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Why exactly is Object.prototype verboten

Why exactly is Object.prototype verboten?

There are several sites claiming that Object.prototype is forbidden. But they don’t really provide a complete explanation of why it is so. Maybe it’s not so forbidden after all? This article takes another stab at this question.

Motivation

Everybody likes the neat iterator methods in Array.prototype. Using these you can write some really short and expressive code:

var persons = [
  {name: "John", age: 10},
  {name: "Mary", age: 28},
  {name: "Alice", age: 16}
];

var averageAge = persons.reduce(function(sum, p){
  return sum + p.age;
}, 0) / persons.length;

// or even shorter
var averageAge = persons.pluck("age").average();

It would be really great to use these same iterator methods on objects:

var ages = {
  John: 10,
  Mary: 28,
  Alice: 16
};

var averageAge = ages.reduce(function(sum, age){
  return sum + age;
}, 0) / ages.length();

// or
var averageAge = ages.average();

// with the added benefit of accessing a particular person
// directly by its name:
if (person.Mary > averageAge) {
  alert("Mary is above average!");
}

Straight-forward Object.prototype extension

So, lets not hold ourselves back and add the average method to Object.prototype:

Object.prototype.average = function() {
  var sum = 0, cnt = 0;
  for (var i in this) {
    sum += this[i];
    cnt++;
  }
  return sum / cnt;
};

And now lets try it out:

var ages = {John: 10, Mary: 20, Alice: 30};
ages.average();

The average of 10,20,30 should be 20, but when we run the above script it prints out NaN. Hei! What happened!

The problem is, that the for in loop iterates over all the properties of an object – including those defined in prototype. That means our average will not iterate over three, but four objects. Three of the objects are numbers and adding them together is no problem, but the fourth is a function (the Object.prototype.average itself) and trying to add that produces NaN.

hasOwnProperty()

Luckily we are able to differenciate between the primary properties of the object and those defined by its prototype using the hasOwnProperty method. Here’s an enhanced version of average:

Object.prototype.average = function() {
  var sum = 0, cnt = 0;
  for (var i in this) {
    if (this.hasOwnProperty(i)) {
      sum += this[i];
      cnt++;
    }
  }
  return sum / cnt;
};

Running the above test code again gives us our expected 20.

But now we have another problem. No more can we use plain for-in loop anywhere in our code – we have to place the hasOwnProperty check inside each one of them. And not only in our code, but also in all the third-party code we use. Even when you are willing to modify the code of your libraries, you might not have enough control to do it (for example when you are using Google Maps library that is hosted directly from Google and isn’t even downloadable).

But lets pause for a minute. Isn’t it so that Object.prototype also contains the built-in hasOwnProperty method, but when we iterate over an object with for-in the hasOwnProperty doesn’t show up, although our average method does? The hasOwnProperty is somehow uniterable! When we could make our average method also to be similarly unitarable, then we could escape from the whole add-hasOwnProperty-inside-every-for-in-loop problem.

ECMAScript 5

And indeed, this is a problem with JavaScript language and a one targeted by the forthcoming 5th edition of ECMAScript (a long and boring PDF, so I suggest you better take a look at the overview provided by John Resig). If we were to be using ECMAScript 5 compatible browser, then we could define our average method as not being enumerable:

Object.defineProperty(Object.prototype, "average", {
  enumerable: false,
  value: function() {
    var sum = 0, cnt = 0;
    for (var i in this) {
      sum += this[i];
      cnt++;
    }
    return sum / cnt;
  }
});

So at least in the future we will be able to extend Object.prototype with some nice generic iterator functions. Happy end? Well... not really.

The fundamental problem with Object.prototype

Consider this code:

var ages = {John: 10, Mary: 20, Alice: 30, average: 15};
ages.average();

The name of one of our subjects happens to be average – the very same as the name of the method we want to use. That new average hides our intended average in the prototype and our code breakes.

The same applies to any method in Object.prototype. Had we adopted the for-in-with-hasOwnProperty approach, then a malicious user could enter “hasOwnProperty” to the right place and bring our system completely down.

Sure, there is a way to ensure, that we are indeed using the exact method defined in prototype:

Object.prototype.average.call(ages);

But this kind of code really isn’t what we strived for.

It only remains to conclude, both for now and for the days ahead:

Object.prototype is verboten.

Kirjutatud 13. septembril 2009.

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