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

#+TITLE: High-Performance React
#+AUTHOR: Thomas Hintz

#+startup: indent
#+tags: noexport sample frontmatter mainmatter backmatter
#+options: toc:nil tags:nil


* Preface
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/preface.markua
  :END:
* Introduction
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/introduction.markua
  :END:
It was the late 90's and I was just a kid visiting my Aunt and Uncle
and their family in Denver. The days were packed with endless playing
and goofing around. I didn't get to see my cousins much and we were
having a good time. But it was the late 90's and the Internet was
booming. And my cousin was in on it.

A "startup", that's what he called it. I didn't understand any of what
he was saying about it. Grown-up stuff. Then he showed us the webpage
for the startup and I thought that was impressive.

"How did you make that"? I asked him. I think he was a little confused
at first about what I was even talking about but he quickly brought me
over to the computer and showed me a screen full of text.

"You just type HTML, that's how you make the webpage." I thought this
was the coolest.

"What do you type that into? What program is it? Can I do that?" He
told me it was easy: just use Notepad. I wasn't going to let him go
without some hook I could grab into this alien world. He told me it's
really easy to learn: do an AOL search for "HTML tutorial".

So began my journey with web development. I AOL searched my way
through as many blinking text tutorials as I could find. It wasn't
long until I was building AJAX. We had IE 5.5 and 6 and Mozilla
Pheonix. And GMail came out. That changed things, now web apps were
"legitimate."

A lot of the technologies and libraries came and went over the years
but one thing remained constant in large web apps: poor
performance. From the very early days I was timing things with my stop
watch. Sometimes things were slow and I had to understand why and how
to fix them. Over the years I learned all about the browser's DOM and
its APIs and how they work. I learned how jQuery worked and
backbone.js and all the rest. I made apps that didn't lag or have
jank.

I was able to do this because I understood the performance
implications of the tools and libraries I was using and I learned how
to measure performance. I had discovered the recipe for
high-performance code.

And that is what this book is: a recipe for producing high-performance
React applications. First, we learn how React works. Then we learn how
to measure performance. And last we learn how to address the
bottlenecks we find. Parts of any technical book will go stale as
technology changes and that is no less true for this book. But what I
hope you learn is not just the technical details but more importantly
the method for writing high-performance code. The API might change but
the method will remain the same.

TODO note that the book references React-DOM but the algorithms should
generally apply to all React implementations.
* Fundamentals: 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.

** React, made of

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 me have to understand its input. The input comes from
~createElement~. While ~createElement~ itself is unlikely to be a
bottleneck it's a 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 the normal way of injecting HTML into the DOM was via
directly utilizing the browser's DOM APIs. 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 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        |

#+BEGIN_SRC
tag name: 'div'
tag prop: 'class'
children: h1..., 'Hello', input...
#+END_SRC

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:

#+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 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 this.

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 our data structure and the
real one that the JSX compiler outputs: 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:

#+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 data-structure and for the
purposes of this book we will use our own simplified data-structure 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 they would behave the same in the ways
that matter to us now.

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 our
JSX-like notation 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 unlikely to be relevant to your application's performance or our
version of React here.

#+BEGIN_SRC javascript
function createElement(node) {
    // an 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 tree of
nodes to actually displaying something on screen?

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

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

Doing the initial render on a tree of elements is quite simple. In
psuedocode 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 we will work on expanding the psuedocode until we build
our own fully functional ~render~ method using the same general
algorithm React uses. Next we will focus on the initial render and
ignore reconciliation.

#+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

We begin 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 they keys setting each property
directly. Then we render each of the children by looping over the
children recursively calling ~render~ on each 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 VDOM or
virtual DOM and the current render 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 and in a lot of ways is that makes React, React.

Conceptually the way this works is that React generates a new element
tree for every render and compares to the newly generated tree to the
tree generated on the previous render. Where it finds differences in
the tree 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 still runs in
O(n^3). This leads to the largest performance related aspect in all of
React.

Since an O(n^3) algorithm isn't going to cut it 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.

According to the 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. Later on we will discuss and implement the ~key~ prop.

TODO https://grfia.dlsi.ua.es/ml/algorithms/references/editsurvey_bille.pdf

The approach we will take here is to integrate the heuristics that
React uses into our render method. This is 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. If you could
only take away one thing from this book it would that.

Here is a more in depth look at the algorithm we will be implementing:

#+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. While its possible that the children will be
able to reuse their associated DOM elements, ~render~ will still be
run on them.

Now, to get started with our render method we must make some
modifications to our previous render methods. 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 need to
re-render something or 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 elements 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

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~. ~preElement~ 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 to the tree than
~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 can'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
swapped out every time we render, only the elements that change
are. When a new DOM element is appended to the tree React would invoke
~componentDidMount~.

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. In React, before removing the element, it invoke
~componentWillUnmount~.

#+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~.

#+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.

#+NAME: Setting DOM Props
#+BEGIN_SRC javascript
function setDOMProps(element, domElement, prevElement) {
    if (prevElement) {
        Object.keys(prevElement.props)
            .filter((key) => key !== 'children')
            .forEach((key) => {
                domElement[key] = '';
            });
    }
    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.
#+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 though.
#+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 current children 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 a list index. 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.

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 are
invoked. And invoking those lifecycle methods, and the whole process
of tearing down and building up a component is expensive. So again you
can see how a bad key would lead to another performance bottleneck
since React will be doing this on all or many of the elements in a
list frequently.

** Fibers

The actual React implementation used to look very similar to what
we've gone through 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. 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.

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 go in to
this more.

** Conclusion

At this point you should have enough information to begin to diagnose
performance issues and prevent them from occurring in the first
place. 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.

TODO maybe a graphic summarizing the heuristics?

* Rendering Model
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/rendering-model.markua
  :END:
  React calls shouldComponentUpdate to know if it should re-render the
  component. by default it returns true.

  generally use PureComponent/React.memo
* Diagnosing Bottlenecks
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/diagnosing-bottlenecks.markua
  :END:
* Reducing Renders
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/reducing-renders.markua
  :END:
* Improving DOM Merge Performance
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/improving-dom-merge-performance.markua
  :END:
* Reducing Number of Components
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/reducing-number-of-components.markua
  :END:
  higher-order components
* Windowing
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/windowing.markua
  :END:
* Performance Tools
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/performance-tools.markua
  :END:
  trace from scheduler/tracing/profiler component
* JS Performance Tools
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/js-performance-tools.markua
  :END:
* Code Splitting
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/code-splitting.markua
  :END:
  React.lazy, suspense

  use on routes
* Server Side Rendering
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/server-side-rendering.markua
  :END:
* Concurrent Rendering
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/concurrent-rendering.markua
  :END:
* UX
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/ux.markua
  :END:
* JS Service Workers
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/js-service-workers.markua
  :END:
* Keys
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/keys.markua
  :END:
* Reconciliation
  :PROPERTIES:
  :EXPORT_FILE_NAME: manuscript/reconciliation.markua
  :END:
  - diffing algorithm based on heuristics. generic algorithm is O(n^3)
  - "Fiber" algorithm notes
    - lists reordering without key means full list output/update
    - type changes cause full re-render
    - keys should be stable, predictable, unique