Isolated test case for the discussion around rest parameters for Node's EventEmitter (https://github.com/nodejs/node/pull/13155#issuecomment-334706721)

bench-event-emitter.js

// Isolated test case for the discussion around rest parameters for Node's EventEmitter
// https://github.com/nodejs/node/pull/13155#issuecomment-334706721
if (typeof console === 'undefined') console = {log:print};

const N = 2e7;
const TESTS = [];

(function() {
  function emitNone(handler, isFn, self) {
    if (isFn)
      handler.call(self);
    else {
      var len = handler.length;
      var listeners = arrayClone(handler, len);
      for (var i = 0; i < len; ++i)
        listeners[i].call(self);
    }
  }
  function emitOne(handler, isFn, self, arg1) {
    if (isFn)
      handler.call(self, arg1);
    else {
      var len = handler.length;
      var listeners = arrayClone(handler, len);
      for (var i = 0; i < len; ++i)
        listeners[i].call(self, arg1);
    }
  }
  function emitTwo(handler, isFn, self, arg1, arg2) {
    if (isFn)
      handler.call(self, arg1, arg2);
    else {
      var len = handler.length;
      var listeners = arrayClone(handler, len);
      for (var i = 0; i < len; ++i)
        listeners[i].call(self, arg1, arg2);
    }
  }
  function emitThree(handler, isFn, self, arg1, arg2, arg3) {
    if (isFn)
      handler.call(self, arg1, arg2, arg3);
    else {
      var len = handler.length;
      var listeners = arrayClone(handler, len);
      for (var i = 0; i < len; ++i)
        listeners[i].call(self, arg1, arg2, arg3);
    }
  }

  function emitMany(handler, isFn, self, args) {
    if (isFn)
      handler.apply(self, args);
    else {
      var len = handler.length;
      var listeners = arrayClone(handler, len);
      for (var i = 0; i < len; ++i)
        listeners[i].apply(self, args);
    }
  }

  const EventEmitter = class EventEmitter {
    constructor() { this._events = Object.create(null); }
    emit(type) {
      const events = this._events;
      const handler = events[type];
      if (!handler) return false;
      var isFn = typeof handler === 'function';
      var len = arguments.length;
      switch (len) {
        // fast cases
        case 1:
          emitNone(handler, isFn, this);
          break;
        case 2:
          emitOne(handler, isFn, this, arguments[1]);
          break;
        case 3:
          emitTwo(handler, isFn, this, arguments[1], arguments[2]);
          break;
        case 4:
          emitThree(handler, isFn, this, arguments[1], arguments[2],
                    arguments[3]);
          break;
        // slower
        default:
          args = new Array(len - 1);
          for (i = 1; i < len; i++)
            args[i - 1] = arguments[i];
          emitMany(handler, isFn, this, args);
      }
      return true;
    }
    on(type, handler) {
      this._events[type] = handler;
    }
  };

  const ee = new EventEmitter();
  for (var k = 0; k < 10; k += 1)
    ee.on('dummy', function() {});

  TESTS.push(
      function testDispatchNode() {
        ee.emit('dummy');
      },
      function testDispatchNodeMultiArgs() {
        ee.emit('dummy', 5, true);
      });
})();

(function() {
  const EventEmitter = class EventEmitter {
    constructor() { this._events = Object.create(null); }
    emit(type, ...args) {
      const events = this._events;
      const handler = events[type];
      if (typeof handler === 'function') {
        handler.call(this, ...args);
      } else if (handler != null) {
        var len = handler.length;
        var listeners = arrayClone(handler, len);
        for (var i = 0; i < len; ++i)
          listeners[i].call(this, ...args);
      }
      return true;
    }
    on(type, handler) {
      this._events[type] = handler;
    }
  };

  const ee = new EventEmitter();
  for (var k = 0; k < 10; k += 1)
    ee.on('dummy', function() {});

  TESTS.push(
      function testDispatchRest() {
        ee.emit('dummy');
      },
      function testDispatchRestMultiArgs() {
        ee.emit('dummy', 5, true);
      });
})();

function test(fn) {
  var result;
  for (var i = 0; i < N; i += 1) result = fn();
  return result;
}

for (var j = 0; j < TESTS.length; ++j) {
  test(TESTS[j]);
}

for (var j = 0; j < TESTS.length; ++j) {
  var startTime = Date.now();
  test(TESTS[j]);
  console.log(TESTS[j].name + ':', (Date.now() - startTime), 'ms.');
}

Results with V8 6.1 and 6.2:

$ ./d8-6.1.534.15 bench-event-emitter.js
testDispatchNode: 525 ms.
testDispatchNodeMultiArgs: 643 ms.
testDispatchRest: 644 ms.
testDispatchRestMultiArgs: 822 ms.
$ ./d8-6.2.414.2 bench-event-emitter.js
testDispatchNode: 421 ms.
testDispatchNodeMultiArgs: 504 ms.
testDispatchRest: 389 ms.
testDispatchRestMultiArgs: 475 ms.