How to loop through selected elements with document.querySelectorAll

for in loop is not recommended for arrays and array-like objects - you see why. There can be more than just number-indexed items, for example the length property or some methods, but for in will loop through all of them. Use either

for (var i = 0, len = checkboxes.length; i < len; i++) {
    //work with checkboxes[i]
}

or

for (var i = 0, element; element = checkboxes[i]; i++) {
    //work with element
}

The second way can't be used if some elements in array can be falsy (not your case), but can be more readable because you don't need to use [] notation everywhere.

My favorite is using spread syntax to convert the NodeList to an array and then use forEach for looping.

var div_list = document.querySelectorAll('div'); // returns NodeList
var div_array = [...div_list]; // converts NodeList to Array
div_array.forEach(div => {

// do something awesome with each div

});

I code in ES2015 and use Babel.js, so there shouldn't be a browser support issue.