React Lazy: a take on preloading views

Our app

Let's consider a React app with multiple routes: a landing page route, another one to get a list of todos, another one to inspect todos. Each route renders a specific view / components. We carefully read the React documentation about code splitting and used React.Lazy and React.Suspense which results in us having a codebase similar to the following:

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import React from 'React';
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import { Route, Router, Switch } from 'react-router-dom';
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const App = React.lazy(() => import('./App'));
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const Todos = React.lazy(() => import('./Todos'));
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const Todo = React.lazy(() => import('./Todo'));
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const routes = [
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  { path: '/', exact: true, component: App },
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  { path: '/todos', exact: true, component: Todos },
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  { path: '/todos/:id', exact: true, component: Todo },
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];
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ReactDOM.render(
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  <Router>
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    <React.Suspense fallback={'Loading'}>
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      <Switch>
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        {routes.map((route) => (
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          <Route
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            key={route.path}
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            exact={route.exact}
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            path={route.path}
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            component={route.component}
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          />
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        ))}
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      </Switch>
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    </React.Suspense>
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  </Router>
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);

If we run our app, we can see in the developer tools of our browser that navigating from one view to another is loading the different "pieces" or "chunks" of our app. Now let's focus on how we could start loading these chunks when the user hovers one of the navigation links instead of loading them after navigating to the new route.

Preload components with React Lazy

To preload the view we'll have to be able to call a preload method on our chunk. This preload method would be able to be called to run the import statement that is passed to React Lazy. While such functionality is available out of the box in React Loadable, React Lazy doesn't seem to provide it and this is why we'll have to implement it from scratch with the following code:

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const ReactLazyPreload = (importStatement) => {
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  const Component = React.lazy(importStatement);
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  Component.preload = importStatement;
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  return Component;
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};

We can now redeclare our code split chunks as follows:

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const App = ReactLazyPreload(() => import('./App'));
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const Todos = ReactLazyPreload(() => import('./Todos'));
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const Todo = ReactLazyPreload(() => import('./Todo'));

With the code above we can now call the preload method at will on any of our components which will result in each of them loading their respective chunks:

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App.preload();
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Todos.preload();
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Todo.preload();

Calling preload on the right component for a given route

While we could specify which component we'd wish to preload when the user hovers a given link, wouldn't it be great if we could "find" which component to preload based on the route? If we take a look at the first code snippet of this post, we can see that we declared a routes object that contains all the properties required by the React Router Route component. This is intentional and will be useful for us to find which component is associated to which route.

Let's declare our findComponentForRoute function:

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import { matchPath } from 'react-router-dom';
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const findComponentForRoute = (path, routes) => {
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  const matchingRoute = routes.find((route) =>
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    matchPath(path, {
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      path: route.path,
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      exact: route.exact,
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    })
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  );
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  return matchingRoute ? matchingRoute.component : null;
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};

React Router comes with a pretty handy method called matchPath which for a given path will return true if the path passed in the second argument matches. The function in the code snippet above uses this method and when a match is found, returns the associated component or null if no route has been found.

Now that we have a way to find the component associalted to a given route, we can create a function to preload it:

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const preloadRouteComponent = (path) => {
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  const component = findComponentForRoute(path, routes);
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  if (component && component.preload) {
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    component.preload();
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  }
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};

Finally, we could just add this function to a onMouseEnter event handler to all our Link components and call it a day but let's make the things we just implemented easier to use. Let's create a LinkWithPreload component that will have the same props as Link but will also use the preloadRouteComponent function:

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import { Link } from 'react-router-dom'
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...
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const LinkWithPreload = ({ to, onPreload, ...rest }) => {
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  return (
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    <Link
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      to={to}
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      onMouseEnter={() => preloadRouteComponent(to)}
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      {...rest}
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    />
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  );
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};
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export default LinkWithPreload;

Now, by using this component instead of Link, as you can see below, hovering any navigational links in our App should load the chunk associated with the route of that link:

<figcaption>Gif showcasing the preloading behavior we implemented using React Lazy and Suspense. When hovering, the chunk will load and the transition to the new route is seemless. However, if we don't hover long enough and click on the link, we will see the fallback component from Suspense until the chunk is fully loaded.</figcaption>

Do you want to see the whole code? I made the app showcased in this article available here!