@parcel/transformer-babel/src/env.js

// @flow strict-local

import type {Config} from '@parcel/types';
import presetEnv from '@babel/preset-env';
import type {BabelConfig} from './types';
import type {Targets as BabelTargets, PresetEnvPlugin} from '@babel/preset-env';

import getBabelTargets from './getBabelTargets';
import {enginesToBabelTargets} from './utils';

/**
 * Generates a @babel/preset-env config for an asset.
 * This is done by finding the source module's target engines, and the app's
 * target engines, and doing a diff to include only the necessary plugins.
 */
export default async function getEnvOptions(
  config: Config,
): Promise<?{|
  config: BabelConfig,
  targets: BabelTargets,
|}> {
  // Only compile if there are engines defined in the environment.
  if (Object.keys(config.env.engines).length === 0) {
    return null;
  }

  // Load the target engines for the app and generate a @babel/preset-env config
  let appBabelTargets = enginesToBabelTargets(config.env);

  // If this is the app module, the source and target will be the same, so just compile everything.
  // Otherwise, load the source engines and generate a babel-present-env config.
  if (!config.isSource) {
    let sourceBabelTargets = await getBabelTargets(config);

    if (
      !sourceBabelTargets ||
      !shouldCompileFurther(sourceBabelTargets, appBabelTargets)
    ) {
      return null;
    }
  }

  return {
    targets: appBabelTargets,
    config: {presets: ['@parcel/babel-preset-env']},
  };
}

function getNeededPlugins(targets: BabelTargets): Array<PresetEnvPlugin> {
  return presetEnv(
    {assertVersion: () => true},
    {targets: targets},
  ).plugins.filter(p => p[0]);
}

function shouldCompileFurther(
  sourceBabelTargets: BabelTargets,
  appBabelTargets: BabelTargets,
): boolean {
  let sourcePlugins = new Set(getNeededPlugins(sourceBabelTargets));
  let appPlugins = getNeededPlugins(appBabelTargets);

  // If there is any app plugin present that was not used to compile the source,
  // then the asset was built to a higher target and will need to be compiled
  // further
  return appPlugins.some(plugin => {
    return !sourcePlugins.has(plugin);
  });
}