Code Splitting

Code splitting and lazy loading is a powerful technique for improving the bundle size and load performance of an application.

  • Reduces the amount of code that needs to be loaded on initial page load
  • Code is loaded on-demand when it is needed
  • Results in more chunks that are smaller in size that can be cached more easily by the browser.

How does TanStack Router split code?

TanStack Router separates code into two categories:

  • Critical Route Configuration - The code that is required to render the current route and kick off the data loading process as early as possible.

    • Path Parsing/Serialization
    • Search Param Validation
    • Loaders, Before Load
    • Route Context
    • Static Data
    • Links
    • Scripts
    • Styles
    • All other route configuration not listed below
  • Non-Critical/Lazy Route Configuration - The code that is not required to match the route, and can be loaded on-demand.

    • Route Component
    • Error Component
    • Pending Component
    • Not-found Component

🧠 Why is the loader not split?

  • The loader is already an asynchronous boundary, so you pay double to both get the chunk and wait for the loader to execute.
  • Categorically, it is less likely to contribute to a large bundle size than a component.
  • The loader is one of the most important preloadable assets for a route, especially if you're using a default preload intent, like hovering over a link, so it's important for the loader to be available without any additional async overhead.

Encapsulating a route's files into a directory

Since TanStack Router's file-based routing system is designed to support both flat and nested file structures, it's possible to encapsulate a route's files into a single directory without any additional configuration.

To encapsulate a route's files into a directory, move the route file itself into a .route file within a directory with the same name as the route file.

For example, if you have a route file named posts.tsx, you would create a new directory named posts and move the posts.tsx file into that directory, renaming it to route.tsx.

Before

  • posts.tsx
  • posts.lazy.tsx

After

  • posts
    • route.tsx
    • route.lazy.tsx

Using the .lazy.tsx suffix

If you're using the recommended File-Based Routing approach, code splitting is as easy as moving your code into a separate file with a .lazy.tsx suffix and use the createLazyFileRoute function instead of the FileRoute class or createFileRoute function.

Here are the options currently supported by the createLazyFileRoute function:

Export NameDescription
componentThe component to render for the route.
errorComponentThe component to render when an error occurs while loading the route.
pendingComponentThe component to render while the route is loading.
notFoundComponentThe component to render if a not-found error gets thrown.

Exceptions to the .lazy.tsx rule

  • The __root.tsx route file does not support code splitting, since it's always rendered regardless of the current route.

❓ Why can't I keep all of my route code in a single file? Can't you just code split that file?

It's true, most popular frameworks support automatic code-extraction, but for now, TanStack Router is going to keep things simple by not getting into the business of parsing and extracting code from files. This also allows you to use any bundler you want, via the CLI, without having to worry about whether or not it supports our code extraction approach.

In practice, we've found that splitting a file into 2 parts (critical and lazy) is the easy part. The harder part by far is wiring it all back together properly using lower-level lazy-loading APIs (not to mention doing that in a type-safe way).

By keeping the splitting process manual and adhering to a simple set of conventions, there's less room for error and less cognitive overhead when working with code splitting.

Example code splitting with .lazy.tsx

When you are using .lazy.tsx you can split your route into two files to enable code splitting:

Before (Single File)

tsx
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'
import { fetchPosts } from './api'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'
import { fetchPosts } from './api'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
  component: Posts,
})

function Posts() {
  // ...
}

After (Split into two files)

This file would contain the critical route configuration:

tsx
// src/routes/posts.tsx

import { createFileRoute } from '@tanstack/react-router'
import { fetchPosts } from './api'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
})
// src/routes/posts.tsx

import { createFileRoute } from '@tanstack/react-router'
import { fetchPosts } from './api'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
})

With the non-critical route configuration going into the file with the .lazy.tsx suffix:

tsx
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}

Using Virtual Routes

You might run into a situation where you end up splitting out everything from a route file, leaving it empty! In this case, simply delete the route file entirely! A virtual route will automatically be generated for you to serve as an anchor for your code split files. This virtual route will live directly in the generated route tree file.

Before (Virtual Routes)

tsx
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  // Hello?
})
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  // Hello?
})
tsx
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}

After (Virtual Routes)

tsx
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.lazy.tsx
import { createLazyFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}

Tada! 🎉

Using automatic code-splitting

Warning

This feature is experimental and currently in development. It is not recommended for production use.

When using the experimental.enableCodeSplitting feature, TanStack Router will automatically code split your route files based on the non-critical route configuration mentioned above.

The automatic code-splitting feature is ONLY available when you are using file-based routing with one of our supported bundlers. This will NOT work if you are only using the CLI (@tanstack/router-cli).

To enable this feature, you can add the following to your tsr.config.json:

json
{
  "experimental": {
    "enableCodeSplitting": true
  }
}
{
  "experimental": {
    "enableCodeSplitting": true
  }
}

If you were previously using the .lazy.tsx suffix, you must remove it and let the automatic code-splitting feature take care of the rest. Your route files will now look like this:

Before (Automatic Code Splitting using the .lazy suffix)

tsx
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
})
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
})
tsx
// src/routes/posts.lazy.tsx
import { createLaztFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.lazy.tsx
import { createLaztFileRoute } from '@tanstack/react-router'

export const Route = createLazyFileRoute('/posts')({
  component: Posts,
})

function Posts() {
  // ...
}

After (Automatic Code Splitting without the .lazy suffix)

tsx
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
  component: Posts,
})

function Posts() {
  // ...
}
// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: fetchPosts,
  component: Posts,
})

function Posts() {
  // ...
}

Code-Based Splitting

Manually Splitting Using route.lazy() and createLazyRoute

If you're not using the file-based routing system, you can still manually split your code using the route.lazy() method and the createLazyRoute function. You'd need to:

Create a lazy route using the createLazyRoute function.

tsx
// src/posts.tsx
export const Route = createLazyRoute('/posts')({
  component: MyComponent,
})

function MyComponent() {
  return <div>My Component</div>
}
// src/posts.tsx
export const Route = createLazyRoute('/posts')({
  component: MyComponent,
})

function MyComponent() {
  return <div>My Component</div>
}

Then, call the .lazy method on the route definition in your app.tsx to import the lazy/code-split route with the non-critical route configuration.

tsx
// src/app.tsx
const postsRoute = createRoute({
  getParent: () => rootRoute,
  path: '/posts',
}).lazy(() => import('./posts.lazy').then((d) => d.Route))
// src/app.tsx
const postsRoute = createRoute({
  getParent: () => rootRoute,
  path: '/posts',
}).lazy(() => import('./posts.lazy').then((d) => d.Route))

Data Loader Splitting

⚠️ Splitting a data loader will incur 2 round trips to the server to retrieve the loader data. One round trip to load the loader code bundle itself and another to execute the loader code and retrieve the data. Do not proceed unless you are VERY sure that your loader is contributing the the bundle size enough to warrant these round trips.

You can code split your data loading logic using the Route's loader option. While this process makes it difficult to maintain type-safety with the parameters passed to your loader, you can always use the generic LoaderContext type to get most of the way there:

tsx
import { LoaderContext } from '@tanstack/react-router'

const route = createRoute({
  path: '/my-route',
  component: MyComponent,
  loader: (...args) => import('./loader').then((d) => d.loader(...args)),
})

// In another file...
export const loader = async (context: LoaderContext) => {
  /// ...
}
import { LoaderContext } from '@tanstack/react-router'

const route = createRoute({
  path: '/my-route',
  component: MyComponent,
  loader: (...args) => import('./loader').then((d) => d.loader(...args)),
})

// In another file...
export const loader = async (context: LoaderContext) => {
  /// ...
}

This process can feel heavy-handed, so TanStack Router exports a utility called lazyFn which is very similar to lazyRouteComponent that can help simplify this process:

tsx
import { lazyFn } from '@tanstack/react-router'

const route = createRoute({
  path: '/my-route',
  component: MyComponent,
  loader: lazyFn(() => import('./loader'), 'loader'),
})

// In another file...a
export const loader = async (context: LoaderContext) => {
  /// ...
}
import { lazyFn } from '@tanstack/react-router'

const route = createRoute({
  path: '/my-route',
  component: MyComponent,
  loader: lazyFn(() => import('./loader'), 'loader'),
})

// In another file...a
export const loader = async (context: LoaderContext) => {
  /// ...
}

Manually accessing Route APIs in other files with the RouteApi class

As you might have guessed, placing your component code in a separate file than your route can make it difficult to consume the route itself. To help with this, TanStack Router exports a handy getRouteApi function that you can use to access a route's type-safe APIs in a file without importing the route itself.

  • my-route.tsx
tsx
import { createRoute } from '@tanstack/react-router'
import { MyComponent } from './MyComponent'

const route = createRoute({
  path: '/my-route',
  loader: () => ({
    foo: 'bar',
  }),
  component: MyComponent,
})
import { createRoute } from '@tanstack/react-router'
import { MyComponent } from './MyComponent'

const route = createRoute({
  path: '/my-route',
  loader: () => ({
    foo: 'bar',
  }),
  component: MyComponent,
})
  • MyComponent.tsx
tsx
import { getRouteApi } from '@tanstack/react-router'

const route = getRouteApi('/my-route')

export function MyComponent() {
  const loaderData = route.useLoaderData()
  //    ^? { foo: string }

  return <div>...</div>
}
import { getRouteApi } from '@tanstack/react-router'

const route = getRouteApi('/my-route')

export function MyComponent() {
  const loaderData = route.useLoaderData()
  //    ^? { foo: string }

  return <div>...</div>
}

The getRouteApi function is useful for accessing other type-safe APIs:

  • useLoaderData
  • useLoaderDeps
  • useMatch
  • useParams
  • useRouteContext
  • useSearch
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