Very weird behavior for in loop javascript

Question

Correct code. it prints in the console:
1
0
https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSCFJNHQ2k1hvNCD3F9zKp9WiPooSBkjUy-ymynu1T0kmiaW-7r
66
which is pretty correct, bacaouse the is only one search_objects item.

            var j;
        for (j=0;j< search_objects.length; j++){
            console.log(j)
            website = search_objects[j].website;
            var rand = Math.floor(Math.random()*3)
            var img_src;
            switch (rand){
                case 0: img_src = "res1.png"; break;
                case 1: img_src = "res2.png"; break;
                case 2: img_src = "https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSCFJNHQ2k1hvNCD3F9zKp9WiPooSBkjUy-ymynu1T0kmiaW-7r"; break;
            }
            console.log(img_src);
            var template_result = '<div class="owl-item " ..........div>'

            console.log(search_objects[j].ID_subr)
            add_carousel_item(template_result)


        }

BUT if i change it to a for in loop it will executed one more time.

        var j; //same thing if i declare like this: for(var j in search_objects)
        for (j in search_objects){
            console.log(j)
            website = search_objects[j].website;
            var rand = Math.floor(Math.random()*3)
            var img_src;
            switch (rand){
                case 0: img_src = "res1.png"; break;
                case 1: img_src = "res2.png"; break;
                case 2: img_src = "https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSCFJNHQ2k1hvNCD3F9zKp9WiPooSBkjUy-ymynu1T0kmiaW-7r"; break;
            }
            console.log(img_src);
            var template_result = '<div class="owl-item " ..........div>'

            console.log(search_objects[j].ID_subr)
            add_carousel_item(template_result)


        }

And this loop in this second case will be executed 2 time when search_objects has only one item and this is proved by printing to the console the objects itself and it's length.
while this is shown in console.
1 //length
0 //j
res1.png //img from math.random
66 //property of the the first item
unique // value of j ??????!!!!!!!!!!!!!
res2.png //img from math.random
undefined //seems to show property of the search_objects[unique]

Notice: this happens most of the times. not 100% of the times. let's say it dont happen when i exe it as the first thing i do after page is refreshed. Anyone have seen such kind of thing before?

Possible duplicate of [Loop through array in JavaScript](http://stackoverflow.com/questions/3010840/loop-through-array-in-javascript) — John Dvorak, Apr 10 '16 at 20:35
Well there's no reason to use a `for ... in` loop on arrays anyway. — Pointy, Apr 10 '16 at 20:38

RJM · Accepted Answer · 2016-04-10T21:36:34.587

for...in will return not just the indexes of the array elements, but will return the names of ALL enumerable properties of the object being iterated over. So it looks like a property named unique is getting added to your search_objects object somewhere else in your code.

From this documentation page

Array indexes are just enumerable properties with integer names and are otherwise identical to general Object properties. There is no guarantee that for...in will return the indexes in any particular order. The for...in loop statement will return all enumerable properties, including those with non–integer names and those that are inherited.

Very weird behavior for in loop javascript

1 Answers1