A JavaScript file can quickly get overloaded with styles if you regularly use the css method to modify element styles. It’s a best practice to group all of the style code in a CSS file, and use jQuery to add and remove the classes from elements — this approach aligns to a design principle called separation of concerns.

Separation of concerns is a design principle stating that code should be separated based on its purpose in a program. In web development, that generally means the structure of a page is defined in an HTML document, styles are stored in a CSS file, and code that defines dynamic behavior is stored in a JavaScript file.

To keep CSS properties in a CSS file, jQuery can add a CSS class to an element with a method named addClass. It’s syntax looks like this:


In the example above:

  • .addClass() is called on the jquery .example-class selector.
  • .addClass() adds the 'active' class to all .example-class elements.
  • Notice that the argument passed to addClass does not have a period preceding it. This is because it expects a class, and therefore only needs the name of the class.



Instead of changing the text color and background color with the .css() method, replace the code in the menu button mouse enter handler with the .addClass() method.

If you navigate to css/styles.css, there’s a class named .button-active, which defines the text color and background color of an element. Use the .addClass() method to add button-active to the .menu-button element when it’s moused over.

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