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Implementation Notes – React

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.

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.

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.

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:

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) {
  
  return (
    Boolean(type.prototype) &&
    Boolean(type.prototype.isReactComponent)
  );
}



function mount(element) {
  var type = element.type;
  var props = element.props;

  
  
  
  var renderedElement;
  if (isClass(type)) {
    
    var publicInstance = new type(props);
    
    publicInstance.props = props;
    
    if (publicInstance.componentWillMount) {
      publicInstance.componentWillMount();
    }
    
    renderedElement = publicInstance.render();
  } else {
    
    renderedElement = type(props);
  }

  
  
  return mount(renderedElement);

  
  
  
}

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 />).

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:

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) {
  
  return (
    Boolean(type.prototype) &&
    Boolean(type.prototype.isReactComponent)
  );
}



function mountComposite(element) {
  var type = element.type;
  var props = element.props;

  var renderedElement;
  if (isClass(type)) {
    
    var publicInstance = new type(props);
    
    publicInstance.props = props;
    
    if (publicInstance.componentWillMount) {
      publicInstance.componentWillMount();
    }
    renderedElement = publicInstance.render();
  } else if (typeof type === 'function') {
    
    renderedElement = type(props);
  }

  
  
  return mount(renderedElement);
}



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);

  
  
  
  var node = document.createElement(type);
  Object.keys(props).forEach(propName => {
    if (propName !== 'children') {
      node.setAttribute(propName, props[propName]);
    }
  });

  
  children.forEach(childElement => {
    
    
    var childNode = mount(childElement);

    
    
    node.appendChild(childNode);
  });

  
  
  return node;
}

function mount(element) {
  var type = element.type;
  if (typeof type === 'function') {
    
    return mountComposite(element);
  } else if (typeof type === 'string') {
    
    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.

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 />);

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 publicInstances, or which DOM nodes 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') {
    
    return new CompositeComponent(element);
  } else if (typeof type === 'string') {
    
    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() {
    
    return this.publicInstance;
  }

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

    var publicInstance;
    var renderedElement;
    if (isClass(type)) {
      
      publicInstance = new type(props);
      
      publicInstance.props = props;
      
      if (publicInstance.componentWillMount) {
        publicInstance.componentWillMount();
      }
      renderedElement = publicInstance.render();
    } else if (typeof type === 'function') {
      
      publicInstance = null;
      renderedElement = type(props);
    }

    
    this.publicInstance = publicInstance;

    
    
    
    var renderedComponent = instantiateComponent(renderedElement);
    this.renderedComponent = renderedComponent;

    
    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() {
    
    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];
    }

    
    var node = document.createElement(type);
    this.node = node;

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

    
    
    
    var renderedChildren = children.map(instantiateComponent);
    this.renderedChildren = renderedChildren;

    
    var childNodes = renderedChildren.map(child => child.mount());
    childNodes.forEach(childNode => node.appendChild(childNode));

    
    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) {
  
  var rootComponent = instantiateComponent(element);

  
  var node = rootComponent.mount();
  containerNode.appendChild(node);

  
  var publicInstance = rootComponent.getPublicInstance();
  return publicInstance;
}

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

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() {
    
    var publicInstance = this.publicInstance;
    if (publicInstance) {
      if (publicInstance.componentWillUnmount) {
        publicInstance.componentWillUnmount();
      }
    }

    
    var renderedComponent = this.renderedComponent;
    renderedComponent.unmount();
  }
}

For DOMComponent, unmounting tells each child to unmount:

class DOMComponent {

  

  unmount() {
    
    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) {
  
  
  var node = containerNode.firstChild;
  var rootComponent = node._internalInstance;

  
  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) {
  
  if (containerNode.firstChild) {
    unmountTree(containerNode);
  }

  
  var rootComponent = instantiateComponent(element);

  
  var node = rootComponent.mount();
  containerNode.appendChild(node);

  
  node._internalInstance = rootComponent;

  
  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.

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);

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.

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;

    
    this.currentElement = nextElement;
    var type = nextElement.type;
    var nextProps = nextElement.props;

    
    var nextRenderedElement;
    if (isClass(type)) {
      
      
      if (publicInstance.componentWillUpdate) {
        publicInstance.componentWillUpdate(nextProps);
      }
      
      publicInstance.props = nextProps;
      
      nextRenderedElement = publicInstance.render();
    } else if (typeof type === '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 (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 />:

    

    
    

    
    var prevNode = prevRenderedComponent.getHostNode();

    
    prevRenderedComponent.unmount();
    var nextRenderedComponent = instantiateComponent(nextRenderedElement);
    var nextNode = nextRenderedComponent.mount();

    
    this.renderedComponent = nextRenderedComponent;

    
    
    
    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() {
    
    
    return this.renderedComponent.getHostNode();
  }
}

class DOMComponent {
  

  getHostNode() {
    return this.node;
  }  
}

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;

    
    Object.keys(prevProps).forEach(propName => {
      if (propName !== 'children' && !nextProps.hasOwnProperty(propName)) {
        node.removeAttribute(propName);
      }
    });
    
    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:

    

    
    var prevChildren = prevProps.children || [];
    if (!Array.isArray(prevChildren)) {
      prevChildren = [prevChildren];
    }
    var nextChildren = nextProps.children || [];
    if (!Array.isArray(nextChildren)) {
      nextChildren = [nextChildren];
    }
    
    var prevRenderedChildren = this.renderedChildren;
    var nextRenderedChildren = [];

    
    var operationQueue = [];

    
    
    

    for (var i = 0; i < nextChildren.length; i++) {
      
      var prevChild = prevRenderedChildren[i];

      
      
      
      if (!prevChild) {
        var nextChild = instantiateComponent(nextChildren[i]);
        var node = nextChild.mount();

        
        operationQueue.push({type: 'ADD', node});
        nextRenderedChildren.push(nextChild);
        continue;
      }

      
      
      
      var canUpdate = prevChildren[i].type === nextChildren[i].type;

      
      
      if (!canUpdate) {
        var prevNode = prevChild.getHostNode();
        prevChild.unmount();

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

        
        operationQueue.push({type: 'REPLACE', prevNode, nextNode});
        nextRenderedChildren.push(nextChild);
        continue;
      }

      
      
      prevChild.receive(nextChildren[i]);
      nextRenderedChildren.push(prevChild);
    }

    
    for (var j = nextChildren.length; j < prevChildren.length; j++) {
      var prevChild = prevRenderedChildren[j];
      var node = prevChild.getHostNode();
      prevChild.unmount();

      
      operationQueue.push({type: 'REMOVE', node});
    }

    
    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:

    

    
    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.

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) {
  
  if (containerNode.firstChild) {
    var prevNode = containerNode.firstChild;
    var prevRootComponent = prevNode._internalInstance;
    var prevElement = prevRootComponent.currentElement;

    
    if (prevElement.type === element.type) {
      prevRootComponent.receive(element);
      return;
    }

    
    unmountTree(containerNode);
  }

  

}

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

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

mountTree(<App />, rootEl);

mountTree(<App />, rootEl);

These are the basics of how React works internally.

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.

• 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.

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.

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

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