What is the purpose of Vite's Dependency Pre-bundling?

Vite's Dependency Pre-bundling is an important optimization mechanism. Its purpose and principle are as follows:

Why Pre-bundling is Needed:

1. CommonJS and UMD Compatibility: Many npm packages use CommonJS or UMD formats, while Vite dev environment uses native ESM. Pre-bundling converts these modules to ESM format so browsers can load them directly.

2. Performance Optimization: Directly loading many fine-grained ESM modules would result in hundreds of HTTP requests, severely affecting performance. Pre-bundling bundles multiple modules into a single chunk, reducing the number of requests.

3. ESM Conversion: Even if some packages are already in ESM format, they may contain features not supported for direct browser loading (like bare module imports), requiring conversion.

Pre-bundling Workflow:

1. Dependency Scanning: Vite scans project source code to identify all imported dependencies.

2. Build with esbuild: Vite uses esbuild to bundle dependencies into the node_modules/.vite directory, generating ESM format modules.

3. Caching Mechanism: Pre-bundling results are cached. Subsequent startups use the cache directly if dependencies haven't changed, significantly improving startup speed.

4. Source Mapping: Generates source maps to ensure debugging can locate original source code.

Configuration Options:

• optimizeDeps.include: Force certain dependencies to be pre-bundled
• optimizeDeps.exclude: Exclude certain dependencies from pre-bundling
• optimizeDeps.esbuildOptions: Customize esbuild configuration

Automatic Re-bundling: Vite automatically re-pre-bundles when:

• Dependencies in package.json change
• vite.config.js configuration file changes
• Manually delete node_modules/.vite cache directory

Performance Advantage: Through dependency pre-bundling, Vite maintains fast development experience while solving ESM ecosystem compatibility and performance issues. This is one of Vite's core advantages over other build tools.