foundations-high-performance-react/foundations-high-performance-react.org

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#+TITLE: Foundations of High-Performance React Applications
#+AUTHOR: Thomas Hintz

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#+tags: noexport sample frontmatter mainmatter backmatter
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* Preface :frontmatter:
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/preface.markua
  :END:

Welcome to /Foundations of High-Performance React/ where we build our
own simplified version of React. We will use our React to gain an
understanding of the real React and how to build high-performance
applications with it.

* Introduction :mainmatter:
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/introduction.markua
  :END:

* Foundations: Building our own React
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/fundamentals--building-our-own-react.markua
  :END:
Baking bread. When I first began to learn how to bake bread the recipe
told me what to do. It listed some ingredients and told me how to
combine them and prescribed times of rest. It gave me an oven
temperature and a period of wait. It gave me mediocre bread of wildly
varying quality. I tried different recipes but the result was always
the same.

Understanding: that's what I was missing. The bread I make is now
consistently good. The recipes I use are simpler and only give ratios
and general recommendations for rests and waits. So why does the bread
turn out better?

Before baking is finished bread is a living organism. The way it grows
and develops and flavors depend on what you feed it and how you feed
it and massage it, care for it. If you have it grow and ferment at a
higher temperature and more yeast it overdevelops producing too much
alcohol. If you give it too much time, acidity will take over the
flavor. The recipes I used initially were missing a critical
ingredient: the rising temperature.

But unlike a lot of ingredients: temperature is hard to control for
the home cook. So the recipe can't just tell you exactly what
temperature to grow the bread at. My initial recipes just silently
made assumptions for the temperature, which rarely turn out to be
true. This means that the only way to consistently make good bread is
to have an understanding of how bread develops so that you can adjust
the other ingredients to complement the temperature. Now the bread can
tell me what to do.

While React isn't technically a living organism that can tell us what
to do, it is, in its whole, a complex, abstract entity. We could learn
basic recipes for how to write high-performance React code but they
wouldn't apply in all cases, and as React and things under it change
our recipes would fall out-of-date. So like the bread, to produce
consistently good results we need to understand how React does what it
does.

** Components of React

Conceptually React is very simple. It starts by walking a tree of
components and building up a tree of their output. Then it compares
that tree to the tree currently in the browser's DOM to find any
differences between them. When it finds differences it updates the
browser's DOM to match its internal tree.

But what does that actually look like? If your app is janky does that
explanation point you towards what is wrong? No. It might make you
wonder if maybe it is too expensive to re-render the tree or if maybe
the diffing React does is slow but you won't really know. When I was
initially testing out different bread recipes I had guesses at why it
wasn't working but I didn't really figure it out until I had a deeper
understanding of how making bread worked. It's time we build up our
understanding of how React works so that we can start to answer our
questions with solid answers.

React is centered around the ~render~ method. The ~render~ method is
what walks our trees, diffs them with the browser's DOM tree, and
updates the DOM as needed. But before we can look at the ~render~
method we have to understand its input. The input comes from
~createElement~. While ~createElement~ itself is unlikely to be a
bottleneck it's good to understand how it works so that we can have a
complete picture of the entire process. The more black-boxes we have
in our mental model the harder it will be for us to diagnose
performance problems.

** Markup in JavaScript: ~JSX~
~createElement~, however, takes as input something that is probably
not familiar to us since we usually work in JSX, which is the last
element of the chain in this puzzle and the first step in solving
it. While not strictly a part of React, it is almost universally used
with it. And if we understand it then ~createElement~ will be less of
a mystery since we will be able to connect all the dots.

JSX is not valid HTML or JavaScript but its own language compiled by a
compiler, like Babel. The output of that compilation is valid
JavaScript that represents the original markup.

Before JSX or similar compilers, the normal way of injecting HTML into
the DOM was via directly utilizing the browser's DOM APIs or by
setting ~innerHTML~. This was very cumbersome.  The code's structure
did not match the structure of the HTML that it output which made it
hard to quickly understand what the output of a piece of code would
be. So naturally programmers have been endlessly searching for better
ways to mix HTML with JavaScript.

And this brings us to JSX. It is nothing new; nothing
complicated. Forms of it have been made and used long before React
adopted it. Now let's see if we can discover JSX for ourselves.

To start with, we need to create a data-structure -- let's call it
JavaScript Markup (JSM) -- that both represents a DOM tree and can
also be used to insert one into the browser's DOM. And to do that we
need to understand what a tree of DOM nodes is constructed of. What
parts do you see here?

#+BEGIN_SRC html
<div class="header">
  <h1>Hello</h1>
  <input type="submit" disabled />
</div>
#+END_SRC

I see three parts: the name of the tag, the tag's properties, and its
children.

|-----------+-----------------------------|
| Name:     | 'div', 'h1', 'input'        |
| Props:    | 'class', 'type', 'disabled' |
| Children: | <h1>, <input>, Hello        |

Now how could we recreate that in JavaScript?

In JavaScript, we store lists of things in arrays, and key/value
properties in objects. Luckily for us, JavaScript even gives us literal
syntax for both so we can easily make a compact DOM tree with our own
notation.

This is what I'm thinking:

#+CAPTION: JSM - JavaScript Markup
#+BEGIN_SRC javascript
['div', { 'className': 'header' },
  [['h1', {}, ['Hello']],
   ['input', { 'type': 'submit', 'disabled': 'disabled' }, []]
  ]
]
#+END_SRC

As you can see, we have a clear mapping from our notation, JSM, to the
original HTML. Our tree is made up of three element arrays. The first
item in the array is the tag, the second is an object containing the
tag's properties, and the third is an array of its children; which are
all made up of the same three element arrays.

The truth is though, if you stare at it long enough, although the
mapping is clear, how much fun would it be to read and write that on a
consistent basis? I can assure you, it is rather not fun. But it has
the advantage of being easy to insert into the DOM. All you need to do
is write a simple recursive function that ingests our data structure
and updates the DOM accordingly. We will get back to that.

So now we have a way to represent a tree of nodes and we
(theoretically) have a way to get those nodes into the DOM. But if we
are being honest with ourselves, while functional, it isn't a pretty
notation nor easy to work with.

And this is where our object of study enters the scene. JSX is just a
notation that a compiler takes as input and outputs in its place a
tree of nodes nearly identical to the notation we came up with! And if
you look back to our notation you can see that you can easily embed
arbitrary JavaScript expressions wherever you want in a node. As you
may have realized, that's exactly what the JSX compiler does when it
sees curly braces!

There are three main differences between JSM and the real output of
the JSX compiler: it uses objects instead of arrays, it inserts calls
to React.createElement on children, and spreads the children instead
of containing them in an array. Here is what real JSX compiler output
looks like:

# #+NAME: foo
# #+CAPTION: foo bar
# #+attr_leanpub: :line-numbers true
#+BEGIN_SRC javascript
React.createElement(
  'div',
  { className: 'header' },
  React.createElement('h1', {}, 'Hello'),
  React.createElement(
    'input',
    { type: 'submit', 'disabled': 'disabled' })
);
#+END_SRC

As you can see, it is very similar to our JSM data-structure and for
the purposes of this book we will use JSM, as it's a bit easier to
work with. A JSX compiler also does some validation and escapes input
to prevent cross-site scripting attacks. In practice though, it would
behave the same in our areas of study and we will keep things simple
by leaving those aspects of the JSX compiler out.

So now that we've worked through JSX we're ready to tackle
~createElement~, the next item on our way to building our own React.

** Getting Ready to Render with ~createElement~

React's ~render~ expects to consume a tree of element objects in a
specific, uniform format. ~createElement~ is the method by which we
achieve that objective. ~createElement~ will take as input JSM and
output a tree of objects compatible with ~render~.

React expects nodes defined as JavaScript objects that look like this:

#+BEGIN_SRC javascript
{
    type: NODE_TYPE,
    props: {
         propA: VALUE,
         propB: VALUE,
         ...
         children: STRING | ARRAY
    }
}
#+END_SRC

That is: an object with two properties: ~type~ and ~props~. The
~props~ property contains all the properties of the node. The node's
~children~ are also considered part of its properties. The full
version of React's ~createElement~ includes more properties but they
are not relevant to our study here.

#+BEGIN_SRC javascript
function createElement(node) {
    // if array (not text, number, or other primitive)
    if (typeof node === 'object') {
        const [ tag, props, children ] = node;
        return {
            type: tag,
            props: {
                ...props,
                children: children.map(createElement)
            }
        };
    }

    // primitives like text or number
    return {
        type: 'TEXT',
        props: {
            nodeValue: node,
            children: []
        }
    };
}
#+END_SRC

Our ~createElement~ has two main parts: complex elements and primitive
elements. The first part tests whether ~node~ is a complex node
(specified by an array) and then generates an ~element~ object based
on the input node. It recursively calls ~createElement~ to generate an
array of children elements. If the node is not complex then we
generate an element of type 'TEXT' which we use for all primitives
like strings and numbers. We call the output of ~createElement~ a tree
of ~elements~ (surprise).

That's it. Now we have everything we need to actually begin the
process of rendering our tree to the DOM!

** Render

There are now only two major puzzles remaining in our quest for our
own React. The next piece is: ~render~. How do we go from our JSM tree
of nodes, to actually displaying something on screen? To do that we
will explore the ~render~ method.

The signature for our ~render~ method should be familiar to you:

#+BEGIN_SRC javascript
function render(element, container)
#+END_SRC

This is the same signature as that of React itself. We begin by just
focusing on the initial render. In pseudocode it looks like this:

#+BEGIN_SRC javascript
function render(element, container) {
    const domElement = createDOMElement(element);
    setProps(element, domElement);
    renderChildren(element, domElement);
    container.appendChild(domElement);
#+END_SRC

Our DOM element is created first. Then we set the properties, render
children elements, and finally append the whole tree to the
container.

Now that we have an idea of what to build we will work on expanding
the pseudocode until we have our own fully functional ~render~ method
using the same general algorithm React uses. In our first pass we will
focus on the initial render and ignore reconciliation.

#+BEGIN_NOTE
Reconciliation is basically React's "diffing" algorithm. We will be
exploring it after we work out the initial render.
#+END_NOTE

#+BEGIN_SRC javascript
function render(element, container) {
    const { type, props } = element;

    // create the DOM element
    const domElement = type === 'TEXT' ?
      document.createTextNode(props.nodeValue) :
      document.createElement(type);

    // set its properties
    Object.keys(props)
      .filter((key) => key !== 'children')
      .forEach((key) => domElement[key] = props[key]);

    // render its children
    props.children.forEach((child) => render(child, domElement));

    // add our tree to the DOM!
    container.appendChild(domElement);
}
#+END_SRC

The ~render~ method starts by creating the DOM element. Then we need
to set its properties. To do this we first need to filter out the
~children~ property and then we simply loop over the keys, setting
each property directly. Following that, we render each of the children
by looping over them and recursively calling ~render~ on each child
with the ~container~ set to the current DOM element (which is each
child's parent).

Now we can go all the way from our JSX-like notation to a rendered
tree in the browser's DOM! But so far we can only add things to our
tree. To be able to remove and modify the tree we need one more part:
reconciliation.

** Reconciliation
A tale of two trees. These are the two trees that people most often
talk about when talking about React's "secret sauce": the virtual DOM
and the browser's DOM tree. This idea is what originally set React
apart. React's reconciliation is what allows you to program
declaratively. Reconciliation is what makes it so we no longer have to
manually update and modify the DOM whenever our own internal state
changes. In a lot of ways, it is what makes React, React.

Conceptually, the way this works is that React generates a new element
tree for every render and compares the newly generated tree to the
tree generated on the previous render. Where it finds differences
between the trees it knows to mutate the DOM state. This is the "tree
diffing" algorithm.

Unfortunately, those researching tree diffing in Computer Science have
not yet produced a generic algorithm with sufficient performance for
use in something like React; as the current best algorithm still [[https://grfia.dlsi.ua.es/ml/algorithms/references/editsurvey_bille.pdf][runs
in O(n^3)]].

Since an O(n^3) algorithm isn't going to cut it in the real-world, the
creators of React instead use a set of heuristics to determine what
parts of the tree have changed. Understanding how the React tree
diffing algorithm works in general and the heuristics currently in use
can help immensely in detecting and fixing React performance
bottlenecks. And beyond that it can help one's understanding of some
of React's quirks and usage. Even though this algorithm is internal to
React and can be changed anytime its details have leaked out in some
ways and are overall unlikely to change in major ways without larger
changes to React itself.

According to the [[https://reactjs.org/docs/reconciliation.html][React documentation]] their diffing algorithm is O(n)
and based on two major components:

  - Elements of differing types will yield different trees
  - You can hint at tree changes with the ~key~ prop.

In this section we will focus on the first part: differing types. In a
later chapter we will discuss and implement the ~key~ prop.

The approach we will take here is to integrate the heuristics that
React uses into our ~render~ method. Our implementation will be very
similar to how React itself does it and we will discuss React's actual
implementation later when we talk about Fibers.

Before we get into the code changes that implement the heuristics it
is important to remember that React /only/ looks at an element's type,
existence, and key. It does not do any other diffing. It does not diff
props. It does not diff sub-trees of modified parents.

While keeping that in mind, here is an overview of the algorithm we
will be implementing in the ~render~ method. ~element~ is the element
from the current tree and ~prevElement~ is the corresponding element
in the tree from the previous render.

#+BEGIN_SRC javascript
    if (!element && prevElement)
      // delete dom element
    else if (element && !prevElement)
      // add new dom element, render children
    else if (element.type === prevElement.type)
      // update dom element, render children
    else if (element.type !== prevElement.type)
      // replace dom element, render children
#+END_SRC

Notice that in every case, except deletion, we still call ~render~ on
the element's children. And while it's possible that the children will
have their associated DOM elements reused, their ~render~ methods will
still be invoked.

Now, to get started with our render method we must make some
modifications to our previous render method. First, we need to be able
to store and retrieve the previous render tree. Then we need to add
code to compare parts of the tree to decide if we can re-use DOM
elements from the previous render tree. And last, we need to return a
tree of elements that can be used in the next render as a comparison
and to reference the DOM elements that we create. These new element
objects will have the same structure as our current elements but we
will add two new properties: ~domElement~ and ~parent~. ~domElement~
is the DOM element associated with our synthetic element and ~parent~
is a reference to the parent DOM element.

Here we begin by adding a global object that will store our last render
tree, keyed by the ~container~.

#+BEGIN_SRC javascript
const renderTrees = {};
function render(element, container) {
    const tree =
      render_internal(element, container, renderTrees[container]);
    // render complete, store the updated tree
    renderTrees[container] = tree;
}
#+END_SRC

As you can see, the change we made is to move the core of our
algorithm into a new function called ~render_internal~ and pass in the
result of our last render to ~render_internal~.

Now that we have stored our last render tree we can go ahead and
update our render method with the heuristics for reusing the DOM
elements. We name it ~render_internal~ because it is what controls the
rendering but takes an additional argument now: the
~prevElement~. ~prevElement~ is a reference to the corresponding
~element~ from the previous render and contains a reference to its
associated DOM element and parent DOM element. If it's the first
render or if we are rendering a new node or branch of the tree then
~prevElement~ will be ~undefined~. If, however, ~element~ is
~undefined~ and ~prevElement~ is defined then we know we need to
delete a node that previously existed.

#+BEGIN_SRC javascript
function render_internal(element, container, prevElement) {
    let domElement, children;
    if (!element && prevElement) {
        removeDOMElement(prevElement);
        return;
    } else if (element && !prevElement) {
        domElement = createDOMElement(element);
    } else if (element.type === prevElement.type) {
        domElement = prevElement.domElement;
    } else { // types don't match
        removeDOMElement(prevElement);
        domElement = createDOMElement(element);
    }
    setDOMProps(element, domElement, prevElement);
    children = renderChildren(element, domElement, prevElement);

    if (!prevElement || domElement !== prevElement.domElement) {
        container.appendChild(domElement);
    }

    return {
        domElement: domElement,
        parent: container,
        type: element.type,
        props: {
            ...element.props,
            children: children
        }
    };
}
#+END_SRC

The only time we shouldn't set DOM properties on our element and
render its children is when we are deleting an existing DOM
element. We use this observation to group the calls for ~setDOMProps~
and ~renderChildren~. Choosing when to append a new DOM element to the
container is also part of the heuristics. If we can reuse an existing
DOM element then we do, but if the element type has changed or if
there was no corresponding existing DOM element then and only then do
we append a new DOM element. This ensures the actual DOM tree isn't
being replaced every time we render, only the elements that change are
replaced.

In the real React, when a new DOM element is appended to the DOM tree,
React would invoke ~componentDidMount~ or schedule ~useEffect~.

Next up we'll go through all the auxiliary methods that complete the
implementation.

Removing a DOM element is straightforward; we just ~removeChild~ on
the parent element. Before removing the element, React would invoke
~componentWillUnmount~ and schedule the cleanup function for
~useEffect~.

#+BEGIN_SRC javascript
function removeDOMElement(prevElement) {
    prevElement.parent.removeChild(prevElement.domElement);
}
#+END_SRC

In creating a new DOM element we just need to branch if we are
creating a text element since the browser API differs slightly. We
also populate the text element's value as the API requires the first
argument to be specified even though later on when we set props we
will set it again. This is where React would invoke
~componentWillMount~ or schedule ~useEffect~.

#+BEGIN_SRC javascript
function createDOMElement(element) {
    return element.type === 'TEXT' ?
      document.createTextNode(element.props.nodeValue) :
      document.createElement(element.type);
}
#+END_SRC

To set the props on an element, we first clear all the existing props
and then loop through the current props, setting them accordingly. Of
course, we filter out the ~children~ prop since we use that elsewhere
and it isn't intended to be set directly.

#+BEGIN_SRC javascript
function setDOMProps(element, domElement, prevElement) {
    if (prevElement) {
        Object.keys(prevElement.props)
            .filter((key) => key !== 'children')
            .forEach((key) => {
                domElement[key] = ''; // clear prop
            });
    }
    Object.keys(element.props)
      .filter((key) => key !== 'children')
      .forEach((key) => {
          domElement[key] = element.props[key];
    });
}
#+END_SRC

#+begin_note
React is more intelligent about only updating or removing props that
need to be updated or removed.
#+end_note

#+begin_warning
This algorithm for setting props does not correctly handle events,
which must be treated specially. For this exercise that detail is not
important and we leave it out for simplicity.
#+end_warning

For rendering children we use two loops. The first loop removes any
elements that are no longer being used. This would happen when the
number of children is decreased. The second loop starts at the first
child and then iterates through all of the children of the parent
element, calling ~render_internal~ on each child. When
~render_internal~ is called the corresponding previous element in that
position is passed to ~render_internal~, or ~undefined~ if there is no
corresponding element, like when the list of children has grown.

#+BEGIN_SRC javascript
function renderChildren(element, domElement, prevElement = { props: { children: [] }}) {
    const elementLen = element.props.children.length;
    const prevElementLen = prevElement.props.children.length;
    // remove now unused elements
    for (let i = elementLen; i < prevElementLen - elementLen; i++) {
        removeDOMElement(element.props.children[i]);
    }
    // render existing and new elements
    return element.props.children.map((child, i) => {
        const prevChild = i < prevElementLen ? prevElement.props.children[i] : undefined;
        return render_internal(child, domElement, prevChild);
    });
}
#+END_SRC

It's very important to understand the algorithm used here because this
is essentially what happens in React when incorrect keys are used,
like using a list index for a key. And this is why keys are so
critical to high performance (and correct) React code. For example, in
our algorithm here, if you removed an item from the front of the list
you may cause every element in the list to be created anew in the DOM
if the types no longer match up. Later on, in the chapter on keys, we
will update this algorithm to incorporate keys. It's actually only a
minor difference in determining which ~child~ gets paired with which
~prevChild~. Otherwise this is effectively the same algorithm React
uses when rendering lists of children.

#+CAPTION: Example of ~renderChildren~ 2nd loop when the 1st element has been removed. In this case the trees for all of the children will be torn down and rebuilt.
| i | child Type | prevChild Type |
|---+------------+----------------|
| 0 | span       | div            |
| 1 | input      | span           |
| 2 | -          | input          |

There are a few things to note here. First, it is important to pay
attention to when React will be removing a DOM element from the tree
and adding a new one as this is when the related lifecycle events or
hooks are invoked. And invoking those lifecycle methods or hooks, and
the whole process of tearing down and building up a component is
expensive. So again, if you use a bad key, like the algorithm here
simulates, you'll be hitting a major performance bottleneck since
React will not only be replacing DOM elements in the browser but also
tearing down and rebuilding the trees of child components.

** Fibers

The actual React implementation used to look very similar to what
we've built so far, but with React 16 this has changed dramatically
with the introduction of Fibers. Fibers are a name that React gives to
discrete units of work during the render process. And the React
reconciliation algorithm was changed to be based on small units of
work instead of one large, potentially long-running call to
~render~. This means that React is now able to process just part of
the render phase, pause to let the browser take care of other things,
and resume again. This is the underlying change the enables the
experimental Concurrent Mode as well as running most hooks without
blocking the render.

But even with such a large change, the underlying algorithms for
deciding how and when to render components is the same. And when not
running in Concurrent Mode the effect is still the same as React does
the render phase in one block still. So using a simplified
interpretation that doesn't include all the complexities of breaking
up the process in to chunks enables us to see more clearly how the
process as a whole works. At this point bottlenecks are much more
likely to occur from the underlying algorithms and not from the Fiber
specific details. In the chapter on Concurrent Mode we will learn more
about Fibers.

** Putting it all together

Throughout the rest of the book we will be building on and using our
React implementation so it would be helpful to see it all put together
and working. At this point the only thing left to do is to create some
components and use them!

#+BEGIN_SRC javascript
  const SayNow = ({ dateTime }) => {
      return ['h1', {}, [`It is: ${dateTime}`]];
  };

  const App = () => {
      return ['div', { 'className': 'header' },
              [SayNow({ dateTime: new Date() }),
               ['input', { 'type': 'submit', 'disabled': 'disabled' }, []]
              ]
             ];
  }

  render(createElement(App()), document.getElementById('root'));
#+END_SRC

We are creating two components, that output JSM, as we defined it
earlier. We create one component prop for the ~SayNow~ component:
~dateTime~. It gets passed from the ~App~ component. The ~SayNow~
component prints out the ~DateTime~ passed in to it. You might notice
that we are passing props the same way one does in the real React, and
it just works!

The next step is to call render multiple times.

#+BEGIN_SRC javascript
setInterval(() =>
  render(createElement(App()), document.getElementById('root')),
  1000);
#+END_SRC

If you run the code above you will see the DateTime display being
updated every second. And if you watch in your dev tools or if you
profile the run you will see that the only part of the DOM that gets
updated or replaced is the part that changes (aside from the DOM
props). We now have a working version of our own React.

#+begin_note
This implementation is designed for teaching purposes and has some
known issues and bugs, like always updating the DOM props, along with
other things. Fundamentally, it functions the same as React but if you
wanted to use it in a more production setting it would take a lot more
development.
#+end_note

** Conclusion

Of course our version of React elides over many details that React
must contend with, like starting a re-render from where state changes
and event handlers. For understanding how to build high-performance
React applications, however, the most important piece to understand is
how and when React renders components, which is what we have learned
in creating our own mini version of React.

At this point you should have an understanding of how React works. In
the rest of the book we are going to be refining this model and
looking at practical applications of it so that we are prepared to
build high performance React applications and diagnose any
bottlenecks.