[node.js] Test process.env with Jest

In my opinion, it's much cleaner and easier to understand if you extract the retrieval of environment variables into a utility (you probably want to include a check to fail fast if an environment variable is not set anyway), and then you can just mock the utility.

// util.js
exports.getEnv = (key) => {
    const value = process.env[key];
    if (value === undefined) {
      throw new Error(`Missing required environment variable ${key}`);
    }
    return value;
};

// app.test.js
const util = require('./util');
jest.mock('./util');

util.getEnv.mockImplementation(key => `fake-${key}`);

test('test', () => {...});

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