treeshaking.js

const path = require('path');
const { rollup } = require('rollup');
const virtual = require('@rollup/plugin-virtual');
const fs = require('fs');

async function main() {
  const input = process.argv[2] || getMainEntry();
	const result = await check(input);
	const relative = path.relative(process.cwd(), input);

	if (result.shaken) {
		console.error(`Success! ${relative} is fully tree-shakeable`);
	} else {
		error(`Failed to tree-shake ${relative}`);
	};
}

function error(msg) {
  console.error(msg);
  process.exit(1);
}

function getMainEntry() {
	if (!fs.existsSync('package.json')) {
		error(`Could not find package.json`);
	}

  const pkg = JSON.parse(fs.readFileSync('package.json'), 'utf-8');

  const unresolved = pkg.module || pkg.main || 'index';
  const resolved = resolve(unresolved);

  if (!resolved) {
    error(`Could not resolve entry point`);
  }

  return resolved;
}

function resolve(file) {
	if (isDirectory(file)) {
		return ifExists(`${file}/index.cjs.js`) || ifExists(`${file}/index.js`);
	}

	return ifExists(file) || ifExists(`${file}.cjs.js`) || ifExists(`${file}.js`);
}

function isDirectory(file) {
	try {
		const stats = fs.statSync(file);
		return stats.isDirectory();
	} catch (err) {
		return false;
	}
}

function ifExists(file) {
  return fs.existsSync(file) ? file : null;
}

async function check(input) {
  const resolved = path.resolve(input);

  const bundle = await rollup({
    input: '__agadoo__',
    plugins: [
      virtual({
        __agadoo__: `import ${JSON.stringify(resolved)}`,
        tslib: `
          const noop = () => {};
          export const __awaiter = noop;
          export const __extends = noop;
          export const __generator = noop;
          export const __spreadArrays = noop;
        `,
      }),
    ],
    onwarn: (warning, handle) => {
      if (warning.code !== 'EMPTY_BUNDLE') handle(warning);
    },
  });

  const o = await bundle.generate({
    format: 'es',
  });

  const output = o.output;
  console.log(output);
  return {
    shaken: output.length === 1 && output[0].code.trim() === '',
  };
}

main();