puppeteer/test/EventEmitter.spec.js

const { EventEmitter } = require('../lib/EventEmitter');
const sinon = require('sinon');
const expect = require('expect');

describe('EventEmitter', () => {
  let emitter;

  beforeEach(() => {
    emitter = new EventEmitter();
  });

  describe('on', () => {
    const onTests = (methodName) => {
      it(`${methodName}: adds an event listener that is fired when the event is emitted`, () => {
        const listener = sinon.spy();
        emitter[methodName]('foo', listener);
        emitter.emit('foo');
        expect(listener.callCount).toEqual(1);
      });

      it(`${methodName} sends the event data to the handler`, () => {
        const listener = sinon.spy();
        const data = {};
        emitter[methodName]('foo', listener);
        emitter.emit('foo', data);
        expect(listener.callCount).toEqual(1);
        expect(listener.firstCall.args[0]).toBe(data);
      });

      it(`${methodName}: supports chaining`, () => {
        const listener = sinon.spy();
        const returnValue = emitter[methodName]('foo', listener);
        expect(returnValue).toBe(emitter);
      });
    };
    onTests('on');
    // we support addListener for legacy reasons
    onTests('addListener');
  });

  describe('off', () => {
    const offTests = (methodName) => {
      it(`${methodName}: removes the listener so it is no longer called`, () => {
        const listener = sinon.spy();
        emitter.on('foo', listener);
        emitter.emit('foo');
        expect(listener.callCount).toEqual(1);
        emitter.off('foo', listener);
        emitter.emit('foo');
        expect(listener.callCount).toEqual(1);
      });

      it(`${methodName}: supports chaining`, () => {
        const listener = sinon.spy();
        emitter.on('foo', listener);
        const returnValue = emitter.off('foo', listener);
        expect(returnValue).toBe(emitter);
      });
    };
    offTests('off');
    // we support removeListener for legacy reasons
    offTests('removeListener');
  });

  describe('once', () => {
    it('only calls the listener once and then removes it', () => {
      const listener = sinon.spy();
      emitter.once('foo', listener);
      emitter.emit('foo');
      expect(listener.callCount).toEqual(1);
      emitter.emit('foo');
      expect(listener.callCount).toEqual(1);
    });

    it('supports chaining', () => {
      const listener = sinon.spy();
      const returnValue = emitter.once('foo', listener);
      expect(returnValue).toBe(emitter);
    });
  });

  describe('emit', () => {
    it('calls all the listeners for an event', () => {
      const listener1 = sinon.spy();
      const listener2 = sinon.spy();
      const listener3 = sinon.spy();
      emitter.on('foo', listener1).on('foo', listener2).on('bar', listener3);

      emitter.emit('foo');

      expect(listener1.callCount).toEqual(1);
      expect(listener2.callCount).toEqual(1);
      expect(listener3.callCount).toEqual(0);
    });

    it('passes data through to the listener', () => {
      const listener = sinon.spy();
      emitter.on('foo', listener);
      const data = {};

      emitter.emit('foo', data);
      expect(listener.callCount).toEqual(1);
      expect(listener.firstCall.args[0]).toBe(data);
    });

    it('returns true if the event has listeners', () => {
      const listener = sinon.spy();
      emitter.on('foo', listener);
      expect(emitter.emit('foo')).toBe(true);
    });

    it('returns false if the event has listeners', () => {
      const listener = sinon.spy();
      emitter.on('foo', listener);
      expect(emitter.emit('notFoo')).toBe(false);
    });
  });

  describe('listenerCount', () => {
    it('returns the number of listeners for the given event', () => {
      emitter.on('foo', () => {});
      emitter.on('foo', () => {});
      emitter.on('bar', () => {});
      expect(emitter.listenerCount('foo')).toEqual(2);
      expect(emitter.listenerCount('bar')).toEqual(1);
      expect(emitter.listenerCount('noListeners')).toEqual(0);
    });
  });

  describe('removeAllListeners', () => {
    it('removes every listener from all events by default', () => {
      emitter.on('foo', () => {}).on('bar', () => {});

      emitter.removeAllListeners();
      expect(emitter.emit('foo')).toBe(false);
      expect(emitter.emit('bar')).toBe(false);
    });

    it('returns the emitter for chaining', () => {
      expect(emitter.removeAllListeners()).toBe(emitter);
    });

    it('can filter to remove only listeners for a given event name', () => {
      emitter
        .on('foo', () => {})
        .on('bar', () => {})
        .on('bar', () => {});

      emitter.removeAllListeners('bar');
      expect(emitter.emit('foo')).toBe(true);
      expect(emitter.emit('bar')).toBe(false);
    });
  });
});