Bootstrap Basic Navbar Collapse, no jQuery or Bootstrap JS

main.js

let collapseElements = document.querySelectorAll('[data-toggle="collapse"]');
const CLASS_SHOW = 'show';
const CLASS_COLLAPSE = 'collapse';
const CLASS_COLLAPSING = 'collapsing';
const CLASS_COLLAPSED = 'collapsed';
const ANIMATION_TIME = 350; // 0.35s

function handleCollapseElementClick(e) {
    let el = e.currentTarget;
    let collapseTargetId = el.dataset.target || el.href || null;
    if (collapseTargetId) {
        let targetEl = document.querySelector(collapseTargetId);
        let isShown = targetEl.classList.contains(CLASS_SHOW) || targetEl.classList.contains(CLASS_COLLAPSING);
        if(!isShown) {
            targetEl.classList.remove(CLASS_COLLAPSE);
            targetEl.classList.add(CLASS_COLLAPSING);
            targetEl.style.height = 0;
            targetEl.classList.remove(CLASS_COLLAPSED);
            setTimeout(() => {
                targetEl.classList.remove(CLASS_COLLAPSING);
                targetEl.classList.add(CLASS_COLLAPSE, CLASS_SHOW);
                targetEl.style.height = '';
            }, ANIMATION_TIME);
            targetEl.style.height = `${targetEl.scrollHeight}px`;
        } else {
            targetEl.style.height = `${targetEl.getBoundingClientRect().height}px`;
            targetEl.offsetHeight; // force reflow
            targetEl.classList.add(CLASS_COLLAPSING);
            targetEl.classList.remove(CLASS_COLLAPSE, CLASS_SHOW);
            targetEl.style.height = '';
            setTimeout(() => {
                targetEl.classList.remove(CLASS_COLLAPSING);
                targetEl.classList.add(CLASS_COLLAPSE);
            }, ANIMATION_TIME);
        }
    }
}

collapseElements.forEach((el) => {
    el.addEventListener('click', handleCollapseElementClick);
});

Thanks for this - it's really helpful!

Fwiw, to make it work in IE 11 I just had to replace this:

collapseElements.forEach((el) => {
    el.addEventListener('click', handleCollapseElementClick);
});

with this:

for (var i = 0; i < collapseElements.length; i++) {
    collapseElements[i].addEventListener('click', handleCollapseElementClick);
};