Need proper syntax

What's the proper syntax to make this work:

document.myForm.myField.value = newValue;
(Where red portions are variables)

I thought it should be something like:
document.(eval(myForm)).eval(myField).value = newValue;

...but it's not working and cannot seem to find a reference for the proper way to write it. Thanks for any input

 

document[myForm][myField].value = newValue;

Code (markup):

 

I don't think you should use that approach because the name attribute in the form tag isn't valid.

 

It isn't in XHTML, that's true. Usually the best thing to do is to call the form fields by theyr id, but there are many people who still does things this way.

 

Thanks for the feedback and suggestions. Should've had morning coffee before posting. I resolved this by using:

document.getElementById(myField).value = newValue;