Runner end event can’t be caught if the grep option matches no tests

[Mr0grog]

Prerequisites

• Checked that your issue isn't already filed by cross referencing issues with the common mistake label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with:
  node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend avoiding the use of globally installed Mocha.

Description

When running Mocha programmatically or in a browser, mocha.run() returns the test runner instance and event handlers can be attached to it. However, when the grep option is used and it matches no tests, ]it emits the end event synchronously](https://github.com/mochajs/mocha/blob/master/lib/runner.js#L621-L623) before the runner is ever returned from mocha.run() and before any third-party code could attach a listener.

I know I can work around this by providing a callback to mocha.run(), but it still seems reasonable to expect that you’d always have an opportunity to attach an event listener before the end event fires.

Steps to Reproduce

If I have a test suite at /single-test.js that looks like:

describe('Test suite', function () {
  it('A single passing test', function() {
    // this passes
  });
});

And I run the following:

const Mocha = require('mocha');
const mocha = new Mocha({ui: 'bdd', grep: /unmatchable/});
mocha.addFile('/single-test.js');
mocha
  .run()
  .on('end', () => console.log('End event handled!'));

Expected behavior: I expect “End event handled!” to be logged to the console.

Actual behavior: The test suite runs, but my handler does not.

Reproduces how often: 100%

Versions

Mocha 4.0.1

[ScottFreeCode]

I wonder if fixing #2753 would automatically fix this too.

[Mr0grog]

Ah! Yes, that definitely would. (Assuming end is guaranteed to come after start :P )

[boneskull]

duplicate of #2753