How do read/write <input> values in an HTML document with JavaScript?

I have an <input> in my document. There I want the user to do some input.

var myVar = document.getElementById('userInput').value;

Then I want this value to be shown in another <input>.

document.getElementById('Preview').value = myVar;

This code somehow doesn't work.

Can anybody help?

Thanks in advance!

Update based on further information in comments before:

<button onClick="calculateThatObscenityDeleted()">"Save"</button >

Submit buttons will submit forms, thus running the JS but immediately blanking the form.

(That might still not be the actual issue, the question is still missing most of the code)

Original answer before it was revealed that the question didn't reflect the problem:

var myVar=document.getElementById('userInput').value;

Don't forget the =.

(And, obviously, you need to use that code in an event handler so it isn't executed only when the document loads and before the user has typed anything.)

Is it form? Try something like this:

oFormObject = document.forms['myform_id'];
oFormObject.elements["element_name"].value = 'Some Value';

Besides the typo, you have to bind an event handler to the first <input>:

var input_a = document.getElementById('userInput');
var input_b = document.getElementById('Preview');

input_a.onkeyup = function(){
    input_b.value = input_a.value;    
}