Testing process.exit with Jest in NodeJS

The scenario

Consider you have the following function to be tested with Jest:

function myFunc() {
    //
    // ...do "stuff"
    //
    if (condition) {
        process.exit(ERROR_CODE);
    }
    //
    // ...do "other stuff"
    //
 }

Setting up the test case

To test the branch where condition is true you have to mock process.exit because otherwise the Jest test process would truly exit and therefore fail. Of course this mocking should have been done in a way that "other stuff" is never executed in this test.

To achieve that we use Jest's spyOn to implement a mock for process.exit that instead of exiting the process will throw en error.

const mockExit = jest.spyOn(process, 'exit')
    .mockImplementation((number) => { throw new Error('process.exit: ' + number); });

This ensures that the execution of our function ends immediately without doing "other stuff" and without ending the Jest test process. Also this mock serves to check if process.exit really was called and what the exit code was. We do this with Jest's toHaveBeenCalledWith test function.

To get the test case up and running we have to wrap our functions execution in an expect( ... ).toThrow() statement because it is now throwing an error by using the mock. Also it is a good practice to restore the original mocked function by calling mockRestore to avoid unintended side-effects.

So we have our final test case looking like that:

it('tests myFunc with process.exit', async () => {
    const mockExit = jest.spyOn(process, 'exit')
        .mockImplementation((number) => { throw new Error('process.exit: ' + number); });
    expect(() => {
        myFunc(true);
    }).toThrow();
    expect(mockExit).toHaveBeenCalledWith(-1);
    mockExit.mockRestore();
});

Try it out

npm install
npm run test

Happy testing ;)