Check a page for dangling links to local anchors

checkanchors.js

let links = document.querySelectorAll("a[href]")
let base = /^[^#]*/.exec(document.location)[0] + "#"
for (let i = 0; i < links.length; i++) {
  let link = links[i], anchor
  if (link.href.indexOf(base) == 0 && !document.getElementById(anchor = decodeURIComponent(link.href.slice(base.length))))
    console.log("Missing anchor: " + anchor)
}