Javascript observable objects with Proxy (ECMA-262 6th Edition, https://mzl.la/203LEg9)

observables.js

ObservableArray = function(array) {
  if (array == null) array = [];
  if (!Array.isArray(array)) throw new TypeError('argument is not an array');

  var listeners = [];

  this.toString = array.toString;
  this.listen = function(listener) {
    listeners.push(listener);
  };
  this.unlisten = function(listener) {
    var index = listeners.indexOf(listener);
    if (index >= 0) listeners.splice(index, 1);
  };

  var fire = function(index, removed, addedCount) {
    listeners.forEach(function(listener) {
      listener(index, removed, addedCount);
    });
  };

  var proxies = {
    push: function() {
      Array.prototype.push.apply(array, arguments);
      fire(array.length - arguments.length, [], arguments.length);
    },
    pop: function() {
      var removed = [array[-1]];
      Array.prototype.pop.apply(array);
      fire(array.length, removed, 0);
    },
    shift: function() {
      var removed = [array[0]];
      Array.prototype.shift.apply(array);
      fire(0, removed, 0);
    },
    unshift: function() {
      Array.prototype.unshift.apply(array, arguments);
      fire(0, [], arguments.length);
    },
    splice: function(index, removeCount) {
      var removed = array.slice(index, index + removeCount);
      Array.prototype.splice.apply(array, arguments);
      fire(index, removed, arguments.length - 2);
    }
  };

  var handler = {
    get: function(target, property) {
      if (property in proxies) return proxies[property];
      return target[property];
    },
    set: function(target, property, value) {
      var removed = [target[property]];
      target[property] = value;
      fire(property, removed, 1);
      return true;
    }
  };

  Object.setPrototypeOf(this, new Proxy(array, handler));
};

Thanks!

1. Never!
2. It would be better to use if (array === undefined) I guess.
3. True
4. True

Further, I should avoid setPrototypeOf. Maybe just return the Proxy and add the listener functions there.