By default, Magento 2 themes have a lot of built-in storefront features. We can further customize them via various JS libraries. Let me give you some info about adding custom JS library to new Magento 2 theme. Magneto 2 brings new feature called RequireJS. New Magetno 2 has better perceived page loading time through the RequireJS because it allows JavaScript to load in the background (asynchronous or “lazy” JS loading). Additional information you can find here.
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