jQuery.classList.js

/*
Usages: 
$(selector).classList() //returns an array of classnames
$(selector).classList('newclass') //replaces the current element's classes
$(selector).classList(['new', 'class', 'names']) //replaces the current element's classes
*/

jQuery.fn.classList = function( classNames ) {
  if ( jQuery.isArray( classNames ) ) {
    // An array was passed, join it into a string.
    classNames = classNames.join(' ')
    
  } else if ( classNames ) {
    // Individual arguments were passed, join them into a string.
    classNames = Array.prototype.join.call( arguments, ' ' );
    
  } else {
    // No arguments were passed, return an array of class names.
    return this.attr( 'class' ).split( /\s+/ );
  }
  
  // Set class names and return the original jQuery object.
  return this.attr( 'class', classNames );
};

It's a common metaphor for jQuery methods to work as both setters and getters, this behavior is the same as other jQuery collection methods like .attr() or .html() or .text().

(some people like it, some people don't, but at least it's consistent)