nice utility to amange scroll in a web page

scroll.js

/**
 * This module allows to execute certain actions during scroll
 * when a certain trigger is triggered
 *  
 */

function init_scroll(opt) {

    let opt_default = {
        scroll_container: 'html, body',       // the element in which we want to scroll
        triggers: {
            "arrived_at_section": {
                type: "section", 
                selector: ".section", 
                top_offset: 0
            },
        },
    }
    opt = Object.assign(opt_default, opt);
    if (opt.scroll_container == 'window') opt.scroll_container = window;

    let $ = jQuery;
    let scrolling = false;
    let events = {}

    // load section triggers
    let section_triggers = [];
    for (let event_name in opt.triggers) {
        if (opt.triggers[event_name].type != 'section') continue;

        $(opt.triggers[event_name].selector).each(function() {
            console.log('got section', $(this).text())
            section_triggers.push({
                event: event_name,
                top: $(this).offset().top - opt.triggers[event_name].top_offset,
                height: $(this).height(),
            })
        })
    }
    for (let i = 0; i < section_triggers.length-1; i++) {
        section_triggers[i].height = section_triggers[i+1].top - section_triggers[i].top;
    }
    section_triggers[section_triggers.length-1].height = $(document).height() - section_triggers[section_triggers.length-1].top;

    // respond to on scroll event to change section
    let curr_section = {};
    let w_height = $(window).height();
    let marge = w_height / 10;
    $(opt.scroll_container).scroll(function() {
        if (scrolling) return;
        let start = $(this).scrollTop();
        let i = 0
        for (let section of section_triggers) {
            let mem_section = curr_section[section];
            if (start > section.top - marge && start < section.top + section.height - marge && curr_section != i) {
                curr_section[section] = i
            }
            if (mem_section != curr_section[section]) {
                dispatch_event(section.event, [curr_section[section], section])
            }
            i++
        }
    })

    function register_event(ev_name, fn) {
        if (ev_name in events) events[ev_name].push(fn);
        else events[ev_name] = [fn];
    }

    function dispatch_event(ev_name, args) {
        if (!events[ev_name]) return;
        for (let fn of events[ev_name]) {
            fn(...args);
        }
    }

    return {
        on: register_event,
        getScrolling: _ => scrolling,
        setScrolling: v => scrolling = v && true,
    }
}