esbuild "Dynamic require of 'fs' is not supported" in CI

esbuild bundled CommonJS code into an ESM output where require does not exist natively. A runtime require(...) of a Node builtin then throws "Dynamic require is not supported".

What this error means

The bundle builds but crashes at runtime in CI with "Dynamic require of 'fs' is not supported". A dependency calls require() dynamically and the ESM output has no require to satisfy it.

node

Error: Dynamic require of "fs" is not supported
    at file:///work/repo/dist/index.js:1:1234

Common causes

CJS bundled to ESM without a require shim

Output format is esm but the bundled dependency still calls require() at runtime, which the ESM output cannot provide.

Node builtins were bundled instead of externalized

Builtins like fs were pulled into the bundle, so esbuild emitted a dynamic require it cannot resolve.

How to fix it

Inject a require shim for ESM output

Recreate require with createRequire and inject it via a banner so bundled CJS can call require at runtime.

build script

await esbuild.build({
  format: 'esm',
  platform: 'node',
  banner: { js: "import{createRequire}from'module';const require=createRequire(import.meta.url);" },
});

Externalize Node builtins and dependencies

Keep builtins (and node_modules) out of the bundle so the runtime resolves them normally.