The document.getElementId()
function returns an object value based on the identifier string (ID) that you supply; therefore if you have an element on your page with the id of foo
then you call
document.getElementById("foo");
to return the object for that element, so that you can manipulate it (changing styles, or attributes etc).
If you want to insert the id into the URL of the test.php page, why not simply pass the identifier string of the element that you pass in with the function?:
function check(id){
url = "test.php?check=1&id=" + id;
}
Then calling
check("123");
will set the url
variable to test.php?check=1&id=123