I'd get rid of checker, leaving this test something like stateVal === false || self.hasClass( className ).

@gibson042 .toggleClass("cls", true) needs to add the class if it's there and avoid removing it. If stateVal is true and the element currently had the class, it would be removed rather than kept. It could be done with a few extra conditionals though, instead of the call.

Ah, right, we'd need two checks against stateVal. It might still be preferable, but I'm now less enthusiastic.

if ( stateVal !== true && self.hasClass( className ) ) {
    self.removeClass( className );
} else if ( stateVal !== false ) {
    self.addClass( className )
}

What is the use of stateVal === false || ?

In .toggleClass() the stateVal can be either true, false or undefined. The boolean cases indicate the caller wants to force the class to be added or removed. For the undefined case, the class should be added if it does not have the class and removed if it does have the class. So if you walk through the three states you can double-check that expression works for all the cases. I added a few extra tests below to catch some of the possible mistakes someone could make in editing this line. 😸

Oh yes, my bad... I realize now the 6th case here, doesn't work without your ===false||, you're right.

$( ".class" ).toggleClass( "class" )        -> removeClass
$( ".class" ).toggleClass( "class", false ) -> removeClass
$( ".class" ).toggleClass( "class", true  ) -> addClass

$( ":not(.class)" ).toggleClass( "class" )        -> addClass
$( ":not(.class)" ).toggleClass( "class", true  ) -> addClass
$( ":not(.class)" ).toggleClass( "class", false ) -> removeClass // need stateVal === false || here

Sorry, I'm not used with the || mixed with && without parentheses ^^ (in C this is throw a warning :D)