How to mock test a Node.js CLI with Jest?

Question

I'm stuck at the very beginning, simply requiring the CLI and capturing its output. I've tried two methods but both don't work.

This is my cli.js:

#!/usr/bin/env node

console.log('Testing...');
process.exit(0);

And this my cli.test.js:

test('Attempt 1', () => {
    let stdout = require("test-console").stdout;
    let output = stdout.inspectSync(function() {
        require('./cli.js');
    });
    expect(output).toBe('Testing...');
});

test('Attempt 2', () => {
    console.log = jest.fn();
    require('./cli.js');
    expect(console.log.calls).toBe(['Testing...']);
});

Doesn't really matter which test is actually being run, the output is always:

$ jest

 RUNS  bin/cli.test.js
Done in 3.10s.

Answer 1

Node.js CLI applications are no different to other applications except their reliance on environment. They are expected to extensively use process members, e.g.:

• process.stdin
• process.stdout
• process.argv
• process.exit

If any of these things are used, they should be mocked and tested accordingly.

Since console.log is called directly for output, there's no problem to spy on it directly, although helper packages like test-console can be used too.

In this case process.exit(0) is called in imported file, so spec file early exits, and next Done output is from parent process. It should be stubbed. Throwing the error is necessary so that code execution is stopped - to mimic the normal behavior:

test('Attempt 2', () => {
    const spy = jest.spyOn(console, 'log');
    jest.spyOn(process, 'exit').mockImplementationOnce(() => {
      throw new Error('process.exit() was called.')
    });

    expect(() => {
      require('./cli.js');
    }).toThrow('process.exit() was called.');
    expect(spy.mock.calls).toEqual([['Testing...']]);
    expect(process.exit).toHaveBeenCalledWith(0);
});