Question

What is the true difference between document.getElementById('SOMEID') and $("[id$='SOMEID']")
?

Answer 1

The $(..) variant uses jQuery, which allows you to use selectors to find what you are searching for (class, id, tag etc). The getElementById() is plain javascript which obviously only searches by element id.

$("#foo") // select elements with id foo
$(".foo") // select elements with class foo
$("foo") // select foo-tags

You can learn more about jQuery’s support for selectors in this official jQuery tutorial.