jquery selector syntax $( [] ) and $("*")

Question

The following line of code comes from the official dialog/#modal-form example

allFields = $( [] ).add( name )

Could someone please clarify what $( [] ) does? Is it the same as $("*")?

Another thing puzzled me is that I did not see allFields being added/appended to anywhere/any object, it is only created and modified. Am I missing something?

Answer 1

$([]) creates an empty jQuery object, just like $(). $('*') creates a jQuery object which holds all elements which match the CSS selector *, which every element will match.

In other words, $([]) gets nothing, $('*') gets everything.

Answer 2

If you take a look at the doco for the jQuery() function - usually seen as the shorthand version $() - you'll see that it accepts several different types and combinations of parameters.

The syntax you asked about:

$([])

is the jQuery( elementArray ) syntax which lets you pass an array of DOM elements where the return will be a jQuery object wrapping those elements. By passing an empty array you basically get an empty jQuery object (just as you would if you passed a selector string that didn't match anything, but without the inefficiency of trying to find a match first).

When created allFields has three DOM elements added to it (where name, email and password are created just before that as jQuery objects containing one DOM element each):

allFields = $( [] ).add( name ).add( email ).add( password )

Presumably the advantage of adding the individual items rather than just doing:

allFields = $("#name,#email,#password")

is that the individual objects for each element were also needed and would've been created anyway, so no need to bother reselecting them via a query string.

Another thing puzzled me is that I did not see allFields being added/appended to anywhere/any object, it is only created and modified, am I missing something?

It is referred to in two other places in the code:

allFields.removeClass( "ui-state-error" );
// and, later
allFields.val( "" ).removeClass( "ui-state-error" );

This is fairly standard jQuery usage to remove a class or set the value of all elements in the jQuery object. No need for it to be added to some other object.