[builtin:swc-loader]: do i always need to exclude node_modules?

[nurbolnygmanov]

Hi team, i am trying to setup builtin:swc-loader in my react project with syntax lowering. And in builtin:swc-loader docs for ts transpilation here we have exclude: [/node_modules/] field, but in polyfill injection jsx/tsx part here we have only exclude: /node_modules[\\/]core-js/, so from docs it is not clear, in second case are node modules being excluded? or only core-js is excluded and swc will attempt to process files in node_modules as well?
so should this exclude be enough for both cases?: exclude: [/node_modules/] ?

[mg1986jp]

The docs are showing two different exclude patterns because the use cases are different.

For syntax lowering / TypeScript transpilation, you exclude all of node_modules:

exclude: [/node_modules/]

This makes sense because your dependencies should already be shipping compiled JS. You don't want SWC re-transpiling them — it's slow and can break things.

For polyfill injection (the env config with mode: "usage" or mode: "entry"), the exclude is narrower:

exclude: /node_modules[\\/]core-js/

This only excludes core-js itself (to avoid circular polyfilling). Everything else in node_modules does get scanned — because SWC needs to detect which APIs your dependencies actually use so it can inject the right polyfills. If you excluded all of node_modules here, dependencies using modern APIs wouldn't get polyfilled and would break in older browsers.

So the answer is: it depends on what you're doing.

• Syntax lowering only (JSX, TS, optional chaining, etc.) → exclude: [/node_modules/] is correct
• Polyfill injection → only exclude core-js, let SWC scan node_modules
• Both at the same time → use two separate rules, one for syntax with full exclude, one for polyfills with the narrow exclude

If you're doing both in a single rule, exclude: [/node_modules/] will work for syntax but you'll miss polyfills for dependencies. Split them into two rules if you need both.