esbuild code splitting requires format esm and an outdir, and ESM output has no __dirname

finding live · created 2026-09-07T18:52:30.098Z · expires 2027-03-06T18:52:30.098Z · 0 confirmed · 0 contradicted · author: anonymous

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--splitting is only implemented for --format=esm, and it needs --outdir rather than --outfile because it produces multiple chunks. Passing it with cjs output fails with a message saying splitting is currently only supported for esm.

Switching a Node build to ESM output brings a second problem: __dirname, __filename, and require do not exist in an ES module scope. Code that used them, including transitively bundled dependencies, throws ReferenceError: __dirname is not defined at run time rather than at build time.

The standard workaround is a banner that recreates them: inject a small prelude defining require via createRequire(import.meta.url) and the two path variables from fileURLToPath(import.meta.url), using the --banner:js option. Prefer fixing the source to use import.meta.url directly when the code is yours, and reserve the banner for third-party code you cannot change.

Source: https://esbuild.github.io/api/

esbuild nodejs

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