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Kent C. Dodds - React Performance

Planted April 08, 2020

This series of workshops has been a bit of a marathon. There has been so much content and so much helpful input. Before I started, I think I would have reckoned I was an intermediate React dev - now, I think I am an intermediate React dev.

My understanding of the whole React ecosystem has been picked up, restructured and placed on new and stronger foundations. I have better clarity about the patterns, questions and expectations I should have of React. I’m finding I’m able to troubleshoot my code and am better equipped to know what to search for when I need a solution to a problem. I have primarily been a backend dev, so this has been really a helpful enhancement of my understanding and appreciation of React.

This is workshop 6 and is all about performance. How can we make our applications more performant?

Code splitting

The secret about performance:

“Performance is all about less code - less lines
and less cycles for your browser to loop over that code.”

Super Simple Start to ESModules

We are probably never going to be free of bundlers, we are going to use dynamic module imports. These are available natively in the browser but for more than 100 modules we lose the efficiencies of this approach. Luckily, bundlers do understand this.

To do this with React the syntax is:

const SmileyFace = React.lazy(() => import('./smiley-face'))

We need to wrap the lazy component in <React.Suspense> boundary and give it a fall back.

We can also prefetch either by using a Magic Comment which will instruct Webpack to load the parcel as prefetched code.

The other way, is to use the useEffect() hook to load the component after the page has rendered.

Cool to see the coverage tool in Chrome Devtools - it shows the proportion of JS and CSS that have been downloaded and used on the page.

Frustratingly, when I tried to implement this on a NextJS project, I found that React.Suspense is not yet supported by ReactDOMServer, so this can’t be used currently in an SSR context.

useMemo for expensive calculations

When you have a calculation that is being run in the render cycle, this doesn’t need to run if the calculations inputs haven’t changed.

We have useMemo, to make sure we don’t recalculate when we don’t need to. We pass in dependencies, a lot like the useEffect hook.

function Distance({ x, y }) {
  const distance = React.useMemo(() => calculateDistance(x, y), [x, y])
  return (
      The distance between {x} and {y} is {distance}.

Use Memo

It was hard to see where the performance gains here were before we came back to the full group. I understood the concept but couldn’t prove it in the dev tools - now I can see how it works. If there is a large calculation, implement useMemo.

This is only useful for synchronously calculated values.

React.memo for reducing unnecessary re-renders

React exists in its current form (in large part) because updating the DOM is the slowest part of this process. By separating us from the DOM, React can perform the most surgically optimal updates to the DOM to speed things up for us big-time.

A React Component can re-render for any of the following reasons:

  1. Its props change
  2. Its internal state changes
  3. It is consuming context values which have changed
  4. Its parent re-renders

Window large lists with react-window

If you have a really long list, there is a cost of trying to calculate whether an item needs to be rendered or not. Even if you’re memoizing this is a potential issue.

A well-accepted solution to this is called windowing - you only render what is in view and maybe a bit beyond. Everything else is set with empty divs with the correct height. This makes things way faster!

This was a cool library that works with long lists and grids of data - this stops the creation of the DOM elements until they are in view or close to being in view.

react-window is a good library for this.

Fix “perf death by a thousand cuts”

This was another demonstration on the importance of co-located state. If state isn’t required globally, then we should feel free to co-locate that state.

If not every component needs all of the data, we should separate the context in to separate domains.

Another approach is to create slices of the state and memoizing that. Then, passing the memoized component will stop a re-render when unrelated context is updated.


Optimize context value

Context triggers a re-render every time the value object updates. The problem here is that sometimes the value doesn’t change but is a new instance of that value.

There are a few different ways to optimise this:

  • memoize the value in the context.
  • separate the setting and getting values into separate context that are provided
    and consumed independently.

Production performance monitoring

Time, it was a-moving on - so this was a demo on the usefulness of the React
profiling component.

You can read up on the capabilities of the <React.Profile /> component here:

Here’s a basic usage example:

  <Profiler id="Navigation" onRender={onRenderCallback}>
    <Navigation {...props} />
  <Main {...props} />

The callback is called with the following arguments

function onRenderCallback(
  id, // the "id" prop of the Profiler tree that has just committed
  phase, // either "mount" (if the tree just mounted) or "update" (if it re-rendered)
  actualDuration, // time spent rendering the committed update
  baseDuration, // estimated time to render the entire subtree without memoization
  startTime, // when React began rendering this update
  commitTime, // when React committed this update
  interactions // the Set of interactions belonging to this update
) {
  // Aggregate or log render timings...

It’s important to note that unless you build your app using react-dom/profiling and scheduler/tracing-profiling this component wont do anything.

Kent has written about production performance monitoring here:
“React Production Performance Monitoring”

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