Multi tool use

Javascript: enable buttons on checkbox' check (both with ElementsByClass)

I am sorry I have to bother you guys with such a noob question, but I am trying and googeling since two days now and I just dont seem to find the point I need, or maybe I just don't understand it.

The case is the following:
I have two rows in a table: 1. row checkbox - 2. row button
I want the button only be enabled, if the checkbox in the same row is enabled.
Picture for better understanding

<input type="checkbox" class="isamed" name="attr_targeted_checkbox1">
<button type='roll' class="isroll" disabled="true">


I used classes and stumbled across getElementsByClassName and the fact, that you get an array/node of objects.
I tried something like this:

var buttons = document.getElementsByClassName('isroll')
var keyselects = document.getElementsByClassName('isaimed');
for (var i = 0; i < keyselects.length; i++) {
if (keyselects[i].checked == true) {
buttons[i].disabled = false;
break;
}
}


and like 20 other codeparts and functions I found on the web, but it's not working. Maybe I have to assign the rows or something like that?
Really I am just utterly desperate and my mind is full of chunk right now.

Thank you very much!

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