document.getElementById(\'frmMain\').elements
can i use like this
document.getElementByName(\'frmMain\').elements
It's getElementsByName() and getElementsByTagName() - note the "s" in "Elements", indicating that both functions return a list of elements, i.e., a NodeList, which you will access like an array. Note that the second function ends with "TagName" not "Tag".
Even if the function only returns one element it will still be in a NodeList of length one. So:
var els = document.getElementsByName('frmMain');
// els.length will be the number of elements returned
// els[0] will be the first element returned
// els[1] the second, etc.
Assuming your form is the first (or only) form on the page you can do this:
document.getElementsByName('frmMain')[0].elements
document.getElementsByTagName('table')[0].elements