title: Using .env files for unit testing with jest
body: <p>Is it possible to load environment variables from an env file for unit testing purposes in Jest? I'm looking to run a series of tests on it like so:</p>
  
  <pre><code>// unit tests for env file
  describe('env', () =&gt; {
      it('should have a client id', () =&gt; {
          expect(process.env.CLIENT_ID).toBeDefined();
      });
      it('should have a client secret', () =&gt; {
          expect(process.env.CLIENT_SECRET).toBeDefined();
      });
      it('should have a host', () =&gt; {
          expect(process.env.HOST).toBeDefined();
      });
      it('should have a scope', () =&gt; {
          expect(process.env.SCOPE).toBeDefined();
      });
      it('should have a response type', () =&gt; {
          expect(process.env.RESPONSE_TYPE).toBeDefined();
      });
      it('should have a redirect uri', () =&gt; {
          expect(process.env.REDIRECT_URI).toBeDefined();
      });
  });
  </code></pre>
  
  <p>Currently, all the above tests will fail, stating that the variables are undefined. Initially I was using a mocha/chai setup, which allowed me to just load all of my env variables via the use of dotenv. This involved running all unit tests through webpack and worked fine. </p>
  
  <p>However, from reading the <a href="https://facebook.github.io/jest/docs/en/webpack.html" rel="noreferrer">documentation</a> Jest doesn't run tests through webpack; instead modules are mocked out via moduleNameMapper. This works fine for everything else, but I can't get the env file variables to load. So far I've tried using the setupFiles option to a js file that calls dotenv.config with the path of the env file given to it like so:</p>
  
  <pre><code>// setup file for jest
  const dotenv = require('dotenv');
  dotenv.config({ path: './env' });
  </code></pre>
  
  <p>This didn't work, so I've now resorted to using just a .env.js file for unit tests and passing this file into the setupFiles option instead. However, for maintainability, and to keep it working with webpack for production, I'd like to just keep it all in one file. Here's an extract of how the .env.js file looks for reference </p>
  
  <pre><code>// .env.js extract example
  process.env.PORT = 3000;
  process.env.HOST = 'localhost';
  process.env.CLIENT_ID = 'your client id';
  process.env.REDIRECT_URI = 'your callback endpoint';
  </code></pre>