js. Oddities with prototype?

Question

JavaScript

Good afternoon.
There is a function

Array.prototype.UContainsCustom = function(closure)
{
    var i, pLen = this.length;
    for (i = 0; i < pLen; i++)
    {
        if (closure(this[i])) { return i; }
    }
    return -1;
}

the problem is that because of this function, strange things happen in the loop:

console.log(currentOffer.IMAGES);
                for (i in currentOffer.IMAGES) {
                    img = currentOffer.IMAGES[i];
                    console.log(img);
                    
                }

first console.log shows

0: "/upload/iblock/0b2/0b223b35e0910f3adb978064abeb335c.jpg"
1: "/upload/iblock/5b7/5b7dfa007d387fab8cca969ca14cdc42.jpg"

everything is ok here.
the second console.log shows already showing weirdness:

/upload/iblock/0b2/0b223b35e0910f3adb978064abeb335c.jpg
/upload/iblock/5b7/5b7dfa007d387fab8cca969ca14cdc42.jpg
function(closure)
{
    var i, pLen = this.length;
    for (i = 0; i < pLen; i++)
    {
        if (closure(this[i])) { return i; }
    }
    return -1;
}

those. on the third iteration, which should not be, the code of this function is displayed.
How can this be explained?

Answer 1

In JavaScript, object properties can be enumerable or non-enumerable...
Enumerated properties participate in iteration in a for...in loop...

https://developer.mozilla.org/en/docs/Web/JavaScript...

Answer 2

for (i in currentOffer.IMAGES) {
  if (!currentOffer.IMAGES.hasOwnProperty(i)) continue; // magic here =))
  img = currentOffer.IMAGES[i];
  console.log(img);
}

Answer 3

Strangeness not with ptototype, but with the study of language features Extending the
prototype of built-in classes (Array, Object, String etc.) is considered bad practice (shit code), but if you really need it, you need to set the flag enumerable = false for the new properties of the prototype object

Object.defineProperty(Array.prototype, 'UContainsCustom ', {
  value: function(closure) {
    var i, pLen = this.length;
    for (i = 0; i < pLen; i++) {
        if (closure(this[i])) return i;
    }
    return -1;
  }
});

https://developer.mozilla.org/en/docs/Web/JavaScript...