Nowadays you should really use scroll-behavior: smooth; in CSS instead of a JavaScript solution.

https://mathiasbynens.be/demo/smooth-scrolling

Find all links to anchors in the same document, and make them scroll smoothly when clicked:

$('html').smoothScroll();

Find all links to anchors in the same document inside #content, and make them scroll smoothly when clicked:

$('#content').smoothScroll();

$('html').smoothScroll(300);

– Mathias