[Feature request] stub emits

[perrin4869]

In addition to returns, yields, throws, etc, when stubbing a method, i.e., stub(obj, 'method'), it would be useful to specify an emits behavior, such as, for example:

const obj = require('obj');
const stub = sinon.stub(obj, 'method').emits('some event', arg1, arg2);
const spy = sinon.spy();

obj.on('some event', spy);
obj.method();
expect(spy.calledOnce).to.equal(true);
expect(spy.firstCall.args).to.deep.equal([arg1, arg2]);

Does this seem feasible? I could try to make an implementation of this, just not sure how feasible it is realistically.

[mantoni]

You can use any event emitter for that. Either stub on or even better, simply call emit on the emitter in your test code.

[mantoni]

Here are two ways how you can do it in your particular example:

const stub = sinon.stub(obj, 'method', function () {
  obj.emit('some event');
});

Alternatively:

const stub = sinon.stub(obj, 'method');
// ...

obj.method();
obj.emit('some event');

It would be out of scope for Sinon to add library or environment specific logic for something like event emitters.

If you have more questions about best practices on how to use Sinon with event emitters, please post questions on the Sinon.JS mailinglist, so the bigger community can help answer your questions.

[perrin4869]

Got it, thanks!!

[aleung]

Another approach, if you have to use sinon.createStubInstance for some reason: make use of npm module extend:

import * as extend from 'extend';
import * as events from 'events';

const stub = sinon.createStubInstance(MyClass);
extend(stub, events.EventEmitter.prototype);

stub.on('event', () => { /* do something */ });
stub.method();
stub.emit('event');