If you're building in esm format, and one of your dependency is written in CJS and you want to bundle it, the result code might contain require call:

var fs = require('fs')

That's because built-in modules can't be resolved and esbuild just leaves it as is, a workaround is to use the banner.js option to make require available in your esm bundle:

// tsup.config.ts
import { defineConfig } from "tsup";

export default defineConfig({
  banner: {
    js: `import {createRequire as __createRequire} from 'module';var require=createRequire(import\.meta.url);`,
  },
});