/**
 * @license
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

module.exports = (api) => {
  const ENV = api.env();
  const DEV = ENV === 'development';
  const TEST = ENV === 'test';
  const CJS = ENV === 'cjs';

  // Options for the @babel/env preset.
  const envOptions = {
    // Transform module syntax if necessary.
    modules: CJS || TEST ? 'commonjs' : false,
    // Set target environment to default browsers.
    targets: 'defaults',
  };

  // Options for the @babel/typescript preset.
  const typescriptOptions = {
    // Opt in to a Babel 8 default.
    allowDeclareFields: true,
    // Be explicit about type-only imports.
    onlyRemoveTypeImports: true,
  };

  // Options for the module-resolver plugin.
  // Used for resolving source files during development.
  let resolverOptions = {
    alias: {
      '^@annotator/(.+)$': '@annotator/\\1/src/index.ts',
    },
  };

  // Options for the @babel/transform-runtime plugin.
  const runtimeOptions = {
    // Use corejs version 3 with shipped proposals.
    corejs: { proposals: true, version: 3 },
    // Use helpers formatted for the target environment.
    useESModules: !CJS && !TEST,
  };

  return {
    plugins: [
      'preserve-comment-header',
      ['@babel/transform-runtime', runtimeOptions],
      ...(DEV ? [['module-resolver', resolverOptions]] : []),
      ...(TEST ? ['istanbul'] : []),
    ],
    presets: [
      ['@babel/env', envOptions],
      ['@babel/typescript', typescriptOptions],
    ],
  };
};