Spies

Mocha does not come equipped with spies, though libraries like Sinon provide this behaviour if desired. The following is an example of Mocha utilizing Sinon and Should.js to test an EventEmitter:

import sinon from "sinon";
import { EventEmitter } from "node:events";

describe("EventEmitter", function () {
  describe("#emit()", function () {
    it("should invoke the callback", function () {
      const spy = sinon.spy(),
        emitter = new EventEmitter();

      emitter.on("foo", spy);
      emitter.emit("foo");
      spy.called.should.equal.true;
    });

    it("should pass arguments to the callbacks", function () {
      const spy = sinon.spy(),
        emitter = new EventEmitter();

      emitter.on("foo", spy);
      emitter.emit("foo", "bar", "baz");
      sinon.assert.calledOnce(spy);
      sinon.assert.calledWith(spy, "bar", "baz");
    });
  });
});

The following is the same test, performed without any special spy library, utilizing the Mocha done([err]) callback as a means to assert that the callback has occurred, otherwise resulting in a timeout. Note that Mocha only allows done() to be invoked once, and will otherwise error.

describe("EventEmitter", function () {
  describe("#emit()", function () {
    it("should invoke the callback", function (done) {
      const emitter = new EventEmitter();

      emitter.on("foo", done);
      emitter.emit("foo");
    });

    it("should pass arguments to the callbacks", function (done) {
      const emitter = new EventEmitter();

      emitter.on("foo", function (a, b) {
        a.should.equal("bar");
        b.should.equal("baz");
        done();
      });
      emitter.emit("foo", "bar", "baz");
    });
  });
});