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React 18 features

What's new in React 18

Concurrent React

Many of the features in React 18 are built on top of the brand-new mechanism – concurrent renderer, that allows to interrupt rendering. It is increase performance of the client rendering overall.

Worth noting, that concurrent rendering automatically enables only for parts, those using it. That's why migration to React 18 will not be a problem.

Batching updates

Batching is the process of grouping multiple updates into one single render. Previously updates inside of promises, setTimeout or native eventHandlers were not batched by default, but now such updates will be batched automatically.

Also, if you are using unstable_batchedUpdates you can stop using it safely.

You can see code examples here.


Transition is a new concept in React to distinguish between urgent and non-urgent updates:

  • urgent updates - direct interaction, like typing, clicking, pressing, and so on;
  • non-urgent updates - transition the UI from one view to another;

Urgent updates supposed to be handled immediately and show updated UI. In opposite, non-urgent updates are not showing every intermediate value on screen, but final result only. Such separation helps to cope with blocked user interactions due to non-urgent updates.

Any updates are urgent by default, but startTransition and useTransition let you mark some updates as not urgent. If an urgent update will happen during a non-urgent one React stop the current rendering and render only the latest update.

As far as you know, content wrapped in Suspense can be suspended for different reasons. To recall:

  • there is lazy component render, wrapped in Suspense;
  • there is data loading in a component, wrapped in Suspense. Tramvai does not support it yet, but you can look into Relay implementation, or origin RFC;
  • content, wrapped in Suspense is waiting for corresponding CSS (currently in development by React team);

So, if a component will be suspended during non-urgent update React will prevent already-visible content from being replaced by a fallback. Instead, React will delay render until data has loaded.

To recap: any update of the state (const [, setState] = useState(true)), wrapped in startTransition will be considered as non-urgent and will not block user input.


Suspense lets to display fallback content for the parts of UI, that not ready to be displayed yet. In Tramvai it is suitable only for lazy components at the moment.

As far as you know Tramvai ships its own way for lazy code loading – @tramvai/lazy. In comparison to classic React.lazy, Tramvai version has some advantages:

  • it optimized for SSR;
  • code of a component loads on the client before hydration;

That's why we recommend using only @tramvai/lazy in Tramvai apps.

Note, that all the content, wrapped in Suspense, even if it not suspends by self reasons, will be suspended.

Also, Suspense usage enables selective hydration. We have a guide about it already. Check it out!

In addition, Suspense allows to intercept server-side and hydration errors.

Strict mode

In order for components to work properly in the future, a new behavior has been added to StrictMode called StrictEffects. The idea is to catch effects that don't work properly during development by React mounting the component twice, i.e.: mount => unmount => mount, calling corresponding effects and unsubscribe functions. It works only in DEV environment. You can enable it such way.

Tramvai integration

When you are using React 18, several features will be allowed to use:

Streaming rendering

We have an article about streaming rendering, check it out.

Selective hydration

hydrateRoot wraps in startTransition on the client automatically. See more.

Suspense in Child App

Each Child App wraps into Suspense automatically.

Tramvai use cases


Use cases

  • displaying a fallback UI, during lazy component load
  • handling server-side errors;

Look to the code example here.


Mark a state update inside it as non-urgent, that allows to not block user input during subsequent updates urgent updates.

Outside the components you should use startTransition function, instead of useTransition hook.

Use cases

  • wrap a navigation between tabs, when you have heavy to render tab components;
  • don't show fallback UI for suspended components;

Take a look to the example. Try to switch to the Slow tab, and then to the Another tab immediately. Navigation will happen straight away, despite slow rendering of the Slow tab. Also, there is no loading state for suspended content, when click to the Switch button.


Hook to generate same identifiers on server and client. Without it, if you will do it by yourself, e.g. just calling uuid, result values will be different on server and client. Also, you can't use simple counter (nextId++) for it, because React does not guarantee the order in which the client components are hydrated. Don't use it for generating keys in a list.

There is an example in our repo. Run the app, go to the /hooks and look to the console. There is a hydration error for the WithoutHook component, and no such error for WithHook.


Lets you defer re-rendering a non-urgent part of the UI.

Look at example.

How it works?

There are two steps for it:

  1. First, when the update happens, React makes a render with updated main state, but with not updated deferred state;
  2. Then in background, React tries to re-render with both main and deferred states updated. However, if it suspends or a new state update received, React will cancel the background render and retry it with a new value.

Use cases

  • defer the rendering of a heavy UI;
  • don't show Suspense fallback for suspended components;

Run the example app and go to the /hooks. Note, that user input does not block by the heavy results render.

Difference from debounce/throttling


  • Designed for the rendering optimizations and integrated with React deeply;
  • There is no fixed time delay, so React will adjust to the user;
  • Can abandon the rendering, by contrast throttle and debounce just postpone the moment when rendering blocks.

However, throttle and debounce are still useful. For example, they can let you fire fewer network requests.

Error handling

Given the new features, handling rendering errors in React 18 looks like this:

  1. If an error happens during SSR, then instead of throwing it to the whole application, React will find nearest Suspense boundary up to tree and include it fallback to the server response.
  2. After that, on the client, React determines what could not be rendered and starts a client-side render from scratch for these parts.
  3. If the client render was successful, React will render it, otherwise it will throw an error that can be caught by the Error Boundary.

Note, that same logic applies to the hydration errors, that is React discard server results to nearest Suspense boundary and renders in on the client from scratch.

That's why we are strongly recommend to wrap in Suspense boundary important parts of your application to improve performance. You can use the next structure as a reference:

<ErrorBoundary fallback>
<Suspense fallback>
<Component />

Note, that Tramvai add ErrorBoundary by default. See more.

Also, Tramvai log the rendering errors and deduplicate them, to avoid noise. In general, we are recommend do not ignore hydration errors, because they are affect performance. As the last resort you can use suppressHydrationWarning={true} on the React component, e.g. to display time.