eventEmitter.spec.js

/* eslint-env jest */
const eventEmitter = require("./eventEmitter");

describe("Event Emitter", () => {
  let emitter;
  const callback1 = jest.fn();
  const callback2 = jest.fn();

  beforeEach(() => {
    emitter = eventEmitter();
    callback1.mockClear();
    callback2.mockClear();
  });

  it("has simple API", () => {
    const keys = Object.keys(emitter).sort().join();
    expect(keys).toBe("emit,on");
  });

  it("passes data to registered callbacks", () => {
    emitter.on("event", callback1);
    emitter.on("click", callback2);

    emitter.emit("event", "foo");
    emitter.emit("click", "bar");

    expect(callback1).toHaveBeenCalledTimes(1);
    expect(callback1).toHaveBeenCalledWith("foo");
    expect(callback2).toHaveBeenCalledTimes(1);
    expect(callback2).toHaveBeenCalledWith("bar");
  });

  it("removes callback when it is unsubscibed", () => {
    const unSub = emitter.on("event", callback1);
    unSub();
    emitter.emit("event", "foo");

    expect(callback1).not.toHaveBeenCalled();
  });

  it("does not call a callback twice", () => {
    emitter.on("event", callback1);
    emitter.on("event", callback1);
    emitter.emit("event", "foo");

    expect(callback1).toHaveBeenCalledTimes(1);
    expect(callback1).toHaveBeenCalledWith("foo");
  });

  it("does not leak on sucessfull emit", () => {
    emitter.on("event", callback1);
    const result = emitter.emit("event", "foo");

    expect(result).toBeUndefined();
  });

  it("returns false on empty emit", () => {
    const result = emitter.emit("event", "foo");

    expect(result).toBe(false);
  });
});