rollup.config.js

import typescript from 'rollup-plugin-typescript2';
import nodeResolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import filesize from 'rollup-plugin-filesize';

const path = require('path');
const basePath = process.cwd();
const packageJson = require(path.resolve(basePath, './package.json'));

const externalDependencies = [
  ...Object.keys(packageJson.dependencies || {}),
  ...Object.keys(packageJson.peerDependencies || {}),
];

const getBaseConfig = (showFilesize = false, noTsDeclaration = false) => {
  const tsconfigOverride = noTsDeclaration
    ? {
        compilerOptions: {
          declaration: false,
          declarationMap: false,
        },
      }
    : undefined;

  return {
    input: 'src/index.ts',
    plugins: [
      nodeResolve({
        preferBuiltins: true,
        browser: true,
      }),
      typescript({
        clean: false,
        tsconfig: 'tsconfig.build.json',
        tsconfigOverride,
      }),
      commonjs(),
      showFilesize ? filesize() : null,
    ],
    external: id => {
      return externalDependencies.some(dep => id.indexOf(dep) === 0);
    },
  };
};

export default commands => {
  const cjsConfig = {
    ...getBaseConfig(true),
    output: {
      file: packageJson.main,
      format: 'cjs',
      exports: 'named',
      sourcemap: true,
    },
  };

  const esConfig = {
    ...getBaseConfig(false, true),
    output: {
      file: packageJson.module,
      format: 'es',
      exports: 'named',
      sourcemap: true,
    },
  };

  if (commands.watch === true) {
    return esConfig;
  } else {
    return [cjsConfig, esConfig];
  }
};