.babelrc

{
  /**
   * When building, we do not want to transpile modules in Babel, and instead handle
   * them via Rollup. When running tests, we want Babel to transpile and convert modules
   * to commonjs, since they're run in Node, and do not use rollup, so we need to use
   * a module import system that Node can understand. We need to make two environments
   * since the options are merged with the top-level configuration.
   */
  "env": {
    "production": {
      "presets": [
        ["env", {
          "targets": {
            "browsers": [
              ">1%",
              "Firefox ESR",
            ]
          },
          "debug": true,
          "modules": false,
          // Disable polyfilling features for now,
          // let consumer handle which features to support. Just
          // handle transpiling of code for the UMD build.
          // "useBuiltIns": "usage",
          "exclude": [
            "web.dom.iterable",
            "web.immediate",
            "web.timers",
            // Ignore @babel/preset-env's async/await transformations,
            // we're using the `fast-async` plugin to reduce build size and
            // not use Regenerator's runtime.
            "transform-regenerator",
            "transform-async-to-generator",
          ],
        }]
      ],
      "plugins": [
        ["fast-async", {
            "spec": true,
            "useRuntimeModule": false,
        }],
        "external-helpers",
      ],
    },
    "test": {
      "presets": [
        ["env", {
          "targets": {
            "node": "8",
          },
          "debug": true,
        }]
      ],
    }
  }
}