UI updating, in its essential, is data change. React offers a straightforward and intuitive way to program front-end Apps as all the moving parts are converged in the form of states. Code review of Apps made with React is a bit more easier to me as I like to start with data structures for a rough expectation of the functionalities and processing logic. From time to time, I was curious about how React works internally, hence this article. Moreover, I think it never hurts to have a deeper understanding down the stack, as it gives me more freedom when I need a new feature, more confidence when I want to contribute and more comfort when I upgrade.

This article will start walking through one of the critical paths of React by rendering a simple component, i.e., a <h1>. Other topics (e.g., Composite components rendering, state driven UI updating and components life cycle) will be discussed in a similar actionable manner in the following articles.

**_Files used in this article:_**  
**isomorphic/React.js**: entry point of ReactElement.createElement()  
**isomorphic/classic/element/ReactElement.js**: workhorse of ReactElement.createElement()  
**renderers/dom/ReactDOM.js**: entry point of ReactDOM.render()  
**renderers/dom/client/ReactMount.js**: workhorse of ReactDom.render()  
**renderers/shared/stack/reconciler/instantiateReactComponent.js**: create different types of ReactComponents based on element type  
**renderers/shared/stack/reconciler/ReactCompositeComponent.js**: ReactComponents wrapper of root element

**_Tags used in the call stack:_**  
\- function call  
\= alias  
~ indirect function call

As the locations of source code files can not be obviously derived from import statement in the [flat module tree](http://thomasboyt.github.io/2014/10/03/react-build.html), I will use @ to help locating the code snippet in demonstration.

Last words, this series is based on React 15.6.2.

### From JSX to \`React.createElement()\`

> After using the function intensively in a React project for a couple of months, I did not even know the existence of this React.createElement() because it is masked by JSX from a developer’s point of view.

In compiling time, components defined in JSX is translated by Babel to React.createElement() called with appropriate parameters. For instance, the default App.js shipped with [create-react-app](https://github.com/facebookincubator/create-react-app#getting-started):

    import React, { Component } from ‘react’;
    import logo from ‘./logo.svg’;
    import ‘./App.css’;

    class App extends Component {
      render() {
        return (
          <div className=”App”>
            <header className=”App-header”>
              <img src={logo} className=”App-logo” alt=”logo” />
              <h1 className=”App-title”>Welcome to React</h1>
            </header>
            <p className=”App-intro”>
              To get started, edit <code>src/App.js</code> and save to reload.
            </p>
          </div>
        );
      }
    }

    export default App;

is compiled to:

    import React, { Component } from ‘react’;
    import logo from ‘./logo.svg’;
    import ‘./App.css’;

    class App extends Component {
      render() {
        return React.createElement(
          ‘div’,
          { className: ‘App’ },
          React.createElement(
            ‘header’,
            { className: ‘App-header’ },
            React.createElement(‘img’, { src: logo, className: ‘App-logo’, alt: ‘logo’ }),
            React.createElement(
              ‘h1’,
              { className: ‘App-title’ },
              ‘Welcome to React’
            )
          ),
          React.createElement(
            ‘p’,
            { className: ‘App-intro’ },
            ‘To get started, edit ‘,
            React.createElement(
              ‘code’,
              null,
              ‘src/App.js’
            ),
            ‘ and save to reload.’
          )
        );
      }
    }

    export default App;

which is the real code executed by a browser. The code above shows a definition of a composite component App, in which JSX, a syntax of interweaving HTML tag in JavaScript code (e.g., <div className=”App”></div>), is translated to React.createElement() calls.

This component will be rendered like this:

    ReactDOM.render(
      <App />,
      document.getElementById(‘root’)
    );

normally by a file named as “index.js”.

This nested components tree is a bit too complicated to be an ideal start point, so let’s forget about the code above for now look at something simpler - that renders a simple HTML element.

    …
    ReactDOM.render(
      <h1 style={{“color”:”blue”}}>hello world</h1>,
      document.getElementById(‘root’)
    );
    …

babeled version

    …
    ReactDOM.render(React.createElement(
      ‘h1’,
      { style: { “color”: “blue” } },
      ‘hello world’
    ), document.getElementById(‘root’));
    …

### \`React.createElement()\` — creating a \`ReactElement\`

The first step does not do much really. It simply constructs an ReactElement instance populated with whatever passed down to the call stack. The data structure is as following:

![](https://hackernoon.com/hn-images/1*E7Qtvd-UlIHE66YeGL6NqQ.png)

The call stack to achieve the purpose of this step is as following:

    React.createElement
    |=ReactElement.createElement(type, config, children)
       |-ReactElement(type,…, props)

1\. React.createElement(type, config, children) is merely an alias of ReactElement.createElement();

    …
    var createElement = ReactElement.createElement;
    …
    var React = {
    …
      createElement: createElement,
    …
    };

    module.exports = React;

    React@isomorphic/React.js

2\. ReactElement.createElement(type, config, children) 1) copies the elements in config to props, 2) copies the children to props.children and 3) copies the type.defaultProps to props;

    …
      // 1)
      if (config != null) {
        …extracting not interesting properties from config…
        // Remaining properties are added to a new props object
        for (propName in config) {
          if (
            hasOwnProperty.call(config, propName) &&
            !RESERVED_PROPS.hasOwnProperty(propName)
          ) {
            props[propName] = config[propName];
          }
        }
      }

      // 2)
      // Children can be more than one argument, and those are transferred onto
      // the newly allocated props object.
      var childrenLength = arguments.length — 2;
      if (childrenLength === 1) {
        props.children = children; // scr: one child is stored as object
      } else if (childrenLength > 1) {
        var childArray = Array(childrenLength);
        for (var i = 0; i < childrenLength; i++) {
          childArray[i] = arguments[i + 2]; // scr: multiple children are stored as array
        }

        props.children = childArray;
      }

      // 3)
      // Resolve default props
      if (type && type.defaultProps) {
        var defaultProps = type.defaultProps;
        for (propName in defaultProps) {
          if (props[propName] === undefined) {
            props[propName] = defaultProps[propName];
          }
        }
      }

      return ReactElement(
        type,
        key,
        ref,
        self,
        source,
        ReactCurrentOwner.current,
        props,
      );
    …

    ReactElement.createElement@isomorphic/classic/element/ReactElement.js

3\. Then ReactElement(type,…, props) copies the type and props as they are to ReactElement and returns the instance.

    …
    var ReactElement = function(type, key, ref, self, source, owner, props) {
      // This tag allow us to uniquely identify this as a React Element
      $$typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
      type: // scr: --------------> ‘h1’
      key:  // scr: --------------> not of interest for now
      ref:  // scr: --------------> not of interest for now
      props: {
        children:     // scr: --------------> ‘hello world’
        …other props: // scr: --------------> style: { “color”: “blue” }
      },

    // Record the component responsible for creating this element.
      _owner:  // scr: --------------> null
    };
    …

    ReactElement@isomorphic/classic/element/ReactElement.js

The fields populated in the newly constructed ReactElement will be used directly by ReactMount.instantiateReactComponent(), which will be explained in detail very soon. Note that the next step will also create a ReactElement object with ReactElement.createElement(), so I will call the ReactElement object of this phase ReactElement\[1\].

### \`ReactDom.render()\` — and render it

#### \`\_renderSubtreeIntoContainer()\` — attach \`TopLevelWrapper\` to the \`ReactElement\[1\]\`

The purpose of the next step is to wrap the ReactElement\[1\] with another ReactElement (we call the instance a \[2\]) and set the ReactElement.type with TopLevelWrapper. The name TopLevelWrapper explains what it does — wrap the top level element (of the DOM hierarchy passed through render()):

![](https://hackernoon.com/hn-images/1*QxlGZWW5gXB-ioJaLCftog.png)

An important definition here is that of TopLevelWrapper:

    …
    var TopLevelWrapper = function() {
      this.rootID = topLevelRootCounter++;
    };
    TopLevelWrapper.prototype.isReactComponent = {};
    TopLevelWrapper.prototype.render = function() { 
    // scr: this function will be used to strip the wrapper later in the // rendering process

      return this.props.child;
    };
    TopLevelWrapper.isReactTopLevelWrapper = true;
    …

    TopLevelWrapper@renderers/dom/client/ReactMount.js

Please note that the entity assigned to ReactElement.type is a type (TopLevelWrapper) which will be instantiated in the following rendering steps. Then ReactElement\[1\] will be extracted from this.props.child.

The call stack to construct the designated object is as following:

    ReactDOM.render
    |=ReactMount.render(nextElement, container, callback)
    |=ReactMount._renderSubtreeIntoContainer(
       parentComponent, // scr: --------------> null
       nextElement,     // scr: -------------->  ReactElement[1]
       container,// scr: --------------> document.getElementById(‘root’)
       callback’ // scr: --------------> undefined
    )

For initial rendering, ReactMount.\_renderSubtreeIntoContainer() is simpler than it seems to be, in fact, most branches (for UI updating) in this function are skipped. The only line that is effective before the logic processes to next step is

    …
      var nextWrappedElement = React.createElement(TopLevelWrapper, {
        child: nextElement,
      });
    …

    _renderSubtreeIntoContainer@renderers/dom/client/ReactMount.js

Now that it should be easy to see how the target object of this step is constructed I will not repeat React.createElement.

#### \`instantiateReactComponent()\` — create a \`ReactCompositeComponent\` using \`ReactElement\[2\]\`

This is step is to create an primitive ReactCompositeComponent for the top level component:

![](https://hackernoon.com/hn-images/1*wvGh2rtNRAlxHxWsMEqq9Q.png)

The call stack of this step is:

    ReactDOM.render
    |=ReactMount.render(nextElement, container, callback)
    |=ReactMount._renderSubtreeIntoContainer()
      |-ReactMount._renderNewRootComponent(
          nextWrappedElement, // scr: ------> ReactElement[2]
          container, // scr: ------> document.getElementById(‘root’)
          shouldReuseMarkup, // scr: null from ReactDom.render()
          nextContext, // scr: emptyObject from ReactDom.render()
        )
        |-instantiateReactComponent(
            node, // scr: ------> ReactElement[2]
            shouldHaveDebugID /* false */
          )
          |-ReactCompositeComponentWrapper(
              element // scr: ------> ReactElement[2]
          );
          |=ReactCompositeComponent.construct(element)

instantiateReactComponent is the only function that is long enough to discuss here. In our context, it check the ReactElement\[2\]’s type (i.e., TopLevelWrapper) and create a ReactCompositeComponent accordingly.

    function instantiateReactComponent(node, shouldHaveDebugID) {
      var instance;
    …
      } else if (typeof node === ‘object’) {
        var element = node;
        var type = element.type;
    …

        // Special case string values
        if (typeof element.type === ‘string’) {
    …
        } else if (isInternalComponentType(element.type)) {
    …
        } else {
          instance = new ReactCompositeComponentWrapper(element);
        }
      } else if (typeof node === ‘string’ || typeof node === ‘number’) {
    …
      } else {
    …
      }

    …
      return instance;
    }

    instantiateReactComponent@renderers/shared/stack/reconciler/instantiateReactComponent.js

One thing worth noting here is that new ReactCompositeComponentWrapper() is a direct call of ReactCompositeComponent constructor:

    …
    // To avoid a cyclic dependency, we create the final class in this module
    var ReactCompositeComponentWrapper = function(element) {
      this.construct(element);
    };
    …

    …
    Object.assign(
      ReactCompositeComponentWrapper.prototype,
      ReactCompositeComponent,
      {
        _instantiateReactComponent: instantiateReactComponent,
      },
    );
    …

    ReactCompositeComponentWrapper@renderers/shared/stack/reconciler/instantiateReactComponent.js

Then the real constructor get called:

    construct: function(element /* scr: ------> ReactElement[2] */) {
      this._currentElement = element;
      this._rootNodeID = 0;
      this._compositeType = null;
      this._instance = null;
      this._hostParent = null;
      this._hostContainerInfo = null;

    // See ReactUpdateQueue
      this._updateBatchNumber = null;
      this._pendingElement = null;
      this._pendingStateQueue = null;
      this._pendingReplaceState = false;
      this._pendingForceUpdate = false;

    this._renderedNodeType = null;
      this._renderedComponent = null;
      this._context = null;
      this._mountOrder = 0;
      this._topLevelWrapper = null;

    // See ReactUpdates and ReactUpdateQueue.
      this._pendingCallbacks = null;

    // ComponentWillUnmount shall only be called once
      this._calledComponentWillUnmount = false;
    },

    ReactCompositeComponent@renderers/shared/stack/reconciler/ReactCompositeComponent.js

The following steps will also create ReactCompositeComponent objects with instantiateReactComponent(), so I will call the object of this phase ReactCompositeComponent\[T\] (T for top).

After the ReactCompositeComponent object is constructed, the next step is to call batchedMountComponentIntoNode and initialize the component instance and mount it, which will be discussed in detail in the next article.

If you like this read please clap for it or follow me on Medium. Thanks, and hope to see you the next time.

_Originally published at_ [_holmeshe.me_](http://holmeshe.me/understanding-react-js-source-code-initial-rendering-I/)_._

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