Add events.once

Node.js 11.13 added an EventEmitter.once method: https://nodejs.org/api/events.html#events_events_once_emitter_name

It returns a Promise that resolves when the requested event is fired. If an ‘error’ event is fired first, the Promise rejects with the error.

The path to implementing this looks a bit like:

• Finding the Node.js implementation (start in lib/events.js in the nodejs/node repository)
• Finding the Node.js tests for this method. Test files are normally named something like test-modulename-functionname.js so you can find them by searching for events-once via the Github UI https://github.com/nodejs/node/find/master
• Copy-paste the test file from Node.js into tests/ and add its require call to tests/index.js. Since this test will require Promise support, it should only be executed if Promises are available. See the require('./symbols.js') call for one approach. (We run browser tests in old IE etc, so Promise support isn’t guaranteed.)
• Copy-paste the implementation code into events.js at the correct location (it should be in the same order in the file as in Node.js, to make things easy to cross-reference).
• Port everything to ES5…! It’s okay for the method itself to fail if Promise doesn’t exist. It should still be possible to use all other events features in environments that do not support Promises, though.
• Document that Promise support or a global polyfill are required for the EventEmitter.once API to work in README.md.

We can then release this as a minor version.

[vweevers]

You can pretty much copy https://github.com/nodejs/node/pull/26078/files, there have been no changes to the once function since that PR

[vweevers]

It really should have gotten a distinct name 😄