for loop index being converted into a string for no reason

Question

I'm writing in JavaScript and detected this strange behavior that I can't explain.

for (i in bubbles){

bubbles[i].string = "some stuff!" // <- no errors here

results[0] = i - 1 
results[1] = i + 1
results[2] = parseInt(i) + 1 


}

when i = 1 this happens

results[0] -> 0
results[1] -> 11
results[2] -> 2

is this even possible?! Maybe it's due to other errors in the code. I tried to isolate the case above but, if you need it, here's the whole code

for (i in bubbles){

            if (bubbles[i].check()){

                // define which boubble has been clicked and start dragging
                bubbleDrag[2] = bubbles[i].check();
                bubbleDrag[1] = i;
                bubbleDrag[0] = true;

                // define where to check to avoid overlapping dates
                if (i != 0 && i < bubbles.length - 1){

                    bubbleDrag[3] = i - 1;
                    bubbleDrag[4] = i + 1;

                } else if (i == 0 && bubbles.lenght > 1){

                    bubbleDrag[3] = i + 1;

                } else if (i == bubbles.lenght - 1){

                    bubbleDrag[3] = i - 1;

                }

            }

        }

What is `bubbles`? A `for -in` loop in JavaScript is intended to iterate over _objects_ `{}`, not over _arrays_ `[]`. The iterator variable will always be a string. — Michael Berkowski, Mar 20 '13 at 16:35
In a `for...in` loop, the loop variable is always a string, because you are iterating over *object properties*. If you are iterating over an array, use a normal `for` loop. See https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/for...in#Description and http://stackoverflow.com/q/2265167/218196. — Felix Kling, Mar 20 '13 at 16:35
bubbles is already an array of objects, I've explicitated that in the code now... My bad! — Saturnix, Mar 20 '13 at 16:38

Brad M · Accepted Answer · 2013-03-20T16:47:56.340

Javascript is interpreting your code.

results[0] = i - 1 
// string minus number, so javascript "assumes" you want "i" as a number

results[1] = i + 1 
// string concatenate with a number, so javascript assumes you want a concatenated string

More examples.

"30" - 10; // echoes number 20
"30" + 10; // echoes string "3010"

Some people like that languages interpret like this, some don't. I find myself in the latter. IMO, string + number should throw an error since intentions aren't explicit. Loose/non-strict interpretations may end up leading to unexpected results. If you read some of Douglas Crockford's code, you'll notice he uses extensively strict type comparison (===, !==) and this is part of the reason why.

for loop index being converted into a string for no reason

1 Answers1