How to mock NODE_ENV in unit test using Jest

Question

I want to set NODE_ENV in one of the unit test but it's always set to test so my tests fails.

loggingService.ts

...

const getTransport = () => {
  if (process.env.NODE_ENV !== "production") {
    let console = new transports.Console({
      format: format.combine(format.timestamp(), format.simple()),
    });
    return console;
  }

  const file = new transports.File({
    filename: "logFile.log",
    format: format.combine(format.timestamp(), format.json()),
  });
  return file;
};

logger.add(getTransport());
const log = (level: string, message: string) => {
  logger.log(level, message);
};

export default log;

loggingService.spec.ts

     ...
    describe("production", () => {
        beforeEach(() => {
          process.env = {
            ...originalEnv,
            NODE_ENV: "production",
          };
    
          console.log("test", process.env.NODE_ENV);
          log(loglevel.INFO, "This is a test");
        });
    
        afterEach(() => {
          process.env = originalEnv;
        });
    
        it("should call log method", () => {
          expect(winston.createLogger().log).toHaveBeenCalled();
        });
    
        it("should not log to the console in production", () => {
          expect(winston.transports.Console).not.toBeCalled();
        });
    
        it("should add file transport in production", () => {
          expect(winston.transports.File).toBeCalledTimes(1);
        });
      });
...

How can I set process.env.NODE_ENV to production in my tests preferably in the beforeEach such that the if block in my service is false and the file transport is returned. I have omitted some code for the sake of brevity.

Answer 1

The core problem you are facing is caused by the fact that once you attempt to import the file that you are trying to test into your test suite - the code within it will be immediately evaluated and the implicitly invoked functions will be executed, meaning that logger.add(getTransport()); will be called before any of the functions like beforeEach have a chance to set the environment variables.

The only way to get around this is to use the following approach:

You will first need to assign the process.env.NODE_ENV environment variable to a const variable within another file. Let's call it environmentVariables.ts , and the following will be its contents:

export const ENVIRONMENT = process.env.NODE_ENV;

We will then have to refactor getTransport to use this variable as follows:

const getTransport = () => {
  if (ENVIRONMENT !== "production") {

In your test suite, you will then have to mock out the const file which will allow you to change what the ENVIRONMENT variable is set to. Note ../src/environmentVariables is an example directory and you will have to actually define what the relevant directory of this file is. Additionally make sure that this is outside of the describe clause, preferably above for readability:

jest.mock('../src/environmentVariables', () => ({
  ENVIRONMENT: 'production',
}));

Your unit tests will then execute with the ENVIRONMENT being production .