# Dependency Pre-Bundling

When you run vite for the first time, you may notice this message:

Optimizable dependencies detected:react, react-domPre-bundling them to speed up dev server page load...(this will be run only when your dependencies have changed)

## The Why

This is Vite performing what we call “dependency pre-bundling”. This process serves two purposes:

1. CommonJS and UMD comaptibility: During development, Vite’s dev serves all code as native ESM. Therefore, Vite must convert dependencies that are shipped as CommonJS or UMD into ESM first.

When converting CommonJS dependencies, Vite performs smart import analysis so that named imports to CommonJS modules will work as expected even if the exports are dynamically assigned (e.g. React):

// works as expectedimportReact,{ useState }from'react'
2. Performance: Vite converts ESM dependencies with many internal modules into a single module to improve subsequent page load performance.

Some packages ship their ES modules builds as many separate files importing one another. For example, lodash-es has over 600 internal modules! When we do import { debounce } from 'lodash-es', the browser fires off 600+ HTTP requests at the same time! Even though the server has no problem handling them, the large amount of requests create a network congestion on the browser side, causing the page to load noticeably slower.

By pre-bundling lodash-es into a single module, we now only need one HTTP request instead!

## Pre-Bundle Criteria

Dependencies are only checked for pre-bundling if it is listed in dependencies of your package.json. It will be eligible for pre-bundling if one of the following is true:

• The dependency’s entry contains no valid ES module export (treated as CommonJS);
• The dependency’s entry contains ES imports to other modules or dependencies (multiple internal modules).

This also means you should avoid placing dependencies that are not meant to be imported in your source code under dependencies (move them to devDependencies instead).

## Deep Imports

Pre-bundled dependencies are bundled into a single, separate module, therefore you should prefer using named exports from the main entry point instead of deep imports to individual modules. For example:

// Bad, will lead to duplicated module instances.// Vite will complain about it.import debounce from'lodash-es/debounce'// Goodimport{ debounce }from'lodash-es'

Some dependencies may be designed to be used via deep imports, e.g. firebase exposes sub modules via firebase/* deep imports. For such dependencies, you can instruct Vite to explicitly include these deep import paths via the optimizeDeps.include option. If you never use the main entry, it is also a good idea to exclude it from pre-bundling.

## Dependency Compatibility

While Vite tries its best to accommodate non-ESM dependencies, there are going to be some dependencies that won’t work out of the box. The most common types are those that import Node.js built-in modules (e.g. os or path) and expect the bundler to automatically shim them. These packages are typically written assuming all users will be consuming it with webpack, but such usage does not make sense when targeting browser environments.

When using Vite, it is strongly recommended to always prefer dependencies that provide ESM formats. This will make your build faster, and results in smaller production bundles due to more efficient tree-shaking.

In a monorepo setup, a dependency may be a linked package from the same repo. Vite automatically detects dependencies that are not resolved from node_modules and treats the linked dep as source code. It will not attempt to bundle the linked dep, and instead will analyze the linked dep’s dependency list instead.

## Customizing the Behavior

The default pre-bundling heuristics may not always be desirable. In cases where you want to explicitly include/exclude dependencies from the list, use the optimizeDeps config options.

## Caching

Vite caches the pre-bundled dependencies in node_modules/.vite. It determines whether it needs to re-run the pre-bundling step based on a few sources:

• The dependencies list in your package.json
• Package manager lockfiles, e.g. package-lock.json, yarn.lock, or pnpm-lock.yaml.
• Your vite.config.js, if present.

The pre-bundling step will only need to be re-run when one of the above has changed.

If for some reason you want to force Vite to re-bundle deps, you can either start the dev server with the --force command line option, or manually delete the node_modules/.vite cache directory.