexts

traverses the contexts (useContext) of a fiber.

import { traverseContexts } from 'bippy';

// ...

traverseContexts(fiber, (next, prev) => {
  console.log(next, prev);
});

setFiberId / getFiberId

set and get a persistent identity for a fiber. by default, fibers are anonymous and have no identity.

import { setFiberId, getFiberId } from 'bippy';

// ...

setFiberId(fiber);
console.log('unique id for fiber:', getFiberId(fiber));

isHostFiber

returns true if the fiber is a host fiber (e.g., a DOM node in react-dom).

import { isHostFiber } from 'bippy';

if (isHostFiber(fiber)) {
  console.log('fiber is a host fiber');
}

isCompositeFiber

returns true if the fiber is a composite fiber. composite fibers represent class components, function components, memoized components, and so on (anything that can actually render output).

import { isCompositeFiber } from 'bippy';

if (isCompositeFiber(fiber)) {
  console.log('fiber is a composite fiber');
}

getDisplayName

returns the display name of the fiber's component, falling back to the component's function or class name if available.

import { getDisplayName } from 'bippy';

console.log(getDisplayName(fiber));

getType

returns the underlying type (the component definition) for a given fiber. for example, this could be a function component or class component.

import { getType } from 'bippy';
import { memo } from 'react';

const RealComponent = () => {
  return <div>hello</div>;
};
const MemoizedComponent = memo(() => {
  return <div>hello</div>;
});

console.log(getType(fiberForMemoizedComponent) === RealComponent);

getNearestHostFiber / getNearestHostFibers

getNearestHostFiber returns the closest host fiber above or below a given fiber. getNearestHostFibers(fiber) returns all host fibers associated with the provided fiber and its subtree.

import { getNearestHostFiber, getNearestHostFibers } from 'bippy';

// ...

function Component() {
  return (
    <>
      <div>hello</div>
      <div>world</div>
    </>
  );
}

console.log(getNearestHostFiber(fiberForComponent)); // <div>hello</div>
console.log(getNearestHostFibers(fiberForComponent)); // [<div>hello</div>, <div>world</div>]

getTimings

returns the self and total render times for the fiber.

// timings don't exist in react production builds
if (fiber.actualDuration !== undefined) {
  const { selfTime, totalTime } = getTimings(fiber);
  console.log(selfTime, totalTime);
}

getFiberStack

returns an array representing the stack of fibers from the current fiber up to the root.

[fiber, fiber.return, fiber.return.return, ...]

getMutatedHostFibers

returns an array of all host fibers that have committed and rendered in the provided fiber's subtree.

import { getMutatedHostFibers } from 'bippy';

console.log(getMutatedHostFibers(fiber));

isValidFiber

returns true if the given object is a valid React Fiber (i.e., has a tag, stateNode, return, child, sibling, etc.).

import { isValidFiber } from 'bippy';

console.log(isValidFiber(fiber));

examples

the best way to understand bippy is to read the source code. here are some examples of how you can use it:

a mini react-scan

here's a mini toy version of react-scan that highlights renders in your app.

import {
  instrument,
  isHostFiber,
  getNearestHostFiber,
  createFiberVisitor,
} from 'bippy'; // must be imported BEFORE react

const highlightFiber = (fiber) => {
  if (!(fiber.stateNode instanceof HTMLElement)) return;
  // fiber.stateNode is a DOM element
  const rect = fiber.stateNode.getBoundingClientRect();
  const highlight = document.createElement('div');
  highlight.style.border = '1px solid red';
  highlight.style.position = 'fixed';
  highlight.style.top = `${rect.top}px`;
  highlight.style.left = `${rect.left}px`;
  highlight.style.width = `${rect.width}px`;
  highlight.style.height = `${rect.height}px`;
  highlight.style.zIndex = 999999999;
  document.documentElement.appendChild(highlight);
  setTimeout(() => {
    document.documentElement.removeChild(highlight);
  }, 100);
};

/**
 * `createFiberVisitor` traverses the fiber tree and determines which
 * fibers have actually rendered.
 *
 * A fiber tree contains many fibers that may have not rendered. this
 * can be because it bailed out (e.g. `useMemo`) or because it wasn't
 * actually rendered (if <Child> re-rendered, then <Parent> didn't
 * actually render, but exists in the fiber tree).
 */
const visit = createFiberVisitor({
  onRender(fiber) {
    /**
     * `getNearestHostFiber` is a utility function that finds the
     * nearest host fiber to a given fiber.
     *
     * a host fiber for `react-dom` is a fiber that has a DOM element
     * as its `stateNode`.
     */
    const hostFiber = getNearestHostFiber(fiber);
    highlightFiber(hostFiber);
  },
});

/**
 * `instrument` is a function that installs the react DevTools global
 * hook and allows you to set up custom handlers for react fiber events.
 */
instrument(
  /**
   * `secure` is a function that wraps your handlers in a try/catch
   * and prevents it from crashing the app. it also prevents it from
   * running on unsupported react versions and during production.
   *
   * this is not required but highly recommended to provide "safeguards"
   * in case something breaks.
   */
  secure({
    /**
     * `onCommitFiberRoot` is a handler that is called when react is
     * ready to commit a fiber root. this means that react is has
     * rendered your entire app and is ready to apply changes to
     * the host tree (e.g. via DOM mutations).
     */
    onCommitFiberRoot(rendererID, root) {
      visit(rendererID, root);
    },
  })
);

a mini why-did-you-render

here's a mini toy version of why-did-you-render that logs why components re-render.

import {
  instrument,
  isHostFiber,
  createFiberVisitor,
  isCompositeFiber,
  getDisplayName,
  traverseProps,
  traverseContexts,
  traverseState,
} from 'bippy'; // must be imported BEFORE react

const visit = createFiberVisitor({
  onRender(fiber) {
    /**
     * `isCompositeFiber` is a utility function that checks if a fiber is a composite fiber.
     * a composite fiber is a fiber that represents a function or class component.
     */
    if (!isCompositeFiber(fiber)) return;

    /**
     * `getDisplayName` is a utility function that gets the display name of a fiber.
     */
    const displayName = getDisplayName(fiber);
    if (!displayName) return;

    const changes = [];

    /**
     * `traverseProps` is a utility function that traverses the props of a fiber.
     */
    traverseProps(fiber, (propName, next, prev) => {
      if (next !== prev) {
        changes.push({
          name: `prop ${propName}`,
          prev,
          next,
        });
      }
    });

    let contextId = 0;
    /**
     * `traverseContexts` is a utility function that traverses the contexts of a fiber.
     * Contexts don't have a "name" like props, so we use an id to identify them.
     */
    traverseContexts(fiber, (next, prev) => {
      if (next !== prev) {
        changes.push({
          name: `context ${contextId}`,
          prev,
          next,
          contextId,
        });
      }
      contextId++;
    });

    let stateId = 0;
    /**
     * `traverseState` is a utility function that traverses the state of a fiber.
     *
     * State don't have a "name" like props, so we use an id to identify them.
     */
    traverseState(fiber, (value, prevValue) => {
      if (next !== prev) {
        changes.push({
          name: `state ${stateId}`,
          prev,
          next,
        });
      }
      stateId++;
    });

    console.group(
      `%c${displayName}`,
      'background: hsla(0,0%,70%,.3); border-radius:3px; padding: 0 2px;'
    );
    for (const { name, prev, next } of changes) {
      console.log(`${name}:`, prev, '!==', next);
    }
    console.groupEnd();
  },
});

instrument(
  secure({
    onCommitFiberRoot(rendererID, root) {
      visit(rendererID, root);
    },
  })
);

glossary

fiber: a "unit of execution" in react, representing a component or dom element
commit: the process of applying changes to the host tree (e.g. DOM mutations)
render: the process of building the fiber tree by executing component function/classes
host tree: the tree of UI elements that react mutates (e.g. DOM elements)
reconciler (or "renderer"): custom bindings for react, e.g. react-dom, react-native, react-three-fiber, etc to mutate the host tree
rendererID: the id of the reconciler, starting at 1 (can be from multiple reconciler instances)
root: a special FiberRoot type that contains the container fiber (the one you pass to ReactDOM.createRoot) in the current property
onCommitFiberRoot: called when react is ready to commit a fiber root
onPostCommitFiberRoot: called when react has committed a fiber root and effects have run
onCommitFiberUnmount: called when a fiber unmounts

development

we use a pnpm monorepo, get started by running:

pnpm install
# create dev builds
pnpm run dev
# run unit tests
pnpm run test

you can ad-hoc test by running pnpm run dev in the /kitchen-sink directory.

cd kitchen-sink
pnpm run dev

misc

we use this project internally in react-scan, which is deployed with proper safeguards to ensure it's only used in development or error-guarded in production.

while i maintain this specifically for react-scan, those seeking more robust solutions might consider its-fine for accessing fibers within react using hooks, or react-devtools-inline for a headful interface.

if you plan to use this project beyond experimentation, please review react-scan's source code to understand our safeguarding practices.

the original bippy character is owned and created by @dairyfreerice. this project is not related to the bippy brand, i just think the character is cute.

Name		Name	Last commit message	Last commit date
Latest commit History 156 Commits
.github		.github
.vscode		.vscode
kitchen-sink		kitchen-sink
scripts		scripts
src		src
.gitignore		.gitignore
LICENSE		LICENSE
README.md		README.md
biome.json		biome.json
package.json		package.json
pnpm-lock.yaml		pnpm-lock.yaml
pnpm-workspace.yaml		pnpm-workspace.yaml
tsconfig.json		tsconfig.json
tsup.config.ts		tsup.config.ts
vitest.config.ts		vitest.config.ts

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bippy

how it works & motivation

how to use

instrument

createFiberVisitor

traverseFiber

traverseProps

traverseState

traverseEffects

traverseContexts

setFiberId / getFiberId

isHostFiber

isCompositeFiber

getDisplayName

getType

getNearestHostFiber / getNearestHostFibers

getTimings

getFiberStack

getMutatedHostFibers

isValidFiber

examples

a mini react-scan

a mini why-did-you-render

glossary

development

misc

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bippy

how it works & motivation

how to use

instrument

createFiberVisitor

traverseFiber

traverseProps

traverseState

traverseEffects

traverseContexts

setFiberId / getFiberId

isHostFiber

isCompositeFiber

getDisplayName

getType

getNearestHostFiber / getNearestHostFibers

getTimings

getFiberStack

getMutatedHostFibers

isValidFiber

examples

a mini react-scan

a mini why-did-you-render

glossary

development

misc

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