Code splitting lets a JavaScript application deliver code in independently loadable bundles, so users can receive what they need for the current page or action without downloading every feature up front. The biggest opportunity is often to defer code that is not needed for the first view—but merely producing extra chunks does not help if the browser requests them all immediately.
What code splitting does—and what it does not
Code splitting divides application code, including third-party dependencies, into bundles that can be loaded separately. A browser can load the code required for the current experience and fetch other code when a user navigates to a route or invokes an optional feature. MDN describes the goal as improving performance, especially on initial load: MDN’s code-splitting definition.
Lazy loading is the decision to treat a resource as non-critical and load it only when it is needed. That point might be a route change, a click, or another interaction. A split chunk is not automatically lazy: if the application requests it as soon as it starts, it still adds work to every initial visit. See MDN’s lazy-loading guidance and webpack’s lazy-loading guide.
The motivation is substantial but not a guarantee of a particular result. MDN reports that median resource weight rose from approximately 100 KB to 400 KB on desktop and 50 KB to 350 KB on mobile between 2011 and 2019. Those are historical figures about resource weight, not current measurements and not an estimate of the improvement code splitting will produce in any particular application.
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Choose a boundary users actually cross
A good split boundary corresponds to code that many users do not need immediately. Begin with production output and common first-use journeys, then choose boundaries that defer substantial work without making important features feel sluggish.
Separate entry points for distinct experiences
If a product has distinct application entry experiences, separate entry points can keep code for one experience out of another. This approach is intuitive but can require more manual configuration. webpack warns that entry-point splitting has pitfalls, including duplicated dependencies; inspect the emitted bundles rather than assuming shared code will be handled as intended. MDN distinguishes entry-point splitting from dynamic splitting in its lazy-loading guidance, and webpack explains the approach in its code-splitting documentation.
Routes as boundaries
Routes are natural boundaries when visitors typically use only part of an application in a session. In React Router framework mode, route modules become bundler entry points: visiting /about, for example, loads that route’s bundle without the unrelated contact route bundle. The framework also documents splitting certain route exports—such as client loaders, actions, middleware, and hydration fallback—into separate chunks. These behaviors and their defaults apply to the documented framework mode; check the documentation for the version in the project before relying on them. See React Router’s automatic code-splitting guide.
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Optional features inside a route
Some code is unnecessary even on a route that users visit. A rich editor, charting tool, or infrequently opened panel may be a better dynamic-import boundary than the whole page. Dynamic import() lets the application request a module when its feature becomes relevant. A click-triggered import is more genuinely deferred than an import fired during startup; webpack’s lazy-loading example demonstrates this distinction.
Implement deferred loading as a user-visible state
A deferred feature has a short period in which its code has not arrived. Plan for that state as part of the interface, rather than treating the network request as invisible.
- Identify a meaningful trigger. Start loading when the user navigates to the route or opens the feature, not unconditionally at application startup.
- Show a suitable loading state. Use a compact placeholder or progress indication appropriate to the feature, so the interface explains the wait without implying that the whole application is unavailable.
- Handle failure. A chunk can fail to load or execute. Provide a user-facing error and a sensible recovery option, such as retrying or reloading when appropriate; do not leave the feature stuck in an indefinite loading state.
In webpack, a failed split-chunk load can surface as ChunkLoadError. Its documentation recommends checking whether the chunk is reachable over the network, verifying publicPath, and inspecting browser-console errors. The bundler does not provide a complete user-facing recovery experience automatically; that behavior belongs in the application. See webpack’s code-splitting troubleshooting guidance.
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Keep shared code from erasing the benefit
Splitting can duplicate dependencies or add request round trips if shared code is arranged poorly. webpack documents entry dependencies and SplitChunksPlugin as ways to avoid duplication. A shared chunk can reduce repeated code, but it also becomes another resource whose loading and caching behavior matters. Check which modules actually land in each emitted chunk.
Request timing depends on the toolchain. Vite documents a case in which a dynamically imported chunk A depends on shared chunk C; a naïve load would fetch A and only then discover C. Vite’s build optimization rewrites the import so A and C can be requested in parallel, removing those extra round trips for traced direct imports. That is documented Vite behavior, not a universal guarantee for every bundler or import graph. See Vite’s build optimizations.
Use preload and prefetch selectively
Speculative loading can make a later transition feel faster, but it spends network capacity before the user has requested the resource. webpack distinguishes the two hints by timing and intent:
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- Prefetch: for a resource likely to be needed on a future navigation. In webpack’s example, the browser requests it during idle time after the parent chunk loads.
- Preload: for a resource needed during the current navigation. webpack’s example requests it in parallel with the parent and at higher priority.
Do not apply either hint indiscriminately: webpack warns that incorrect preload use can hurt performance. Vite separately documents modulepreload directives for entry chunks and direct imports, plus a preload step for dynamic imports that helps fetch common dependencies in parallel. Treat these as tool-specific behaviors and consult the documentation for the build setup in use: webpack code splitting and Vite build optimizations.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Remember CSS and other resources
JavaScript is only one part of the initial experience. MDN notes that CSS is render-blocking by default until the CSS object model is constructed, and describes splitting JavaScript, CSS, and HTML into smaller chunks as part of lazy-loading strategies. A JavaScript split will not by itself resolve a bottleneck caused by critical CSS or other resources. See MDN’s lazy-loading guidance.
Vite documents that CSS used by an asynchronous chunk is extracted into a separate file and loaded with that chunk; evaluation waits for the CSS so the feature does not flash unstyled. This is documented Vite behavior, not a general property of all build systems. See Vite’s build optimizations.
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Evaluate the split in the production build
There is no universal percentage improvement to expect. The value depends on what the initial experience used to download, how users navigate, which dependencies remain shared, and how the browser and network handle the resulting requests. Compare the application before and after the change along these dimensions:
- Initial code: What JavaScript is transferred and parsed for the first view?
- Requests: How many chunks are requested, and when? Does a feature introduce a dependency waterfall?
- User journeys: How often do users reach the deferred route or feature, and how quickly does it appear when they do?
- Dependencies and caching: Did shared libraries get duplicated, or did a shared chunk improve reuse without adding costly requests?
- Experience under delay or failure: Is the loading state understandable, and can a failed chunk be diagnosed and recovered from?
A practical sequence is to inspect the existing production bundle, identify code outside common first-use flows, split at route or meaningful feature boundaries, then rebuild and exercise the same user journeys. webpack points to its official bundle analysis tool and other visualizers for seeing where modules ended up: webpack’s code-splitting guide.
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