The way I did it can be found in this Stack Overflow question.

It is important to use resetModules before each test and then dynamically import the module inside the test:

describe('environmental variables', () => {
  const OLD_ENV = process.env;

  beforeEach(() => {
    jest.resetModules() // Most important - it clears the cache
    process.env = { ...OLD_ENV }; // Make a copy
  });

  afterAll(() => {
    process.env = OLD_ENV; // Restore old environment
  });

  test('will receive process.env variables', () => {
    // Set the variables
    process.env.NODE_ENV = 'dev';
    process.env.PROXY_PREFIX = '/new-prefix/';
    process.env.API_URL = 'https://new-api.com/';
    process.env.APP_PORT = '7080';
    process.env.USE_PROXY = 'false';

    const testedModule = require('../../config/env').default

    // ... actual testing
  });
});

If you look for a way to load environment values before running the Jest look for the answer below. You should use setupFiles for that.

Jest automatically defines environment variable NODE_ENV as test(see https://jestjs.io/docs/environment-variables), as you can confirm from your error message:

console.error node_modules/config/lib/config.js:1727
WARNING: NODE_ENV value of 'test' did not match any deployment config file names. 

What you can do is simply create config/test.json and include the contents {}, which is an empty valid JSON object.

See https://github.com/lorenwest/node-config/wiki/Strict-Mode

Note: the aforementioned error occurs when you use the config package, and meanwhile you don't have the test.json file in the config directory.

UPDATE: Jest 27 is now released and now supports this. If you are using an older version of Jest, update to Jest 27 to be able to use CustomEnvironment as typescript https://jestjs.io/blog/2021/05/25/jest-27#features-coming-with-breaking-changes

Modules used in the following configuration options are now transformed like the rest of your code, which may be breaking if you relied on them being loaded as-is:

• testEnvironment
• runner
• testRunner
• snapshotResolver

Alternatively, you can continue to use the workaround below for prior versions.

This isn't supported in Jest 26 and is slated to be in Jest 27 https://github.com/facebook/jest/pull/8751#issuecomment-699049851

For now the solution is to use setupFilesAfterEnv files in my jest.config https://jestjs.io/docs/en/configuration#setupfilesafterenv-array.

From there I could put all my my setup/teardown in beforeAll() and afterAll() blocks. This was effectively equivalent to using the Node Environment and the setupFilesAfterEnv files is compatible with typescript.

//jest.setup.ts
import {LocalObject} from "./object/local-object.helper";

let localObject: LocalObject;

beforeAll(async () => {
  //Start my environment or seed data to DB or whatever
  localObject = await LocalObject.init()
}


afterAll(async () => {
  //teardown or clean things started in setup my environment
  await localObject.teardown()
}