vite-plugin-entry-shaking

vite-plugin-entry-shaking

Mimic tree-shaking behaviour when importing code from an entry file in development mode.

[!NOTE] The main execution logic of this plugin only applies to development mode because it addresses an issue which is specific to development mode. It may be used in test environments that rely on Vite (e.g. Vitest) but should be used with caution as stated in limitations.

Install

# Using npm
npm i -D vite vite-plugin-entry-shaking
# Using Yarn
yarn add -D vite vite-plugin-entry-shaking
# Using pnpm
pnpm add -D vite vite-plugin-entry-shaking

Usage

Setup the plugin in your Vite configuration file.

import { resolve } from 'path';
import { defineConfig } from 'vite';
import EntryShakingPlugin from 'vite-plugin-entry-shaking';

export default defineConfig({
  plugins: [
    EntryShakingPlugin({
      targets: [
        // Using direct paths.
        resolve(__dirname, 'src/entry-a'),
        // Or using glob patterns.
        {
          glob: 'src/utils/*.ts',
          globOptions: { ignore: ['excluded.ts'] },
        }
      ],
    }),
  ],
});

Plugin options

Option name	Type	Default	Description
targets (required)	string[] { path: string }[] { glob: string; globOptions: import('fast-glob').Options }[]	N/A	You need to list all of the entry points you want this plugin to process.
extensions	string[]	['js', 'jsx', 'mjs', 'ts', 'tsx', 'mts']	This specifies which file extensions you want this plugin to process.
ignorePatterns	(string | RegExp)[]	[/node_modules/]	This specifies RegExp/string whose matched paths must be ignored by the plugin.
maxWildcardDepth	number	0	How deep should this plugin run static analysis when encountering wildcards?
diagnostics	boolean | DiagnosticsConfig	true	Toggles all diagnostics when set as a boolean, or specific diagnostics when set as an object.
debug	boolean	false	Toggles debug mode. This will print debug logs if Vite's logLevel is set to any other value than 'silent'

Diagnostics configuration

Diagnostics are recommendations/warnings emitted by the plugin as it analyzes the specified entry files. They aim to help you improve the plugin's efficiency by suggesting changes and provide explanation on why they were emitted. See RESOURCES for a more in-depth insight about possible diagnostics.

Name	Default	Description
definedWithinEntry	true	Emit warning if an entry file defines code that may prevent tree-shaking.
maxDepthReached	true	Emits a warning when max analysis depth was reached when handling wildcard imports.

Examples

Examples illustrating usage and benefits can be found here. Feel free to fork and play around. For instance, you can toggle the plugin on and off their respective vite config file while serving in development mode and see how it affects the amount of requests made by your browser.

This repository provides a simple CLI to help you quickly scaffold a new example, simply run the following command from the project's root to get started:

pnpm generate-example

Debugger (experimental)

Version 0.4.0 introduces a debugger as an optional dependency. When installed and attached to the plugin, a debugger application will open alongside your actual application in dev mode. This can help you get an overall idea of what the plugin is processing and its impacts on performances.

[!NOTE] This requires installing vite-plugin-entry-shaking-debugger and setting the debug option to true.

Motivation

The problem this plugin tries to address is well described by this Vite's github issue, so we'll stick to its author's example. Suppose your codebase contains an index file which is basically used as an entry point to dispatch code imported from other files. This is a rather common pattern which may be handy and avoid writing a lot of individual import statements:

// shared/index.ts
export { a } from './a';
export { b } from './b';
export { c } from './c';

Let's pretend you have a module which imports c from that entry point:

// module.ts
import { c } from './shared';

In development mode, when Vite serves the module.ts file, the browser loads the shared/index.ts file which initiates requests for all of the three a, b and c modules it relies on. The thing is, we actually only needed the c module, this results in both a and b requests being unnecessary!

As your projet and entry points grow, you end up paying the price for the ever-increasing amount of unnecessary requests and HMR updates, which consumes time and resources. Well, that escalated quickly. Let's try to work around this…

The idea

The main idea is to rewrite imports from a target entry point and replace them by individual imports of the actual module. Back with the above example:

// module.ts
import { c } from './shared';

// gets rewritten as…
import { c } from './shared/c';

This way, the shared/index.ts file is not loaded by the browser anymore and no additional requests are initiated.

How it works

First of all, the plugin reads all of the target entry files listed in the plugin's targets option. For each entry file :

• It uses es-module-lexer to get a list of imports and exports.
• It stores named exports that are re-exports of code imported from another module and the path they resolve to. It also stores whether this re-exported code is the default or a named export of its origin. This lets us correctly rewrite the import using the adequate statement.
• It also tracks a mutated version of the entry file where these stored named exports are removed. This is required because we might still import code which is actually defined within the entry file, rather than exported from another module. To make these work, we'll still need to serve this mutated version of the entry file so that this kind of code can be reached.

Whenever Vite serves a file which includes an import which resolved to one of the targets, this file is transformed to rewrite the relevant import statements. It extracts the list of entities imported from that entry file, and for each imported entity :

• it removes the import statement of that entry file.
• if it has a matching stored named export, it adds a direct import to the relevant module's absolute path, taking into consideration whether it imports a named export or a default export.
• If it has no matching stored named export, it is some code which is actually defined within the entry file. These are batched and eventually add a recomposed import of the target.

When encountering the above latest case, we have the browser still loading the shared/index.ts, which could therefore trigger unnecessary requests, as described earlier. To prevent this kind of scenario, any import of an entry point is caught and is forced to serve its mutated version we stored while parsing the entry point file. This ensures the entry point only imports what it needs to make the code it explicitly defines work.

Please refer to RESOURCES.md for a more precise overview of what happens under the hood.

Limitations

See es-module-lexer's own limitations.

Behaviour

• Import statements are not cleaned up from analyzed targets. This means if you import code that was defined within a target, you might still load unnecessary modules. This is by design because getting rid of unused imports would require us to traverse each target's AST to make sure it is indeed not used, which would end up quite expensive.

• By default, tree-shaking wildcard imports only work when imported path is part of target list. Other wildcard imports may be handled by setting the maxWildcardDepth option. Read more

Unsupported syntaxes

You should not expect errors using these. Instead, it just means the content they intend to import won't be tree-shaken by the plugin:

• dynamic imports
• import json from './json.json' assert { type: 'json' }

Using with Vite-based test runners

As a Vite user, you may be using a test runner that relies on Vite's dev server and your usual vite.config file. Vitest, for example, would - by default - still use this plugin when running tests. This means tested/imported modules and even test files themselves may be transformed. This could be an issue when some of your tests rely on mocks whose references are affected by this plugin (read more on this).

To properly benefit from this plugin within test files, one have to understand how the plugin behaves, how vite's resolver behaves and have a clear overview of their codebase.

Unless you're confident about the above statement, it is recommended to disable this plugin when running tests.

JSX

[!WARNING] This feature is still experimental

This plugin supports JSX and TSX syntaxes by pre-transforming them using esbuild which is included by Vite.

Useful links

• Vite's documentation
• Contribution guide
• Resources