missing-anchor-ids.js

const currURL = `${window.location.protocol}//${window.location.host}${window.location.pathname}${window.location.search}`
let outputString = ''
for (var idx in document.links) {
    const ancString = `${document.links[idx]}`
    if (ancString.includes('#')) {
        const anchParts = ancString.split('#')
        if (anchParts[0] === currURL) {
​
            if (!document.getElementById(anchParts[1])) {
                outputString += `${document.links[idx]}\n`
            }
        }
    }
​
}
​
if (outputString !== '') {
    const thisComment = document.createComment(`\n+-- Missing Anchors: [\n"${outputString}"]  ---+\n`);
    document.head.insertBefore(thisComment, document.head.firstChild);
}