Estimate JavaScript bundle sizes with tree-shaking simulation and package size comparison.
Bundle Size Analyzer frames dependency choice as a comparison of estimated JavaScript bundle sizes. It models how excluding unused exports could change a package's contribution, then lets competing packages be judged on the same size-oriented question. The result is useful before committing to a dependency or import pattern. The important surprise is that a tree-shaken estimate is conditional: production output also depends on package structure, side effects, bundler configuration, code splitting, and the exact package version.
The comparison treats each package as a candidate source of bundle code and presents estimated sizes for judging their relative cost. This answers whether one dependency is plausibly lighter than another, not whether the smaller package is functionally equivalent. Differences in features, transitive dependencies, versions, and import patterns still matter when interpreting the comparison.
The simulation models the reduction possible when exports that are not used can be excluded from a bundle. Tree shaking is most reliable when module relationships can be determined statically and modules do not require discarded code for side effects. The simulated remainder is therefore a scenario estimate, not a promise about every bundler or build configuration.
No. It provides an estimate for comparing package and tree-shaking scenarios. Exact output requires building the real application with its selected versions, imports, target environments, bundler configuration, and optimization settings. Use the analyzer to narrow choices, then verify the preferred choice against the generated production assets.
No. A named import can give a bundler a statically identifiable dependency, but elimination still depends on the package's module form, re-exports, side effects, and build configuration. Some unused code may remain because executing the module is observable or because the dependency structure cannot be safely reduced.
The behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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