jQuery what is faster: selectors or methods?

Question

Let's say I have $('mySelector:first'); and $('mySelector').first();. Which way is the most efficient? I looked in the source, but still couldn't figure it out.

It looks like in the first case jQuery goes through every item until gets the first one:

CHILD: function( elem, match ) {
        var type = match[1],
        node = elem;
        switch ( type ) {
            ...
         case "first":
          while ( (node = node.previousSibling) )  {
           if ( node.nodeType === 1 ) { 
            return false; 
           }
          }
          if ( type === "first" ) { 
           return true; 
          }
          node = elem;
                ...
        }
}

In second case jQuery slices the collection, but I am not sure how efficient it is:

function first() {
  return this.eq( 0 );
};

function eq( i ) {
  return i === -1 ?
    this.slice( i ) :
    this.slice( i, +i + 1 );
};

Answer 1

The current accepted answer is not consistent with tests across many browsers comparing :first and :eq(0) to .first() and .eq(0).

For the current major desktop browsers:
$('foo').first() is almost four times faster than $('foo:first')

If you want to inspect the methodology, here are the tests and their current results.

Answer 2

The second would have to fetch ALL the items in the selector before getting the first. So the if the selector was 10,000 items it would fetch all 10,000 then the first from that group. I would hope the first would be better in this regard since it would filter as it searches (and stopping after the first was found). Probably trivial in most cases, though.

Of course if you are chaining functions then it may be unavoidable:

$('.someclass').addClass('otherClass').first().addClass('firstClass');