smoothScroll.js

$(function() {
    smoothScroll(300);
});

function smoothScroll(duration) {
    $('a[href^="#"]').on("click", function(event) {
        var target = $($(this).attr("href"));

        if (target.length) {
            event.preventDefault();
            $("html, body").animate({
                scrollTop: target.offset().top
            }, duration);
        }
    });
}