code: function toggleClass( element, className, forceFlag ) {
    let hasClass = false;
    if ( hasClassList ) {
      hasClass = element.classList.toggle( className );
    } else if ( forceFlag === false || contains( element, className ) ) {
      removeClass( element, forceFlag );
    } else {
      addClass( element, className );
      hasClass = true;
    }
  
    return hasClass;
  }
docs: Toggle CSS class on an element.
  
  @param {HTMLNode} element - A DOM element.
  @param {string} className - CSS selector.
  @param {boolean} forceFlag - True if `className` class
  should be forcibly removed.
  @returns {boolean} True if the flag existed, false otherwise.