file.js

(function (global) {
    'use strict';

    function isHeaderFixed() {
        return $('.b-header--fixed').length > 0;
    }

    function getOffsetY(y) {
        var originalY = isHeaderFixed() ? y + 220 : y;

        if (originalY > 300) {
            return originalY - 300;
        }
        else {
            return originalY - 80;
        }
    }

    function doScroll(y) {
        $('html, body').animate({
            scrollTop: getOffsetY(y)
        });
    }

    var scroll = {

        scrollToY: function (y) {
            doScroll(y);
        },

        scrollToElem: function (elem) {
            var $elem;

            if (typeof elem === 'string') {
                $elem = $(elem);
            }
            else{
                $elem = elem;
            }

            if ($elem.length === 0) {
                return false;
            }

            return this.scrollToY($elem.first().offset().top);
        },

        init: function () {
            var that = this;

            $('body').on('click', '.js-scroll[data-target]', function () {
                that.scrollToElem($(this).data('target'));
                return false;
            });
        }
    };

    $(function () {
        scroll.init();
    });

    global.Scroll = scroll;
})(window);