Webpack is far more than a simple bundler—it’s a modular build pipeline. Under the hood, its execution can be divided into several deep technical phases that determine how source code becomes an optimized bundle.
The Resolution phase comes first. Here, Webpack interprets every import or require statement and maps it to an actual file path. This process is powered by the enhanced-resolve package, which supports aliases, custom extensions, and module fallbacks. Multiple entry points can each trigger independent dependency graphs that Webpack merges later, deduplicating shared modules across entries.
Once files are resolved, the Evaluation phase begins. Each file runs through a chain of loaders—executed from right to left—to transform the content into JavaScript modules. For example, a .scss file may pass through sass-loader, css-loader, and style-loader, turning styles into JS-compatible strings. During this step, plugins can intercept runtime events and alter the output or even extract assets, as seen in MiniCssExtractPlugin.
Next, the Optimization and Code Splitting stage groups modules into chunks using SplitChunksPlugin, isolates vendor dependencies, and optionally separates the Webpack runtime—a small script that manages module loading in the browser. Dynamic import() statements create asynchronous chunks that the runtime loads only when needed, improving initial load time.
Finally, Webpack’s Output phase writes everything to disk, generating hashed filenames and a manifest that maps chunk identifiers to their physical files. The runtime uses this manifest at execution time to locate and load modules dynamically.
In essence, Webpack is not just bundling—it’s constructing, transforming, optimizing, and orchestrating the runtime behavior of modern web applications.
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