returns the header of the section containing a given element

getSections.js

function getSectionHeader(element, headerDepth = 6){
    const headerSelector = Array(headerDepth).fill().map((i,idx) => `h${idx+1}`).join(',');
    console.log(headerSelector);
    if (element.matches(headerSelector)) {
        return element;
    }
    const tempAttr = 'data-getSectionInTitleElement';
    element.setAttribute(tempAttr, "");
    const els = document.querySelectorAll(`${headerSelector},[${tempAttr}]`);
    let headerIndex;
    Array.from(els).some((el, idx)=>{
        if (el.matches(`[${tempAttr}]`)){
            headerIndex = idx - 1;
            return true;
        }
        return false;
    });
    const headerEl = els.item(headerIndex);
    element.removeAttribute(tempAttr);
    return headerEl;
}

function getSectionTitle(element){
    const header = getSectionHeader(element);
    if (header) return header.textContent;
    return document.title;
}

function getSections(element){
    let sections = '';
    let header = getSectionHeader(element);
    if (header){
        let maxLevel = Number(header.tagName.substr(1))
        sections = `${maxLevel}.${header.textContent}`;
        let currentLevel = maxLevel - 1
        while (currentLevel){
            header = getSectionHeader(element, currentLevel);
            if (header){
                sections = `${currentLevel}.${header.textContent} - ${sections}`
            }
            currentLevel --;
        }
    }
    return sections || document.title;
}

function findPrecedingMatch(el, selector){
    const siblingMatch = (el, selector) => {
        while (el) {
            el = el.previousElementSibling;
            if (!el || el.matches(selector)) return el
        }
    }

    const matchingSib = siblingMatch(el, selector);
    if (matchingSib) return matchingSib;

    // level up.

    const nextContentParent = (el) => {
        let parent = el.parentElement;
        while (parent){
            if (parent.children.length > 1) return parent;
            parent = parent.parentElement;
        }
    }

    let levelEl = nextContentParent(el);
    
    while(levelEl){
        let currentEl = levelEl;
        while(currentEl){
            let matches = currentEl.querySelectorAll(selector);
            if (matches.length){ 
                return matches[matches.length - 1] 
            };
            currentEl = currentEl.previousElementSibling;
        }
        levelEl = nextContentParent(levelEl);
    }
    return null;
}