rollup.base.mjs

import { nodeResolve } from '@rollup/plugin-node-resolve';
import swc from '@rollup/plugin-swc';
import { readFileSync } from 'node:fs';
import { relative, resolve } from 'node:path';

const pkg = JSON.parse(readFileSync('./package.json', 'utf-8'));
const cwd = process.cwd();
const deps = new Set([...Object.keys(pkg.dependencies || {}), ...Object.keys(pkg.peerDependencies || {})]);

export default {
  input: 'build/esm/index.js',
  external: (id, parent) => {
    // this function decides if a module should be bundled or not (external)
    // a module is not bundled if it points to one of our dependencies
    // if it's not a dependency but still resolves outside the build/esm dir, an error is thrown
    const path = normalizePath(id, parent);
    const depId = npmPackage(id);
    if (deps.has(depId)) {
      console.log('exclude:', path);
      return true;
    }
    if (!path.startsWith('build/esm/')) {
      // this will throw if we accidentally import something that isn't local and wasn't listed among dependencies
      throw new Error(`Attempt to bundle external dependency ${path} for import ${id}. Are we missing a dependency?`);
    }
    console.log('include:', path);
    return false;
  },
  output: {
    file: 'dist/index.js',
    format: 'es',
    sourcemap: true,
    generatedCode: 'es2015',
  },
  plugins: [
    nodeResolve(),
    swc({
      swc: {
        jsc: {
          target: 'es2015',
        },
      },
    }),
  ],
};

/**
 * Return the path relative to cwd for a give import. Npm package imports will be returned as if they were directly in cwd.
 *
 * @param id the imported path
 * @param parent absolute path of the file containing the import, or undefined if it's the entry point
 * @returns path relative to cwd
 */
function normalizePath(id, parent = '') {
  const absolute = id.startsWith('.') ? resolve(parent, id) : id;
  return relative(cwd, absolute);
}

/**
 * Return the npm package name of an import, if applicable.
 *
 * @param id an import path
 * @returns the npm package name if id is determined to point to an npm package, '' otherwise.
 */
function npmPackage(id) {
  // relative or absolute ids are not npm packages
  if (id.startsWith('.') || id.startsWith('/')) return '';
  // we're only ever interested in the first two parts the id (the second is needed if the first part specifies a scope)
  const parts = id.split('/', 2);
  if (parts[0].startsWith('@')) {
    // it's a scoped package, so it will include one backslash
    return parts.join('/');
  }
  return parts[0];
}