babel.config.js

'use strict'
// @ts-check

/** @type {babel.ConfigFunction} */
module.exports = (api) => ({
  presets: [
    // Transpile modern JavaScript into code compatible with njs.
    // This is used only for building the dist bundle with Rollup.
    ...api.env('njs') ? [
      'babel-preset-njs',
    ] : [],
    // Parse TypeScript syntax and transform it to JavaScript (i.e. it strips
    // type annotations, but does not perform type checking).
    ['@babel/preset-typescript', {
      allowDeclareFields: true,
    }],
  ],

  plugins: [
    ...!api.caller(c => c && c.supportsStaticESM) ? [
      // Transform ES modules to CommonJS if needed  needed for Mocha tests).
      // Mocha, babel-node, babel/register etc. don't understand ES module
      // syntax, so we have to transform it to CommonJS.
      // This is not used with Rollup.
      '@babel/plugin-transform-modules-commonjs'
    ] : [],
    ...api.env('mocha') ? [
      // Transform power-assert. This is used only for Mocha tests.
      'babel-plugin-empower-assert',
      'babel-plugin-espower',
    ] : [],
  ],
})