> [!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

``jsx
import { onCommitFiberRoot, traverseFiber } from 'bippy';

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

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:

`typescript
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:

- traverseRenderedFibers 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:

`typescript
interface __REACT_DEVTOOLS_GLOBAL_HOOK__ {
// list of renderers (react-dom, react-native, etc.)
renderers: Map;

// 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)_
- traverseRenderedFibers 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.

`shell
npm install bippy
`

or, use via script tag:

`html

`

> 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.

$3

a utility function that wraps the instrument function and sets the onCommitFiberRoot hook.

`typescript
import { onCommitFiberRoot } from 'bippy';

onCommitFiberRoot((root) => {
console.log('root ready to commit', root);
});
`

$3

> the underlying implementation for the onCommitFiberRoot() function. this is optional, unless you want to plug into more less common, advanced functionality.

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.

`typescript
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);
},
})
);
`

$3

returns the window.__REACT_DEVTOOLS_GLOBAL_HOOK__ object. great for advanced use cases, such as accessing or modifying the renderers property.

`typescript
import { getRDTHook } from 'bippy';

const hook = getRDTHook();
console.log(hook);
`

$3

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

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

instrument(
secure({
onCommitFiberRoot(rendererID, root) {
traverseRenderedFibers(root, (fiber) => {
console.log('fiber rendered', fiber);
});
},
})
);
`

$3

calls a callback on every fiber in the fiber tree.

`typescript
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);
});
},
})
);
`

$3

traverses the props of a fiber.

`typescript
import { traverseProps } from 'bippy';

// ...

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

$3

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

`typescript
import { traverseState } from 'bippy';

// ...

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

$3

traverses the contexts (useContext) of a fiber.

`typescript
import { traverseContexts } from 'bippy';

// ...

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

$3

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

`typescript
import { setFiberId, getFiberId } from 'bippy';

// ...

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

$3

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

`typescript
import { isHostFiber } from 'bippy';

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

$3

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).

`typescript
import { isCompositeFiber } from 'bippy';

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

$3

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

`typescript
import { getDisplayName } from 'bippy';

console.log(getDisplayName(fiber));
`

$3

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

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

const RealComponent = () => {
return

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

$3

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.

`jsx
import { getNearestHostFiber, getNearestHostFibers } from 'bippy';

// ...

function Component() {
return (
<>


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

$3

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

`typescript
[fiber, fiber.return, fiber.return.return, ...]
`

$3

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

`typescript
import { getMutatedHostFibers } from 'bippy';

console.log(getMutatedHostFibers(fiber));
`

$3

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

`typescript
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:

$3

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

`javascript
import {
instrument,
isHostFiber,
getNearestHostFiber,
traverseRenderedFibers,
} 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);
};

/**
* 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) {
/**
* traverseRenderedFibers 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 re-rendered, then didn't
* actually render, but exists in the fiber tree).
*/
traverseRenderedFibers(root, (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);
});
},
})
);
`

$3

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

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

instrument(
secure({
onCommitFiberRoot(rendererID, root) {
traverseRenderedFibers(root, (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();
});
},
})
);
`

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

pre-requisite: you should understand how react works internally. if you don't, please give this series of articles a read.

we use a pnpm monorepo, get started by running:

`shell
pnpm install


create dev builds

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

`shell
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.