Can't read value of checkbox with value, innerHTML or checked

I'm doing a basic (becuse I'm not skilled and just like to play around bulidning small things to learn) database project, setting things by checking checkboxes. However, when trying to check a box and read if it is checked or not checked it always reads "null". I've tried .value, .innerHTML and .checked. Nothing gets me there. All I'm trying to do is to pick up the input (i.e. checked or not checked) with getElementById and store it in a var, and later on compare if the boolean is true och false to set the value in the databas accordingly.

checkbox = document.createElement("input");
checkbox.type = "checkbox";
var valueOfId = document.getElementById("idString").checked;

Error messege: Cannot read property 'checked' of null.

Any suggestions?