Understanding The React Source Code - Initial Rendering (Class Component) V

Understanding The React Source Code

Last time we completed the upper half of the class component rendering logic which is similar to albeit different from simple component rendering in the following respects: 1) it instantiates one additional ReactCompositeComponent to represent the class component (App); and 2) it calls App.render() that triggers cascading React.createElement()s to establish a ReactElement tree.

This time we are going to explore more branches in lower half by examining how the ReactElements in the tree is transformed to their respective ReactDOMComponents, and eventually, to real DOM objects.

Files used in this article:

renderers/dom/shared/ReactDOMComponent.js: offers the one of the methods that this post focuses on, _createInitialChildren

renderers/dom/client/utils/setTextContent.js: DOM operation, setting text

renderers/dom/client/utils/DOMLazyTree.js: DOM operation, appending child

renderers/shared/stack/reconciler/ReactMultiChild.js: the intermediate method to traverseAllChildren as well as the other method in focus, mountChildren()

shared/utils/traverseAllChildren.js: method that iterates direct sub ReactElements and instantiates their respective ReactDOMComponents

Notations used in the call stack:
↻ loop
? condition

I use {} to reference the previous post that is relevant to the methods (or logic process) being discussed.

The process discussed in this post occurs majorly within ReactDOMComponent[6].mountComponent(). The major task of this method, that derives a DOM object from ReactDOMComponent[6], is covered in {post three}. This task is numbered as 0) for later reference.

In this post, we are going to address one of the methods that we overlooked on purpose last time, _createInitialChildren which is used for handling the newly introduced ReactElement tree as the class component’s children. It was used in a niche case, a text child, in {post three *7} and only a side branch was triggered. This side branch, as well as the entire method will be discussed in detail in this post.

_createInitialChildren is our protagonist today; please search *7 in post three if you want to check its role in the simple component rendering. The other overlooked method _updateDOMProperties in {post three *6} will be discussed in later articles.

To be more specific, this method 1) transforms ReactElements to their corresponding ReactDOMComonents; 2) (recursively) calls ReactDOMComponent[*].mountComponent() to create the DOM objects; and 3) appends them to the root DOM node that is created in 0).

So firstly let’s recap step 0) in the context of class component.

ReactDOMComponent[6].mountComponent() (before _createInitialChildren)—create the DOM element[6]

time-saving hint: this paragraph is here to keep the post self-contained, the detail of the ReactDOMComponent creation process has been covered in {post three}

Designated data structure:

Call stack in action:

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...
|~mountComponentIntoNode()                                    |
  |-ReactReconciler.mountComponent()                          |
    |-ReactCompositeComponent[T].mountComponent()             |
      |-ReactCompositeComponent[T].performInitialMount()  upper half
        |-ReactReconciler.mountComponent()                    |
          |-ReactCompositeComponent[ins].mountComponent()     |
            |-this.performInitialMount()                      |
              |-this._renderValidatedComponent()              |
              |-instantiateReactComponent()                  _|_ 
                (we are here)                                 |
              |-ReactDOMComponent[6].mountComponent(          |
                  transaction, // scr: -----> not of interest |
                  hostParent,  // scr: -----> null            |
                  hostContainerInfo,// scr:---------------------> ReactDOMContainerInfo[ins]                                lower half
                  context      // scr: -----> not of interest |
                )                                             |
...

This step creates a DOM object with ReactDOMComponent[6], and set up its attributes.

To recap: it 1) initializes properties of ReactDOMComponent[6]; 2) creates a div DOM element using document.createElement(); 3) creates a double link between ReactDOMComponent[6] and the DOM object; 4) & 5) set the properties and attributes of the newly created DOM object; and 6) embeds the DOM object in DOMLazyTree[1].

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mountComponent: function (
  transaction,
  hostParent,
  hostContainerInfo,
  context
) {

// scr: --------------------------------------------------------> 1)
  this._rootNodeID = globalIdCounter++;
  this._domID = hostContainerInfo._idCounter++;
  this._hostParent = hostParent;
  this._hostContainerInfo = hostContainerInfo; // scr: ------------> ReactDOMContainerInfo[ins]

  var props = this._currentElement.props;

  switch (this._tag) { // scr: ---> no condition is met here
...
  }

... // scr: -----> sanity check

// We create tags in the namespace of their parent container, except HTML
// tags get no namespace.
  var namespaceURI;
  var parentTag;

  if (hostParent != null) { // scr: -----> it is null
...
  } else if (hostContainerInfo._tag) {
    namespaceURI = hostContainerInfo._namespaceURI; // scr: -------> "http://www.w3.org/1999/xhtml"
    parentTag = hostContainerInfo._tag;        // scr: ------> "div"
  }
  if (namespaceURI == null || 
      namespaceURI === DOMNamespaces.svg && 
      parentTag === 'foreignobject'
  ) { // scr: -----> no
...
  }

  if (namespaceURI === DOMNamespaces.html) {
    if (this._tag === 'svg') {               // scr: -----> no
...
    } else if (this._tag === 'math') {       // scr: -----> no
...
    }
  }

  this._namespaceURI = namespaceURI;  // scr: ---------------------> "http://www.w3.org/1999/xhtml"

... // scr: ------> DEV code

  var mountImage;

  if (transaction.useCreateElement) { // scr: ---------------------> transaction related logic, we assume it is true
    var ownerDocument = hostContainerInfo._ownerDocument;
    var el;

    if (namespaceURI === DOMNamespaces.html) {
      if (this._tag === 'script') {         // scr: -----> no
...
      } else if (props.is) {                // scr: -----> no
...
      } else {
        // Separate else branch instead of using `props.is || undefined` above becuase of a Firefox bug.
        // See discussion in https://github.com/facebook/react/pull/6896
        // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240

// scr: --------------------------------------------------------> 2)
        // scr: ---------> HTML DOM API
        el = ownerDocument.createElement(this._currentElement.type);
      }
    } else { // scr: ------> no
...
    }

// scr: --------------------------------------------------------> 3)
    ReactDOMComponentTree.precacheNode(this, el); // scr: --------> doubly link (._hostNode & .internalInstanceKey)
    this._flags |= Flags.hasCachedChildNodes; // scr: ------------>
bit wise its flags

// scr: --------------------------------------------------------> 4)
    if (!this._hostParent) { // scr: ------> it is the root element
      DOMPropertyOperations.setAttributeForRoot(el); // scr: -----> data-reactroot
    }

// scr: --------------------------------------------------------> 5)
    this._updateDOMProperties( //*6
      null,
      props,
      transaction
    ); // scr: --------------------------> style:{ “color”: “blue” }

// scr: --------------------------------------------------------> 6)
    var lazyTree = DOMLazyTree(el); // scr: ------> DOMLazyTree[ins]
    this._createInitialChildren(transaction, props, context, lazyTree);
...
  } // if (transaction.useCreateElement)

  return mountImage;
}

ReactDOMComponent@renderers/dom/shared/ReactDOMComponent.js

ReactDOMComponent[6] gets its DOM node, its children are next in the line

ReactDOMComponent[6]._createInitialChildren() —create DOM elements[2-5]

Designated data structure:

As mentioned before, this method was used to create the string child node (‘hello world’) in {post three}. We will reuse this side branch in this post when rendering similar nodes (i.e., [3] and [5]), and we name the branch {1}.

In the case of class component, route {2} is hit when this method is accessed the first time. To be specific, this branch handles the ReactElement tree. As mentioned, it 1) transforms ReactElements to ReactDOMComponents (a), generates DOM nodes with ReactDOMComponents (b), and 2) insert DOM nodes to the root node generated with ReactDOMComponent[6] in last step.

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_createInitialChildren: function (
  transaction, // scr: not of interest
  props,       // scr: -------------------> ReactElement[6].props
  context,     // scr: not of interest
  lazyTree     // scr: -------------------> DOMLazyTree[ins]
) {
  // Intentional use of != to avoid catching zero/false.
  // scr: it is named as 'dangerous', let's avoid touching it
  var innerHTML = props.dangerouslySetInnerHTML;
  if (innerHTML != null) { // scr: so no innerHTML
...
  } else {
    var contentToUse = CONTENT_TYPES[typeof props.children] ? props.children : null;

    var childrenToUse = contentToUse != null ? null : props.children;
    // scr: some comments
    if (contentToUse != null) {
      // scr: some comments
      if (contentToUse !== '') { // scr: ----------------> route {1}
...// scr: DEV code
        DOMLazyTree.queueText(lazyTree, contentToUse);
      }
    } else if (childrenToUse != null) { // scr: ---------> route {2}
      var mountImages = this.mountChildren(childrenToUse, transaction, context);  // scr: --------------------------------> 1)
      for (var i = 0; i < mountImages.length; i++) { scr: ------> 2)
        DOMLazyTree.queueChild(lazyTree, mountImages[i]);
      }
    }
  }
},

ReactDOMComponent@renderers/dom/shared/ReactDOMComponent.js

The call hierarchy and iteration is a bit complex from now on, so this time I’ll first establish an overview of the big picture before diving into any detail.

The static call stack:

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...                                            (outer recursion)
ReactDOMComponent[6].mountComponent()    <-------------------------|
    (we are here)                                                  |
  |-this._createInitialChildren()                                  |
  ?{1}                                                             |
    |-DOMLazyTree.queueText()                                      |
  ?{2}                                                             |
    |-this.mountChildren()        // scr: ---------------> 1)(a)   |
      |-this._reconcilerInstantiateChildren()                      |
        |-ReactChildReconciler.instantiateChildren()               |
          |-traverseAllChildren()                                  |
            |-traverseAllChildrenImpl()  <------|inner             |
              |↻traverseAllChildrenImpl() ------|recursion         |
                |-instantiateChild()                               |
                  |-instantiateReactComponent()                    |
      |↻ReactDOMComponent.mountComponent()      // scr: -> 1)(b)---|
    |↻DOMLazyTree.queueChild()    // scr: ---------------> 2)
...

First we examine the bottom of the (complex) stack that operates on the DOM level (by understanding the end purpose, we can be a bit relieved in facing the complex call graph).

DOMLazyTree.queueText() and DOMLazyTree.queueChild()

DOMLazyTree.queueText() has only one effective line in this walk-through:

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function queueText(tree, text) {
  if (enableLazy) { // scr: NO, I mean, false
...
  } else {
    setTextContent(tree.node, text);
  }
}

queueText@renderers/dom/client/utils/DOMLazyTree.js

var setTextContent = function (node, text) {
  if (text) {
    var firstChild = node.firstChild;

  if (firstChild && firstChild === node.lastChild && firstChild.nodeType === 3) { // scr: false
...
    }
  }
  node.textContent = text; // scr: the only effective line
};

setTextContent@renderers/dom/client/utils/setTextContent.js

Node.textContent is a DOM standard property that represents, well, text content of a node. Apparently, it is the end purpose of route {1}.

DOMLazyTree.queueChild() has one line as well:

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function queueChild(parentTree, childTree) {
  if (enableLazy) { // scr: again, false
...
  } else {
    parentTree.node.appendChild(childTree.node);
  }
}

queueChild@renderers/dom/client/utils/DOMLazyTree.js

Here Node.appendChild() is yet another DOM standard API that inserts a node to another as child. Apparently, it is the last stop of route {2}.

Now we can replace these two methods with their respective essence line.

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...                                          (outer recursion)
ReactDOMComponent[6].mountComponent()    <-------------------------|
  |-this._createInitialChildren()                                  |
  ?{1}                                                             |
    |-node.textContent = text;                                     |
  ?{2}                                                             |
    |-this.mountChildren()        // scr: ---------------> 1)(a)   |
      |-this._reconcilerInstantiateChildren()                      |
        |-ReactChildReconciler.instantiateChildren()               |
          |-traverseAllChildren()                                  |
            |-traverseAllChildrenImpl()  <------|inner             |
              |↻traverseAllChildrenImpl() ------|recursion         |
                |-instantiateChild()                               |
                  |-instantiateReactComponent()                    |
      |↻ReactDOMComponent.mountComponent() // scr: ------> 1)(b)---|
    |↻node.appendChild()                   // scr: ------> 2)
...

Extrapolate the big picture

To do so, we start from methods that we already know.

First, instantiateReactComponent() which instantiates a ReactDOMComponent from ReactElement (we do not have any “composite” ReactElement in the tree now so all the components being created are ReactDOMComponents), which is also the end of the deeply nested call hierarchy {post two}

Second, ReactDOMComponent.mountComponent() which initializes the ReactDOMComponents created in the previous step and create the corresponding DOM nodes based on them. {post three} & {beginning}

Considering the above two operations an {OG} (operation group), it is now easier to explain how the rest of the ReactElement tree is processed.

Here is a high-level explanation:

  1. when outer recursion of ReactDOMComponent.mountComponent() is called for non-leaf node, branch {2} will be taken to trigger {OG} for each of the component’s children;

  2. when outer recursion of ReactDOMComponent.mountComponent() is called for leaf node that contains text, branch {1} will be in action, which set node.textContent;

  3. when outer recursion of ReactDOMComponent.mountComponent() is called for leaf node that does not contain text, _createInitialChildren() will not be called at all.

Please note that in this process ReactDOMComponent.mountComponent() is used repeatedly to create DOM node for the respective ReactDOMComponent instance, so you might need to check its implementation in the beginning of this text if it is not in your (brain) cache.

It’s time to draw the call stack in action:

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...
ReactDOMComponent[6].mountComponent()
  |-this._createInitialChildren()
    |-this.mountChildren() 
...           |↻instantiateReactComponent()[4,5]
      |-ReactDOMComponent[5].mountComponent()
        |-this._createInitialChildren()
          |-node.textContent = text; // scr: [5] done
      |-ReactDOMComponent[4].mountComponent()
        |-this._createInitialChildren()
          |-this.mountChildren() 
...                 |↻instantiateReactComponent()[2,3]
          |-ReactDOMComponent[2].mountComponent() // scr: [2] done
          |-ReactDOMComponent[3].mountComponent()
            |-this._createInitialChildren()
              |-node.textContent = text; // scr: [3] done
        |↻node[4].appendChild()[2,3] // scr: [4] done

    |↻node[6].appendChild()[4,5] // scr: [6] done
...

In this call stack I omit the deep nested call hierarchy that is used to instantiate the ReactDOMComponent from ReactElement as we are going to examine it next.

The deep nested loop of instantiateReactComponent()

In particular, we need to pay attention to the arguments to keep track the input & output across the fairly deep and complex chain that enlists recursion and callback.

Starting from inside ReactDOMComponent._createInitialChildren():

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var mountImages = this.mountChildren(
  childrenToUse, // scr:----------> ReactElement[6].props.children
  transaction,   // scr: not of interest
  context        // scr: not of interest
);
...

Next we look at the implementation of ReactDOMComponent.mountChildren(). As mentioned before, it 1) instantiates all the children of ReactDOMComponents; and 2) initializes those ReactDOMComponent by calling ReactDOMComponent.mountComponent().

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mountChildren: function (
  nestedChildren, // scr:----------> ReactElement[6].props.children
  transaction,    // scr: not of interest
  context         // scr: not of interest
) {
  
  // scr: ------------------------------------------------------> 1)
  var children = this._reconcilerInstantiateChildren(nestedChildren, transaction, context);
  this._renderedChildren = children;
  var mountImages = [];
  var index = 0;
  for (var name in children) {
    if (children.hasOwnProperty(name)) {
      var child = children[name];
      var selfDebugID = 0;
...// scr: DEV code
                                                   (outer recursion)      
      // scr: --------------------------------------------------> 2)
      var mountImage = ReactReconciler.mountComponent(child, transaction, this, this._hostContainerInfo, context, selfDebugID);
      
      child._mountIndex = index++;
      mountImages.push(mountImage);
    }
  }
...// scr: DEV code
  return mountImages;
},

ReactDOMComponent@renderers/dom/shared/ReactDOMComponent.js

2) was referred to as “outer recursion” before and is yet another ReactReconciler.mountComponent() {post two}, so we focus on 1)

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_reconcilerInstantiateChildren: function (
  nestedChildren, // scr:----------> ReactElement[6].props.children
  transaction,    // scr: not of interest
  context         // scr: not of interest
) {
...// scr: DEV code
  return ReactChildReconciler.instantiateChildren(nestedChildren, transaction, context);
},

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

which is a direct call of

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instantiateChildren: function (
  nestedChildNodes, // scr: --------> ReactElement[6].props.children
  transaction,      // scr: not of interest
  context,          // scr: not of interest
  selfDebugID
) // 0 in production and for roots {
  if (nestedChildNodes == null) {
    return null;
  }
  var childInstances = {};
  if (process.env.NODE_ENV !== 'production') {
...// scr: DEV code
  } else {
    traverseAllChildren(nestedChildNodes, instantiateChild, childInstances);
  }
  return childInstances;
},

instantiateChildren@renderers/shared/stack/reconciler/ReactChildReconciler.js

which is, again, a direct call to traverseAllChildren(). Note that instantiateChild is the callback method which is invoked for each child.

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function instantiateChild(
  childInstances, // scr: ---> the output parameter childInstances is passed all the way down here
  child,      // scr: --> a ReactElement
  name,       // scr: --> unique name for indexing in childInstances
  selfDebugID // scr: --> undefined
) {
... // scr: DEV code
  }
  if (child != null && keyUnique) {
    childInstances[name] = instantiateReactComponent(child, true);
  }
}

instantiateChild@renderers/shared/stack/reconciler/ReactChildReconciler.js

It in turn calls instantiateReactComponent() {post one} directly.

We move on to traverseAllChildren()

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function traverseAllChildren(
  children, // scr: ---------> ReactElement[6].props.children
  callback, // scr: ---------> instantiateChild
  traverseContext // scr: ---> output parameter, initialized as {}
) {
  if (children == null) {
    return 0;
  }
  return traverseAllChildrenImpl(children, '', callback, traverseContext);
}

traverseAllChildren@shared/utils/traverseAllChildren.js

yet another direct call to traverseAllChildrenImpl()

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function traverseAllChildrenImpl(
  children,  // scr: ---------> ReactElement[6].props.children
  nameSoFar, // scr: ---------> ''
  callback,  // scr: ---------> instantiateChild
  traverseContext // scr: ---> output parameter, initialized as {}
) {
  var type = typeof children;
  if (type === 'undefined' || type === 'boolean') {
    // All of the above are perceived as null.
    children = null;
  }
// scr: -------------------------------------------------------> {a}
  if (children === null || type === 'string' || type === 'number' || type === 'object' && children.$$typeof === REACT_ELEMENT_TYPE) {
    callback(traverseContext, children,
    // If it's the only child, treat the name as if it was wrapped in an array
    // so that it's consistent if the number of children grows.
    nameSoFar === '' ? SEPARATOR + getComponentKey(children, 0) : nameSoFar);
    return 1;
  }
  var child;
  var nextName;
  var subtreeCount = 0; // Count of children found in the current subtree.
  var nextNamePrefix = nameSoFar === '' ? SEPARATOR : nameSoFar + SUBSEPARATOR;
// scr: -------------------------------------------------------> {b}
  if (Array.isArray(children)) {
    for (var i = 0; i < children.length; i++) {
      child = children[i];
      nextName = nextNamePrefix + getComponentKey(child, i);
      subtreeCount += traverseAllChildrenImpl(child, nextName, callback, traverseContext);
    }
  } else {
... // scr: this branch will not be called here
  }
  return subtreeCount;
}

traverseAllChildrenImpl@shared/utils/traverseAllChildren.js

After those directly one-line calls of another method, traverseAllChildrenImpl() is the true workhouse. this method was also referred to as “inner recursion” soon before.

The logic of traverseAllChildrenImpl() is quite straight forward: when it is called the first time (and the type of children parameter is array), it calls itself for every ReactElement within the array; when it is called successively (children is ReactElement), it invokes the aforementioned callback that internally relies on instantiateReactComponent() {post one} to transform a ReactElement to an empty and uninitialized ReactDOMComonent.

Note that “inner recursion” works on DIRECT children only while the “outer recursion” traverse the ENTIRE ReactElement tree.

After all the ReactElements are transformed to ReactDOMComonents, the output is returned all the way back to ReactDOMComponent.mountChildren() and complete the circle.

To better understand the full circle, you might need to refer to different pieces of the puzzle back and forth, for example, the beginning of this text where ReactDOMComponent.mountComponent() is discussed; the two DOM operations (Node.appendChild, Node.textContent) that define the stack bottom; the discussion of the big picture; as well as this section.

At last, after all the DOM nodes are generated, the logic returns back to ReactReconciler.mountComponent() and the new node tree is inserted to the designated div container. {post three}

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...
|~mountComponentIntoNode()                                    |
  |-ReactReconciler.mountComponent()                          |
    |-ReactCompositeComponent[T].mountComponent()             |
      |-ReactCompositeComponent[T].performInitialMount()  upper half
        |-ReactReconciler.mountComponent()                    |
          |-ReactCompositeComponent[ins].mountComponent()     |
            |-this.performInitialMount()                      |
              |-this._renderValidatedComponent()              |
              |-instantiateReactComponent()                  _|_ 
              |-ReactDOMComponent[6].mountComponent(          |
                  transaction, // scr: -----> not of interest |
                  hostParent,  // scr: -----> null            |
                  hostContainerInfo,// scr:---------------------> ReactDOMContainerInfo[ins]                                    | 
                  context      // scr: -----> not of interest |
                )                                             |
                                                              |
... // the content of this section                        lower half
        |-_mountImageIntoNode()                  (HTML DOM specific)
            markup,    // scr: --> DOMLazyTree[ins]           |
            container, // scr: --> document.getElementById(‘root’)
            wrapperInstance, // scr:----> same                |
            shouldReuseMarkup, // scr:--> same                |
            transaction, // scr: -------> same                |
          )                                                  _|_

That's it. Did I make a serious mistake? or miss out on anything important? Or you simply like the read. Link me on -- I'd be chuffed to hear your feedback.