Backbone BackToTopView

backToTop.js

define([
    'jquery',
    'underscore',
    'backbone',
], function ($, _, Backbone) {
    'use strict';

    var BackToTopView = Backbone.View.extend({
        initialize: function (options) {
            _(this.options).extend(options);
            $(window).on('scroll', _(this.moveToPosition.bind(this)).debounce(200));
            this.moveToPosition();
        },

        options: {
            start_offset: 0,
        },
        moveToPosition: function () {
            var parentBottom = this.$el.parent().offset().top + this.$el.parent().outerHeight();
            var viewportBottom = window.scrollY + window.innerHeight;

            this.$el.css({
                bottom: window.scrollY > this.options.start_offset ? Math.max(parentBottom - viewportBottom, 0) : 0,
            });
        },
    });

    return BackToTopView;
});