This section will give you an overview of the React codebase organization, its conventions, and the implementation.

If you want to contribute to React we hope that this guide will help you feel more comfortable making changes.

We don’t necessarily recommend any of these conventions in React apps. Many of them exist for historical reasons and might change with time.

Top-Level Folders

After cloning the React repository, you will see a few top-level folders in it:

• packages contains metadata (such as package.json ) and the source code ( src subdirectory) for all packages in the React repository. If your change is related to the code, the src subdirectory of each package is where you’ll spend most of your time.


• fixtures contains a few small React test applications for contributors.


• build is the build output of React. It is not in the repository but it will appear in your React clone after you build it for the first time.

The documentation is hosted in a separate repository from React.

There are a few other top-level folders but they are mostly used for the tooling and you likely won’t ever encounter them when contributing.

Colocated Tests

We don’t have a top-level directory for unit tests. Instead, we put them into a directory called __tests__ relative to the files that they test.

For example, a test for setInnerHTML.js is located in __tests__/setInnerHTML-test.js right next to it.

Warnings and Invariants

The React codebase uses console.error to display warnings:

if (__DEV__) {
  console.error('Something is wrong.');
}

Warnings are only enabled in development. In production, they are completely stripped out. If you need to forbid some code path from executing, use invariant module instead:

var invariant = require('invariant');

invariant(
  2 + 2 === 4,
  'You shall not pass!'
);

The invariant is thrown when the invariant condition is false .

“Invariant” is just a way of saying “this condition always holds true”. You can think about it as making an assertion.

It is important to keep development and production behavior similar, so invariant throws both in development and in production. The error messages are automatically replaced with error codes in production to avoid negatively affecting the byte size.

Development and Production

You can use __DEV__ pseudo-global variable in the codebase to guard development-only blocks of code.

It is inlined during the compile step, and turns into process.env.NODE_ENV !== 'production' checks in the CommonJS builds.

For standalone builds, it becomes true in the unminified build, and gets completely stripped out with the if blocks it guards in the minified build.

if (__DEV__) {
  // This code will only run in development.
}

Flow

We recently started introducing Flow checks to the codebase. Files marked with the @flow annotation in the license header comment are being typechecked.

We accept pull requests adding Flow annotations to existing code. Flow annotations look like this:

ReactRef.detachRefs = function(
  instance: ReactInstance,
  element: ReactElement | string | number | null | false,
): void {
  // ...
}

When possible, new code should use Flow annotations.
You can run yarn flow locally to check your code with Flow.

Multiple Packages

React is a monorepo. Its repository contains multiple separate packages so that their changes can be coordinated together, and issues live in one place.

React Core

The “core” of React includes all the top-level React APIs, for example:

• React.createElement()


• React.Component


• React.Children

React core only includes the APIs necessary to define components. It does not include the reconciliation algorithm or any platform-specific code. It is used both by React DOM and React Native components.

The code for React core is located in packages/react in the source tree. It is available on npm as the react package. The corresponding standalone browser build is called react.js , and it exports a global called React .

Renderers

React was originally created for the DOM but it was later adapted to also support native platforms with React Native. This introduced the concept of “renderers” to React internals.

Renderers manage how a React tree turns into the underlying platform calls.

Renderers are also located in packages/ :

• React DOM Renderer renders React components to the DOM. It implements top-level ReactDOM APIs and is available as react-dom npm package. It can also be used as standalone browser bundle called react-dom.js that exports a ReactDOM global.


• React Native Renderer renders React components to native views. It is used internally by React Native.


• React Test Renderer renders React components to JSON trees. It is used by the Snapshot Testing feature of Jest and is available as react-test-renderer npm package.

The only other officially supported renderer is react-art . It used to be in a separate GitHub repository but we moved it into the main source tree for now.

Note:

Technically the react-native-renderer is a very thin layer that teaches React to interact with React Native implementation. The real platform-specific code managing the native views lives in the React Native repository together with its components.

Reconcilers

Even vastly different renderers like React DOM and React Native need to share a lot of logic. In particular, the reconciliation algorithm should be as similar as possible so that declarative rendering, custom components, state, lifecycle methods, and refs work consistently across platforms.

To solve this, different renderers share some code between them. We call this part of React a “reconciler”. When an update such as setState() is scheduled, the reconciler calls render() on components in the tree and mounts, updates, or unmounts them.

Reconcilers are not packaged separately because they currently have no public API. Instead, they are exclusively used by renderers such as React DOM and React Native.

Stack Reconciler

The “stack” reconciler is the implementation powering React 15 and earlier. We have since stopped using it, but it is documented in detail in the next section.

Fiber Reconciler

The “fiber” reconciler is a new effort aiming to resolve the problems inherent in the stack reconciler and fix a few long-standing issues. It has been the default reconciler since React 16.

Its main goals are:

• Ability to split interruptible work in chunks.


• Ability to prioritize, rebase and reuse work in progress.


• Ability to yield back and forth between parents and children to support layout in React.


• Ability to return multiple elements from render() .


• Better support for error boundaries.

You can read more about React Fiber Architecture here and here. While it has shipped with React 16, the async features are not enabled by default yet.

Its source code is located in packages/react-reconciler .

Event System

React implements a layer over native events to smooth out cross-browser differences. Its source code is located in packages/react-dom/src/events .

What Next?

Read the next section to learn about the pre-React 16 implementation of reconciler in more detail. We haven’t documented the internals of the new reconciler yet.

This section is a collection of implementation notes for the stack reconciler.

It is very technical and assumes a strong understanding of React public API as well as how it’s divided into core, renderers, and the reconciler. If you’re not very familiar with the React codebase, read the codebase overview first.

It also assumes an understanding of the differences between React components, their instances, and elements.

The stack reconciler was used in React 15 and earlier. It is located at src/renderers/shared/stack/reconciler.

Video: Building React from Scratch

Paul O’Shannessy gave a talk about building React from scratch that largely inspired this document.

Both this document and his talk are simplifications of the real codebase so you might get a better understanding by getting familiar with both of them.

Overview

The reconciler itself doesn’t have a public API. Renderers like React DOM and React Native use it to efficiently update the user interface according to the React components written by the user.

Mounting as a Recursive Process

Let’s consider the first time you mount a component:

const root = ReactDOM.createRoot(rootEl);
root.render(<App />);

root.render will pass <App /> along to the reconciler. Remember that <App /> is a React element, that is, a description of what to render. You can think about it as a plain object:

console.log(<App />);
// { type: App, props: {} }

The reconciler will check if App is a class or a function.

If App is a function, the reconciler will call App(props) to get the rendered element.

If App is a class, the reconciler will instantiate an App with new App(props) , call the componentWillMount() lifecycle method, and then will call the render() method to get the rendered element.

Either way, the reconciler will learn the element App “rendered to”.

This process is recursive. App may render to a <Greeting /> , Greeting may render to a <Button /> , and so on. The reconciler will “drill down” through user-defined components recursively as it learns what each component renders to.

You can imagine this process as a pseudocode:

function isClass(type) {
  // React.Component subclasses have this flag
  return (
    Boolean(type.prototype) &&
    Boolean(type.prototype.isReactComponent)
  );
}

// This function takes a React element (e.g. <App />)
// and returns a DOM or Native node representing the mounted tree.
function mount(element) {
  var type = element.type;
  var props = element.props;

  // We will determine the rendered element
  // by either running the type as function
  // or creating an instance and calling render().
  var renderedElement;
  if (isClass(type)) {
    // Component class
    var publicInstance = new type(props);
    // Set the props
    publicInstance.props = props;
    // Call the lifecycle if necessary
    if (publicInstance.componentWillMount) {
      publicInstance.componentWillMount();
    }
    // Get the rendered element by calling render()
    renderedElement = publicInstance.render();
  } else {
    // Component function
    renderedElement = type(props);
  }

  // This process is recursive because a component may
  // return an element with a type of another component.
  return mount(renderedElement);

  // Note: this implementation is incomplete and recurses infinitely!
  // It only handles elements like <App /> or <Button />.
  // It doesn't handle elements like <div /> or <p /> yet.
}

var rootEl = document.getElementById('root');
var node = mount(<App />);
rootEl.appendChild(node);

Note:

This really is a pseudo-code. It isn’t similar to the real implementation. It will also cause a stack overflow because we haven’t discussed when to stop the recursion.

Let’s recap a few key ideas in the example above:

• React elements are plain objects representing the component type (e.g. App ) and the props.


• User-defined components (e.g. App ) can be classes or functions but they all “render to” elements.


• “Mounting” is a recursive process that creates a DOM or Native tree given the top-level React element (e.g. <App /> ).

Mounting Host Elements

This process would be useless if we didn’t render something to the screen as a result.

In addition to user-defined (“composite”) components, React elements may also represent platform-specific (“host”) components. For example, Button might return a <div /> from its render method.

If element’s type property is a string, we are dealing with a host element:

console.log(<div />);
// { type: 'div', props: {} }

There is no user-defined code associated with host elements.

When the reconciler encounters a host element, it lets the renderer take care of mounting it. For example, React DOM would create a DOM node.

If the host element has children, the reconciler recursively mounts them following the same algorithm as above. It doesn’t matter whether children are host (like <div><hr /></div> ), composite (like <div><Button /></div> ), or both.

The DOM nodes produced by the child components will be appended to the parent DOM node, and recursively, the complete DOM structure will be assembled.

Note:

The reconciler itself is not tied to the DOM. The exact result of mounting (sometimes called “mount image” in the source code) depends on the renderer, and can be a DOM node (React DOM), a string (React DOM Server), or a number representing a native view (React Native).

If we were to extend the code to handle host elements, it would look like this:

function isClass(type) {
  // React.Component subclasses have this flag
  return (
    Boolean(type.prototype) &&
    Boolean(type.prototype.isReactComponent)
  );
}

// This function only handles elements with a composite type.
// For example, it handles <App /> and <Button />, but not a <div />.
function mountComposite(element) {
  var type = element.type;
  var props = element.props;

  var renderedElement;
  if (isClass(type)) {
    // Component class
    var publicInstance = new type(props);
    // Set the props
    publicInstance.props = props;
    // Call the lifecycle if necessary
    if (publicInstance.componentWillMount) {
      publicInstance.componentWillMount();
    }
    renderedElement = publicInstance.render();
  } else if (typeof type === 'function') {
    // Component function
    renderedElement = type(props);
  }

  // This is recursive but we'll eventually reach the bottom of recursion when
  // the element is host (e.g. <div />) rather than composite (e.g. <App />):
  return mount(renderedElement);
}

// This function only handles elements with a host type.
// For example, it handles <div /> and <p /> but not an <App />.
function mountHost(element) {
  var type = element.type;
  var props = element.props;
  var children = props.children || [];
  if (!Array.isArray(children)) {
    children = [children];
  }
  children = children.filter(Boolean);

  // This block of code shouldn't be in the reconciler.
  // Different renderers might initialize nodes differently.
  // For example, React Native would create iOS or Android views.
  var node = document.createElement(type);
  Object.keys(props).forEach(propName => {
    if (propName !== 'children') {
      node.setAttribute(propName, props[propName]);
    }
  });

  // Mount the children
  children.forEach(childElement => {
    // Children may be host (e.g. <div />) or composite (e.g. <Button />).
    // We will also mount them recursively:
    var childNode = mount(childElement);

    // This line of code is also renderer-specific.
    // It would be different depending on the renderer:
    node.appendChild(childNode);
  });

  // Return the DOM node as mount result.
  // This is where the recursion ends.
  return node;
}

function mount(element) {
  var type = element.type;
  if (typeof type === 'function') {
    // User-defined components
    return mountComposite(element);
  } else if (typeof type === 'string') {
    // Platform-specific components
    return mountHost(element);
  }
}

var rootEl = document.getElementById('root');
var node = mount(<App />);
rootEl.appendChild(node);

This is working but still far from how the reconciler is really implemented. The key missing ingredient is support for updates.

Introducing Internal Instances

The key feature of React is that you can re-render everything, and it won’t recreate the DOM or reset the state:

root.render(<App />);
// Should reuse the existing DOM:
root.render(<App />);

However, our implementation above only knows how to mount the initial tree. It can’t perform updates on it because it doesn’t store all the necessary information, such as all the publicInstance s, or which DOM node s correspond to which components.

The stack reconciler codebase solves this by making the mount() function a method and putting it on a class. There are drawbacks to this approach, and we are going in the opposite direction in the ongoing rewrite of the reconciler. Nevertheless this is how it works now.

Instead of separate mountHost and mountComposite functions, we will create two classes: DOMComponent and CompositeComponent .

Both classes have a constructor accepting the element , as well as a mount() method returning the mounted node. We will replace a top-level mount() function with a factory that instantiates the correct class:

function instantiateComponent(element) {
  var type = element.type;
  if (typeof type === 'function') {
    // User-defined components
    return new CompositeComponent(element);
  } else if (typeof type === 'string') {
    // Platform-specific components
    return new DOMComponent(element);
  }  
}

First, let’s consider the implementation of CompositeComponent :

class CompositeComponent {
  constructor(element) {
    this.currentElement = element;
    this.renderedComponent = null;
    this.publicInstance = null;
  }

  getPublicInstance() {
    // For composite components, expose the class instance.
    return this.publicInstance;
  }

  mount() {
    var element = this.currentElement;
    var type = element.type;
    var props = element.props;

    var publicInstance;
    var renderedElement;
    if (isClass(type)) {
      // Component class
      publicInstance = new type(props);
      // Set the props
      publicInstance.props = props;
      // Call the lifecycle if necessary
      if (publicInstance.componentWillMount) {
        publicInstance.componentWillMount();
      }
      renderedElement = publicInstance.render();
    } else if (typeof type === 'function') {
      // Component function
      publicInstance = null;
      renderedElement = type(props);
    }

    // Save the public instance
    this.publicInstance = publicInstance;

    // Instantiate the child internal instance according to the element.
    // It would be a DOMComponent for <div /> or <p />,
    // and a CompositeComponent for <App /> or <Button />:
    var renderedComponent = instantiateComponent(renderedElement);
    this.renderedComponent = renderedComponent;

    // Mount the rendered output
    return renderedComponent.mount();
  }
}

This is not much different from our previous mountComposite() implementation, but now we can save some information, such as this.currentElement , this.renderedComponent , and this.publicInstance , for use during updates.

Note that an instance of CompositeComponent is not the same thing as an instance of the user-supplied element.type . CompositeComponent is an implementation detail of our reconciler, and is never exposed to the user. The user-defined class is the one we read from element.type , and CompositeComponent creates an instance of it.

To avoid the confusion, we will call instances of CompositeComponent and DOMComponent “internal instances”. They exist so we can associate some long-lived data with them. Only the renderer and the reconciler are aware that they exist.

In contrast, we call an instance of the user-defined class a “public instance”. The public instance is what you see as this in the render() and other methods of your custom components.

The mountHost() function, refactored to be a mount() method on DOMComponent class, also looks familiar:

class DOMComponent {
  constructor(element) {
    this.currentElement = element;
    this.renderedChildren = [];
    this.node = null;
  }

  getPublicInstance() {
    // For DOM components, only expose the DOM node.
    return this.node;
  }

  mount() {
    var element = this.currentElement;
    var type = element.type;
    var props = element.props;
    var children = props.children || [];
    if (!Array.isArray(children)) {
      children = [children];
    }

    // Create and save the node
    var node = document.createElement(type);
    this.node = node;

    // Set the attributes
    Object.keys(props).forEach(propName => {
      if (propName !== 'children') {
        node.setAttribute(propName, props[propName]);
      }
    });

    // Create and save the contained children.
    // Each of them can be a DOMComponent or a CompositeComponent,
    // depending on whether the element type is a string or a function.
    var renderedChildren = children.map(instantiateComponent);
    this.renderedChildren = renderedChildren;

    // Collect DOM nodes they return on mount
    var childNodes = renderedChildren.map(child => child.mount());
    childNodes.forEach(childNode => node.appendChild(childNode));

    // Return the DOM node as mount result
    return node;
  }
}

The main difference after refactoring from mountHost() is that we now keep this.node and this.renderedChildren associated with the internal DOM component instance. We will also use them for applying non-destructive updates in the future.

As a result, each internal instance, composite or host, now points to its child internal instances. To help visualize this, if a function <App> component renders a <Button> class component, and Button class renders a <div> , the internal instance tree would look like this:

[object CompositeComponent] {
  currentElement: <App />,
  publicInstance: null,
  renderedComponent: [object CompositeComponent] {
    currentElement: <Button />,
    publicInstance: [object Button],
    renderedComponent: [object DOMComponent] {
      currentElement: <div />,
      node: [object HTMLDivElement],
      renderedChildren: []
    }
  }
}

In the DOM you would only see the <div> . However the internal instance tree contains both composite and host internal instances.

The composite internal instances need to store:

• The current element.


• The public instance if element type is a class.


• The single rendered internal instance. It can be either a DOMComponent or a CompositeComponent .

The host internal instances need to store:

• The current element.


• The DOM node.


• All the child internal instances. Each of them can be either a DOMComponent or a CompositeComponent .

If you’re struggling to imagine how an internal instance tree is structured in more complex applications, React DevTools can give you a close approximation, as it highlights host instances with grey, and composite instances with purple:

To complete this refactoring, we will introduce a function that mounts a complete tree into a container node and a public instance:

function mountTree(element, containerNode) {
  // Create the top-level internal instance
  var rootComponent = instantiateComponent(element);

  // Mount the top-level component into the container
  var node = rootComponent.mount();
  containerNode.appendChild(node);

  // Return the public instance it provides
  var publicInstance = rootComponent.getPublicInstance();
  return publicInstance;
}

var rootEl = document.getElementById('root');
mountTree(<App />, rootEl);

Unmounting

Now that we have internal instances that hold onto their children and the DOM nodes, we can implement unmounting. For a composite component, unmounting calls a lifecycle method and recurses.

class CompositeComponent {

  // ...

  unmount() {
    // Call the lifecycle method if necessary
    var publicInstance = this.publicInstance;
    if (publicInstance) {
      if (publicInstance.componentWillUnmount) {
        publicInstance.componentWillUnmount();
      }
    }

    // Unmount the single rendered component
    var renderedComponent = this.renderedComponent;
    renderedComponent.unmount();
  }
}

For DOMComponent , unmounting tells each child to unmount:

class DOMComponent {

  // ...

  unmount() {
    // Unmount all the children
    var renderedChildren = this.renderedChildren;
    renderedChildren.forEach(child => child.unmount());
  }
}

In practice, unmounting DOM components also removes the event listeners and clears some caches, but we will skip those details.

We can now add a new top-level function called unmountTree(containerNode) that is similar to ReactDOM.unmountComponentAtNode() :

function unmountTree(containerNode) {
  // Read the internal instance from a DOM node:
  // (This doesn't work yet, we will need to change mountTree() to store it.)
  var node = containerNode.firstChild;
  var rootComponent = node._internalInstance;

  // Unmount the tree and clear the container
  rootComponent.unmount();
  containerNode.innerHTML = '';
}

In order for this to work, we need to read an internal root instance from a DOM node. We will modify mountTree() to add the _internalInstance property to the root DOM node. We will also teach mountTree() to destroy any existing tree so it can be called multiple times:

function mountTree(element, containerNode) {
  // Destroy any existing tree
  if (containerNode.firstChild) {
    unmountTree(containerNode);
  }

  // Create the top-level internal instance
  var rootComponent = instantiateComponent(element);

  // Mount the top-level component into the container
  var node = rootComponent.mount();
  containerNode.appendChild(node);

  // Save a reference to the internal instance
  node._internalInstance = rootComponent;

  // Return the public instance it provides
  var publicInstance = rootComponent.getPublicInstance();
  return publicInstance;
}

Now, running unmountTree() , or running mountTree() repeatedly, removes the old tree and runs the componentWillUnmount() lifecycle method on components.

Updating

In the previous section, we implemented unmounting. However React wouldn’t be very useful if each prop change unmounted and mounted the whole tree. The goal of the reconciler is to reuse existing instances where possible to preserve the DOM and the state:

var rootEl = document.getElementById('root');

mountTree(<App />, rootEl);
// Should reuse the existing DOM:
mountTree(<App />, rootEl);

We will extend our internal instance contract with one more method. In addition to mount() and unmount() , both DOMComponent and CompositeComponent will implement a new method called receive(nextElement) :

class CompositeComponent {
  // ...

  receive(nextElement) {
    // ...
  }
}

class DOMComponent {
  // ...

  receive(nextElement) {
    // ...
  }
}

Its job is to do whatever is necessary to bring the component (and any of its children) up to date with the description provided by the nextElement .

This is the part that is often described as “virtual DOM diffing” although what really happens is that we walk the internal tree recursively and let each internal instance receive an update.

Updating Composite Components

When a composite component receives a new element, we run the componentWillUpdate() lifecycle method.

Then we re-render the component with the new props, and get the next rendered element:

class CompositeComponent {

  // ...

  receive(nextElement) {
    var prevProps = this.currentElement.props;
    var publicInstance = this.publicInstance;
    var prevRenderedComponent = this.renderedComponent;
    var prevRenderedElement = prevRenderedComponent.currentElement;

    // Update *own* element
    this.currentElement = nextElement;
    var type = nextElement.type;
    var nextProps = nextElement.props;

    // Figure out what the next render() output is
    var nextRenderedElement;
    if (isClass(type)) {
      // Component class
      // Call the lifecycle if necessary
      if (publicInstance.componentWillUpdate) {
        publicInstance.componentWillUpdate(nextProps);
      }
      // Update the props
      publicInstance.props = nextProps;
      // Re-render
      nextRenderedElement = publicInstance.render();
    } else if (typeof type === 'function') {
      // Component function
      nextRenderedElement = type(nextProps);
    }

    // ...

Next, we can look at the rendered element’s type . If the type has not changed since the last render, the component below can also be updated in place.

For example, if it returned <Button color="red" /> the first time, and <Button color="blue" /> the second time, we can just tell the corresponding internal instance to receive() the next element:

    // ...

    // If the rendered element type has not changed,
    // reuse the existing component instance and exit.
    if (prevRenderedElement.type === nextRenderedElement.type) {
      prevRenderedComponent.receive(nextRenderedElement);
      return;
    }

    // ...

However, if the next rendered element has a different type than the previously rendered element, we can’t update the internal instance. A <button> can’t “become” an <input> .

Instead, we have to unmount the existing internal instance and mount the new one corresponding to the rendered element type. For example, this is what happens when a component that previously rendered a <button /> renders an <input /> :

    // ...

    // If we reached this point, we need to unmount the previously
    // mounted component, mount the new one, and swap their nodes.

    // Find the old node because it will need to be replaced
    var prevNode = prevRenderedComponent.getHostNode();

    // Unmount the old child and mount a new child
    prevRenderedComponent.unmount();
    var nextRenderedComponent = instantiateComponent(nextRenderedElement);
    var nextNode = nextRenderedComponent.mount();

    // Replace the reference to the child
    this.renderedComponent = nextRenderedComponent;

    // Replace the old node with the new one
    // Note: this is renderer-specific code and
    // ideally should live outside of CompositeComponent:
    prevNode.parentNode.replaceChild(nextNode, prevNode);
  }
}

To sum this up, when a composite component receives a new element, it may either delegate the update to its rendered internal instance, or unmount it and mount a new one in its place.

There is another condition under which a component will re-mount rather than receive an element, and that is when the element’s key has changed. We don’t discuss key handling in this document because it adds more complexity to an already complex tutorial.

Note that we needed to add a method called getHostNode() to the internal instance contract so that it’s possible to locate the platform-specific node and replace it during the update. Its implementation is straightforward for both classes:

class CompositeComponent {
  // ...

  getHostNode() {
    // Ask the rendered component to provide it.
    // This will recursively drill down any composites.
    return this.renderedComponent.getHostNode();
  }
}

class DOMComponent {
  // ...

  getHostNode() {
    return this.node;
  }  
}

Updating Host Components

Host component implementations, such as DOMComponent , update differently. When they receive an element, they need to update the underlying platform-specific view. In case of React DOM, this means updating the DOM attributes:

class DOMComponent {
  // ...

  receive(nextElement) {
    var node = this.node;
    var prevElement = this.currentElement;
    var prevProps = prevElement.props;
    var nextProps = nextElement.props;    
    this.currentElement = nextElement;

    // Remove old attributes.
    Object.keys(prevProps).forEach(propName => {
      if (propName !== 'children' && !nextProps.hasOwnProperty(propName)) {
        node.removeAttribute(propName);
      }
    });
    // Set next attributes.
    Object.keys(nextProps).forEach(propName => {
      if (propName !== 'children') {
        node.setAttribute(propName, nextProps[propName]);
      }
    });

    // ...

Then, host components need to update their children. Unlike composite components, they might contain more than a single child.

In this simplified example, we use an array of internal instances and iterate over it, either updating or replacing the internal instances depending on whether the received type matches their previous type . The real reconciler also takes element’s key in the account and track moves in addition to insertions and deletions, but we will omit this logic.

We collect DOM operations on children in a list so we can execute them in batch:

    // ...

    // These are arrays of React elements:
    var prevChildren = prevProps.children || [];
    if (!Array.isArray(prevChildren)) {
      prevChildren = [prevChildren];
    }
    var nextChildren = nextProps.children || [];
    if (!Array.isArray(nextChildren)) {
      nextChildren = [nextChildren];
    }
    // These are arrays of internal instances:
    var prevRenderedChildren = this.renderedChildren;
    var nextRenderedChildren = [];

    // As we iterate over children, we will add operations to the array.
    var operationQueue = [];

    // Note: the section below is extremely simplified!
    // It doesn't handle reorders, children with holes, or keys.
    // It only exists to illustrate the overall flow, not the specifics.

    for (var i = 0; i < nextChildren.length; i++) {
      // Try to get an existing internal instance for this child
      var prevChild = prevRenderedChildren[i];

      // If there is no internal instance under this index,
      // a child has been appended to the end. Create a new
      // internal instance, mount it, and use its node.
      if (!prevChild) {
        var nextChild = instantiateComponent(nextChildren[i]);
        var node = nextChild.mount();

        // Record that we need to append a node
        operationQueue.push({type: 'ADD', node});
        nextRenderedChildren.push(nextChild);
        continue;
      }

      // We can only update the instance if its element's type matches.
      // For example, <Button size="small" /> can be updated to
      // <Button size="large" /> but not to an <App />.
      var canUpdate = prevChildren[i].type === nextChildren[i].type;

      // If we can't update an existing instance, we have to unmount it
      // and mount a new one instead of it.
      if (!canUpdate) {
        var prevNode = prevChild.getHostNode();
        prevChild.unmount();

        var nextChild = instantiateComponent(nextChildren[i]);
        var nextNode = nextChild.mount();

        // Record that we need to swap the nodes
        operationQueue.push({type: 'REPLACE', prevNode, nextNode});
        nextRenderedChildren.push(nextChild);
        continue;
      }

      // If we can update an existing internal instance,
      // just let it receive the next element and handle its own update.
      prevChild.receive(nextChildren[i]);
      nextRenderedChildren.push(prevChild);
    }

    // Finally, unmount any children that don't exist:
    for (var j = nextChildren.length; j < prevChildren.length; j++) {
      var prevChild = prevRenderedChildren[j];
      var node = prevChild.getHostNode();
      prevChild.unmount();

      // Record that we need to remove the node
      operationQueue.push({type: 'REMOVE', node});
    }

    // Point the list of rendered children to the updated version.
    this.renderedChildren = nextRenderedChildren;

    // ...

As the last step, we execute the DOM operations. Again, the real reconciler code is more complex because it also handles moves:

    // ...

    // Process the operation queue.
    while (operationQueue.length > 0) {
      var operation = operationQueue.shift();
      switch (operation.type) {
      case 'ADD':
        this.node.appendChild(operation.node);
        break;
      case 'REPLACE':
        this.node.replaceChild(operation.nextNode, operation.prevNode);
        break;
      case 'REMOVE':
        this.node.removeChild(operation.node);
        break;
      }
    }
  }
}

And that is it for updating host components.

Top-Level Updates

Now that both CompositeComponent and DOMComponent implement the receive(nextElement) method, we can change the top-level mountTree() function to use it when the element type is the same as it was the last time:

function mountTree(element, containerNode) {
  // Check for an existing tree
  if (containerNode.firstChild) {
    var prevNode = containerNode.firstChild;
    var prevRootComponent = prevNode._internalInstance;
    var prevElement = prevRootComponent.currentElement;

    // If we can, reuse the existing root component
    if (prevElement.type === element.type) {
      prevRootComponent.receive(element);
      return;
    }

    // Otherwise, unmount the existing tree
    unmountTree(containerNode);
  }

  // ...

}

Now calling mountTree() two times with the same type isn’t destructive:

var rootEl = document.getElementById('root');

mountTree(<App />, rootEl);
// Reuses the existing DOM:
mountTree(<App />, rootEl);

These are the basics of how React works internally.

What We Left Out

This document is simplified compared to the real codebase. There are a few important aspects we didn’t address:

• Components can render null , and the reconciler can handle “empty slots” in arrays and rendered output.


• The reconciler also reads key from the elements, and uses it to establish which internal instance corresponds to which element in an array. A bulk of complexity in the actual React implementation is related to that.


• In addition to composite and host internal instance classes, there are also classes for “text” and “empty” components. They represent text nodes and the “empty slots” you get by rendering null .


• Renderers use injection to pass the host internal class to the reconciler. For example, React DOM tells the reconciler to use ReactDOMComponent as the host internal instance implementation.


• The logic for updating the list of children is extracted into a mixin called ReactMultiChild which is used by the host internal instance class implementations both in React DOM and React Native.


• The reconciler also implements support for setState() in composite components. Multiple updates inside event handlers get batched into a single update.


• The reconciler also takes care of attaching and detaching refs to composite components and host nodes.


• Lifecycle methods that are called after the DOM is ready, such as componentDidMount() and componentDidUpdate() , get collected into “callback queues” and are executed in a single batch.


• React puts information about the current update into an internal object called “transaction”. Transactions are useful for keeping track of the queue of pending lifecycle methods, the current DOM nesting for the warnings, and anything else that is “global” to a specific update. Transactions also ensure React “cleans everything up” after updates. For example, the transaction class provided by React DOM restores the input selection after any update.

Jumping into the Code

• ReactMount is where the code like mountTree() and unmountTree() from this tutorial lives. It takes care of mounting and unmounting top-level components. ReactNativeMount is its React Native analog.


• ReactDOMComponent is the equivalent of DOMComponent in this tutorial. It implements the host component class for React DOM renderer. ReactNativeBaseComponent is its React Native analog.


• ReactCompositeComponent is the equivalent of CompositeComponent in this tutorial. It handles calling user-defined components and maintaining their state.


• instantiateReactComponent contains the switch that picks the right internal instance class to construct for an element. It is equivalent to instantiateComponent() in this tutorial.


• ReactReconciler is a wrapper with mountComponent() , receiveComponent() , and unmountComponent() methods. It calls the underlying implementations on the internal instances, but also includes some code around them that is shared by all internal instance implementations.


• ReactChildReconciler implements the logic for mounting, updating, and unmounting children according to the key of their elements.


• ReactMultiChild implements processing the operation queue for child insertions, deletions, and moves independently of the renderer.


• mount() , receive() , and unmount() are really called mountComponent() , receiveComponent() , and unmountComponent() in React codebase for legacy reasons, but they receive elements.


• Properties on the internal instances start with an underscore, e.g. _currentElement . They are considered to be read-only public fields throughout the codebase.

Future Directions

Stack reconciler has inherent limitations such as being synchronous and unable to interrupt the work or split it in chunks. There is a work in progress on the new Fiber reconciler with a completely different architecture. In the future, we intend to replace stack reconciler with it, but at the moment it is far from feature parity.

Next Steps

Read the next section to learn about the guiding principles we use for React development.

We wrote this document so that you have a better idea of how we decide what React does and what React doesn’t do, and what our development philosophy is like. While we are excited to see community contributions, we are not likely to choose a path that violates one or more of these principles.

Note:

This document assumes a strong understanding of React. It describes the design principles of React itself , not React components or applications.

For an introduction to React, check out Thinking in React instead.

Composition

The key feature of React is composition of components. Components written by different people should work well together. It is important to us that you can add functionality to a component without causing rippling changes throughout the codebase.

For example, it should be possible to introduce some local state into a component without changing any of the components using it. Similarly, it should be possible to add some initialization and teardown code to any component when necessary.

There is nothing “bad” about using state or lifecycle methods in components. Like any powerful feature, they should be used in moderation, but we have no intention to remove them. On the contrary, we think they are integral parts of what makes React useful. We might enable more functional patterns in the future, but both local state and lifecycle methods will be a part of that model.

Components are often described as “just functions” but in our view they need to be more than that to be useful. In React, components describe any composable behavior, and this includes rendering, lifecycle, and state. Some external libraries like Relay augment components with other responsibilities such as describing data dependencies. It is possible that those ideas might make it back into React too in some form.

Common Abstraction

In general we resist adding features that can be implemented in userland. We don’t want to bloat your apps with useless library code. However, there are exceptions to this.

For example, if React didn’t provide support for local state or lifecycle methods, people would create custom abstractions for them. When there are multiple abstractions competing, React can’t enforce or take advantage of the properties of either of them. It has to work with the lowest common denominator.

This is why sometimes we add features to React itself. If we notice that many components implement a certain feature in incompatible or inefficient ways, we might prefer to bake it into React. We don’t do it lightly. When we do it, it’s because we are confident that raising the abstraction level benefits the whole ecosystem. State, lifecycle methods, cross-browser event normalization are good examples of this.

We always discuss such improvement proposals with the community. You can find some of those discussions by the “big picture” label on the React issue tracker.

Escape Hatches

React is pragmatic. It is driven by the needs of the products written at Facebook. While it is influenced by some paradigms that are not yet fully mainstream such as functional programming, staying accessible to a wide range of developers with different skills and experience levels is an explicit goal of the project.

If we want to deprecate a pattern that we don’t like, it is our responsibility to consider all existing use cases for it and educate the community about the alternatives before we deprecate it. If some pattern that is useful for building apps is hard to express in a declarative way, we will provide an imperative API for it. If we can’t figure out a perfect API for something that we found necessary in many apps, we will provide a temporary subpar working API as long as it is possible to get rid of it later and it leaves the door open for future improvements.

Stability

We value API stability. At Facebook, we have more than 50 thousand components using React. Many other companies, including Twitter and Airbnb, are also heavy users of React. This is why we are usually reluctant to change public APIs or behavior.

However we think stability in the sense of “nothing changes” is overrated. It quickly turns into stagnation. Instead, we prefer the stability in the sense of “It is heavily used in production, and when something changes, there is a clear (and preferably automated) migration path.”

When we deprecate a pattern, we study its internal usage at Facebook and add deprecation warnings. They let us assess the impact of the change. Sometimes we back out if we see that it is too early, and we need to think more strategically about getting the codebases to the point where they are ready for this change.

If we are confident that the change is not too disruptive and the migration strategy is viable for all use cases, we release the deprecation warning to the open source community. We are closely in touch with many users of React outside of Facebook, and we monitor popular open source projects and guide them in fixing those deprecations.

Given the sheer size of the Facebook React codebase, successful internal migration is often a good indicator that other companies won’t have problems either. Nevertheless sometimes people point out additional use cases we haven’t thought of, and we add escape hatches for them or rethink our approach.

We don’t deprecate anything without a good reason. We recognize that sometimes deprecations warnings cause frustration but we add them because deprecations clean up the road for the improvements and new features that we and many people in the community consider valuable.

For example, we added a warning about unknown DOM props in React 15.2.0. Many projects were affected by this. However fixing this warning is important so that we can introduce the support for custom attributes to React. There is a reason like this behind every deprecation that we add.

When we add a deprecation warning, we keep it for the rest of the current major version, and change the behavior in the next major version. If there is a lot of repetitive manual work involved, we release a codemod script that automates most of the change. Codemods enable us to move forward without stagnation in a massive codebase, and we encourage you to use them as well.

You can find the codemods that we released in the react-codemod repository.

Interoperability

We place high value in interoperability with existing systems and gradual adoption. Facebook has a massive non-React codebase. Its website uses a mix of a server-side component system called XHP, internal UI libraries that came before React, and React itself. It is important to us that any product team can start using React for a small feature rather than rewrite their code to bet on it.

This is why React provides escape hatches to work with mutable models, and tries to work well together with other UI libraries. You can wrap an existing imperative UI into a declarative component, and vice versa. This is crucial for gradual adoption.

Scheduling

Even when your components are described as functions, when you use React you don’t call them directly. Every component returns a description of what needs to be rendered, and that description may include both user-written components like <LikeButton> and platform-specific components like <div> . It is up to React to “unroll” <LikeButton> at some point in the future and actually apply changes to the UI tree according to the render results of the components recursively.

This is a subtle distinction but a powerful one. Since you don’t call that component function but let React call it, it means React has the power to delay calling it if necessary. In its current implementation React walks the tree recursively and calls render functions of the whole updated tree during a single tick. However in the future it might start delaying some updates to avoid dropping frames.

This is a common theme in React design. Some popular libraries implement the “push” approach where computations are performed when the new data is available. React, however, sticks to the “pull” approach where computations can be delayed until necessary.

React is not a generic data processing library. It is a library for building user interfaces. We think that it is uniquely positioned in an app to know which computations are relevant right now and which are not.

If something is offscreen, we can delay any logic related to it. If data is arriving faster than the frame rate, we can coalesce and batch updates. We can prioritize work coming from user interactions (such as an animation caused by a button click) over less important background work (such as rendering new content just loaded from the network) to avoid dropping frames.

To be clear, we are not taking advantage of this right now. However the freedom to do something like this is why we prefer to have control over scheduling, and why setState() is asynchronous. Conceptually, we think of it as “scheduling an update”.

The control over scheduling would be harder for us to gain if we let the user directly compose views with a “push” based paradigm common in some variations of Functional Reactive Programming. We want to own the “glue” code.

It is a key goal for React that the amount of the user code that executes before yielding back into React is minimal. This ensures that React retains the capability to schedule and split work in chunks according to what it knows about the UI.

There is an internal joke in the team that React should have been called “Schedule” because React does not want to be fully “reactive”.

Developer Experience

Providing a good developer experience is important to us.

For example, we maintain React DevTools which let you inspect the React component tree in Chrome and Firefox. We have heard that it brings a big productivity boost both to the Facebook engineers and to the community.

We also try to go an extra mile to provide helpful developer warnings. For example, React warns you in development if you nest tags in a way that the browser doesn’t understand, or if you make a common typo in the API. Developer warnings and the related checks are the main reason why the development version of React is slower than the production version.

The usage patterns that we see internally at Facebook help us understand what the common mistakes are, and how to prevent them early. When we add new features, we try to anticipate the common mistakes and warn about them.

We are always looking out for ways to improve the developer experience. We love to hear your suggestions and accept your contributions to make it even better.

Debugging

When something goes wrong, it is important that you have breadcrumbs to trace the mistake to its source in the codebase. In React, props and state are those breadcrumbs.

If you see something wrong on the screen, you can open React DevTools, find the component responsible for rendering, and then see if the props and state are correct. If they are, you know that the problem is in the component’s render() function, or some function that is called by render() . The problem is isolated.

If the state is wrong, you know that the problem is caused by one of the setState() calls in this file. This, too, is relatively simple to locate and fix because usually there are only a few setState() calls in a single file.

If the props are wrong, you can traverse the tree up in the inspector, looking for the component that first “poisoned the well” by passing bad props down.

This ability to trace any UI to the data that produced it in the form of current props and state is very important to React. It is an explicit design goal that state is not “trapped” in closures and combinators, and is available to React directly.

While the UI is dynamic, we believe that synchronous render() functions of props and state turn debugging from guesswork into a boring but finite procedure. We would like to preserve this constraint in React even though it makes some use cases, like complex animations, harder.

Configuration

We find global runtime configuration options to be problematic.

For example, it is occasionally requested that we implement a function like React.configure(options) or React.register(component) . However this poses multiple problems, and we are not aware of good solutions to them.

What if somebody calls such a function from a third-party component library? What if one React app embeds another React app, and their desired configurations are incompatible? How can a third-party component specify that it requires a particular configuration? We think that global configuration doesn’t work well with composition. Since composition is central to React, we don’t provide global configuration in code.

We do, however, provide some global configuration on the build level. For example, we provide separate development and production builds. We may also add a profiling build in the future, and we are open to considering other build flags.

Beyond the DOM

We see the value of React in the way it allows us to write components that have fewer bugs and compose together well. DOM is the original rendering target for React but React Native is just as important both to Facebook and the community.

Being renderer-agnostic is an important design constraint of React. It adds some overhead in the internal representations. On the other hand, any improvements to the core translate across platforms.

Having a single programming model lets us form engineering teams around products instead of platforms. So far the tradeoff has been worth it for us.

Implementation

We try to provide elegant APIs where possible. We are much less concerned with the implementation being elegant. The real world is far from perfect, and to a reasonable extent we prefer to put the ugly code into the library if it means the user does not have to write it. When we evaluate new code, we are looking for an implementation that is correct, performant and affords a good developer experience. Elegance is secondary.

We prefer boring code to clever code. Code is disposable and often changes. So it is important that it doesn’t introduce new internal abstractions unless absolutely necessary. Verbose code that is easy to move around, change and remove is preferred to elegant code that is prematurely abstracted and hard to change.

Optimized for Tooling

Some commonly used APIs have verbose names. For example, we use componentDidMount() instead of didMount() or onMount() . This is intentional. The goal is to make the points of interaction with the library highly visible.

In a massive codebase like Facebook, being able to search for uses of specific APIs is very important. We value distinct verbose names, and especially for the features that should be used sparingly. For example, dangerouslySetInnerHTML is hard to miss in a code review.

Optimizing for search is also important because of our reliance on codemods to make breaking changes. We want it to be easy and safe to apply vast automated changes across the codebase, and unique verbose names help us achieve this. Similarly, distinctive names make it easy to write custom lint rules about using React without worrying about potential false positives.

JSX plays a similar role. While it is not required with React, we use it extensively at Facebook both for aesthetic and pragmatic reasons.

In our codebase, JSX provides an unambiguous hint to the tools that they are dealing with a React element tree. This makes it possible to add build-time optimizations such as hoisting constant elements, safely lint and codemod internal component usage, and include JSX source location into the warnings.

Dogfooding

We try our best to address the problems raised by the community. However we are likely to prioritize the issues that people are also experiencing internally at Facebook. Perhaps counter-intuitively, we think this is the main reason why the community can bet on React.

Heavy internal usage gives us the confidence that React won’t disappear tomorrow. React was created at Facebook to solve its problems. It brings tangible business value to the company and is used in many of its products. Dogfooding it means that our vision stays sharp and we have a focused direction going forward.

This doesn’t mean that we ignore the issues raised by the community. For example, we added support for web components and SVG to React even though we don’t rely on either of them internally. We are actively listening to your pain points and address them to the best of our ability. The community is what makes React special to us, and we are honored to contribute back.

After releasing many open source projects at Facebook, we have learned that trying to make everyone happy at the same time produced projects with poor focus that didn’t grow well. Instead, we found that picking a small audience and focusing on making them happy brings a positive net effect. That’s exactly what we did with React, and so far solving the problems encountered by Facebook product teams has translated well to the open source community.

The downside of this approach is that sometimes we fail to give enough focus to the things that Facebook teams don’t have to deal with, such as the “getting started” experience. We are acutely aware of this, and we are thinking of how to improve in a way that would benefit everyone in the community without making the same mistakes we did with open source projects before.

How can I make an AJAX call?

You can use any AJAX library you like with React. Some popular ones are Axios, jQuery AJAX, and the browser built-in window.fetch.

Where in the component lifecycle should I make an AJAX call?

You should populate data with AJAX calls in the componentDidMount lifecycle method. This is so you can use setState to update your component when the data is retrieved.

Example: Using AJAX results to set local state

The component below demonstrates how to make an AJAX call in componentDidMount to populate local component state.

The example API returns a JSON object like this:

{
  "items": [
    { "id": 1, "name": "Apples",  "price": "$2" },
    { "id": 2, "name": "Peaches", "price": "$5" }
  ] 
}

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      error: null,
      isLoaded: false,
      items: []
    };
  }

  componentDidMount() {
    fetch("https://api.example.com/items")
      .then(res => res.json())
      .then(
        (result) => {
          this.setState({
            isLoaded: true,
            items: result.items
          });
        },
        // Note: it's important to handle errors here
        // instead of a catch() block so that we don't swallow
        // exceptions from actual bugs in components.
        (error) => {
          this.setState({
            isLoaded: true,
            error
          });
        }
      )
  }

  render() {
    const { error, isLoaded, items } = this.state;
    if (error) {
      return <div>Error: {error.message}</div>;
    } else if (!isLoaded) {
      return <div>Loading...</div>;
    } else {
      return (
        <ul>
          {items.map(item => (
            <li key={item.id}>
              {item.name} {item.price}
            </li>
          ))}
        </ul>
      );
    }
  }
}

Here is the equivalent with Hooks:

function MyComponent() {
  const [error, setError] = useState(null);
  const [isLoaded, setIsLoaded] = useState(false);
  const [items, setItems] = useState([]);

  // Note: the empty deps array [] means
  // this useEffect will run once
  // similar to componentDidMount()
  useEffect(() => {
    fetch("https://api.example.com/items")
      .then(res => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setItems(result);
        },
        // Note: it's important to handle errors here
        // instead of a catch() block so that we don't swallow
        // exceptions from actual bugs in components.
        (error) => {
          setIsLoaded(true);
          setError(error);
        }
      )
  }, [])

  if (error) {
    return <div>Error: {error.message}</div>;
  } else if (!isLoaded) {
    return <div>Loading...</div>;
  } else {
    return (
      <ul>
        {items.map(item => (
          <li key={item.id}>
            {item.name} {item.price}
          </li>
        ))}
      </ul>
    );
  }
}

Do I need to use JSX with React?

No! Check out “React Without JSX” to learn more.

Do I need to use ES6 (+) with React?

No! Check out “React Without ES6” to learn more.

How can I write comments in JSX?

<div>
  {/* Comment goes here */}
  Hello, {name}!
</div>

<div>
  {/* It also works 
  for multi-line comments. */}
  Hello, {name}! 
</div>