This example shows how the sideEffects flag for library authors works.
The example contains a large library, big-module. big-module contains multiple child modules: a, b and c. The exports from the child modules are re-exported in the entry module (index.js) of the library. A consumer uses some of the exports, importing them from the library via import { a, b } from "big-module". According to the EcmaScript spec, all child modules must be evaluated because they could contain side effects.
The "sideEffects": false flag in big-module's package.json indicates that the package's modules have no side effects (on evaluation) and only expose exports. This allows tools like webpack to optimize re-exports. In the case import { a, b } from "big-module-with-flag" is rewritten to import { a } from "big-module-with-flag/a"; import { b } from "big-module-with-flag/b".
The example contains two variants of big-module. big-module has no sideEffects flag and big-module-with-flag has the sideEffects flag. The example client imports a and b from each of the variants.
After being built by webpack, the output bundle contains index.js a.js b.js c.js from big-module, but only a.js and b.js from big-module-with-flag.
Advantages:
- Smaller bundles
- Faster boot up
_{{example.js}}__{{node_modules/big-module/package.json}}__{{node_modules/big-module-with-flag/package.json}}__{{node_modules/big-module-with-flag/index.js}}__{{dist/output.js}}__{{stdout}}_
_{{production:stdout}}_