Using ES6 `import` in tests with Jest and Rollup

Today while working on a new version of Preliminaries using ES6 I was trying to get both Rollup working to bundle my code and Jest to run tests when importing my code.

Rollup recommends* something like the following .babelrc:

{
  "ignore": [
    "node_modules/**"
  ],
  "presets": [
    ["es2015", { "modules": false }],
    "stage-0"
  ],
  "plugins": [
    "external-helpers"
  ]
}

which worked great for bundling my code, so I wrote a simple test:

import Preliminaries from "../src/Preliminaries";

const preliminaries = new Preliminaries();

describe("Constructor:", () => {
  it("should return something", () => {
    expect(new Preliminaries()).not.toBeNull();
  });
});

however, when running jest I was greeted with the following error:

 FAIL  tests/test.test.js
  ● Test suite failed to run

    /Users/Joseph/Workspace/preliminaries/preliminaries/tests/test.test.js:12
    import Preliminaries from "../src/index";
    ^^^^^^
    SyntaxError: Unexpected token import

      at transformAndBuildScript (node_modules/jest-runtime/build/transform.js:320:12)
      at process._tickCallback (internal/process/next_tick.js:109:7)

It turns out modules are required for tests, so the following config works for jest:

{
  "ignore": [
    "node_modules/**"
  ],
  "presets": [
    "es2015",
    "stage-0"
  ]
}

but not for rollup of course!

🚨   (babel plugin) It looks like your Babel configuration specifies a module transformer. Please disable it. See https://github.com/rollup/rollup-plugin-babel#configuring-babel for more information
src/index.js

I was able to work around this by combining both these approaches with Babel’s env option:

{
  "ignore": [
    "node_modules/**"
  ],
  "env": {
    "development": {
      "presets": [
        "flow",
        "es2015",
        "stage-0"
      ]
    },
    "production": {
      "presets": [
        "flow",
        ["es2015", { "modules": false }],
        "stage-0"
      ],
      "plugins": [
        "external-helpers"
      ]
    }
  }
}

and setting NODE_ENV in the scripts in my package.json:

  "scripts": {
    "test": "NODE_ENV=development jest",
    "build": "NODE_ENV=production rollup -c"
  }

Success!