Fix: listen to 'start' and 'end' events when using Mocha programmatically

[juergba]

Description

mocha.run() returns a runner instance. Since the EVENT_RUN_BEGIN (start) event is emitted synchronously, it's impossible to register an EVENT_RUN_BEGIN listener in time. The event is emitted, but never catched.

When no tests are provided - eg. by a too restrictive grep expression - then neither EVENT_RUN_BEGIN nor EVENT_RUN_END (end) events can be listened to.

var mocha = new Mocha();

mocha.addFile("../eventStart.js");

mocha.run()
  .on(EVENT_RUN_BEGIN, () => console.log('start'))    // never runs
  .on(EVENT_RUN_END, () => console.log('end'));       // never runs when no tests are provided

Description of Change

We delay the start of the root Suite by Runner.immediately. The order of the event emissions remains unchanged.

Applicable issues

#3395 (backed out)
#3331
#3065
closes #2753

[coveralls]

Coverage increased (+0.07%) to 92.97% when pulling 1f31f4e on juergba:juergba/issue/3414 into 9cbb6f6 on mochajs:master.

[juergba]

I messed up my branch, now everything is ok. Hopefully this will reopen my PR.

[plroebuck]

There is no way to register EventListener to the runner instance when Mocha runs programmatically with mocha.run().

Isn't this already possible with delayed root suite?

[juergba]

@plroebuck
I don't expect the user to know the answer, if I (we) don't know it without analyzing code and try-and-error testing. It wouldn't be an user-friendly solution.

yes, but... : if there are test files found, which include the (delay) run() function. (no: waiting)
no: if there are no test files found (grep)
no: waiting event doesn't work at all, which is emitted before start event

--delay does alter the event firing. start and end events are emitted twice, probably one additional set while the files get loaded.

[boneskull]

It seems we could instantiate the Runner in the Mocha constructor instead of in Mocha#run(), and then make it a property of the Mocha instance:

function Mocha() {
  // create root suite, process options
  this.runner = new Runner(rootSuite, options.delay);
}

Then users could:

const mocha = new Mocha();
mocha.runner.on('stuff', () => {})

Needs conflicts resolved

Outdated, I started a new approach to fix this issue