For loop index variable modified with helper method?

Question

I'm using the following for loop and as it is written below it alerts 0, 1, 2, 3, 4, 5, 6, 7, 8 (like it should) but when I uncomment set(buttons, i) it alerts 0, 2, 4, 6, 8.

for(i = 0; i < layerOrder.length; i++){
    alert(i);
    //set(buttons, i);
}

I'm very confused by this behavior because I have always assumed that integers were passed by value and not reference.

Is it possible to modify the for loop index variable in a function method like in my set() method? If so I can focus my time on debugging that.

primitives are passed by value. what's the code of `set()`? does it use a variable `i`? have you declared it inside the function? — Thomas, May 27 '16 at 20:47
The code that you've provided in your question won't reproduce the problem you're having. You probably ought to declare `i` in the `for` loop itself, rather than referencing a global variable `i`. — Robert Harvey, May 27 '16 at 20:49
There's quite a bit of code involved to reproduce the problem. I followed it back to this for loop but I understand now it was a question of scope. Adding `var i = 0` did actually solve the problem. — spencer.sm, May 27 '16 at 20:53

score 3 · Accepted Answer · edited May 23 '17 at 12:24

3

This looks like something that is caused by leaving out the var. Please update your first line to read:

for(var i = 0; i < layerOrder.length; i++){

Please refer to this page for more info: "var" or no "var" in JavaScript's "for-in" loop?

edited May 23 '17 at 12:24

Community

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answered May 27 '16 at 20:47

Kenny Grage

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Also the `set` function needs to declare `i` as a local variable. – Barmar May 27 '16 at 21:45

score 1 · Answer 2 · edited May 27 '16 at 21:11

1

You must be incrementing i in set method.

Since you have not described i as local variable by prefixing var keyword it is treated as a global variable in for loop. And hence is modified in set method. In javascript to declare a variable locally prefix it with var or else they are defined as global variables.

edited May 27 '16 at 21:11

GMchris

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answered May 27 '16 at 20:47

Aditya

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For loop index variable modified with helper method?

2 Answers2