How can I use a javascript array with getElementByID?

First, by explaining what you want to do...
GetElementById is used to return a reference to an HTML element with a specific id.
Nothing directly related to an array here...

document.getElementById(var[index]).value = another var[index];

I want to use a var to identify an elment by it's ID in order to set it's value property.

And your question is ?
Beside .value being correct only for a input element, of type "text", "password" or a textarea, what you wrote above is correct.

Yeah, only problem is it does not work, it crashes the script!

... And without seeing the page or the code, nobody can do anything.