simple-email/node_modules/npm/html/static/toc.js

;(function () {
  var wrapper = document.getElementById('wrapper')
  var els = Array.prototype.slice.call(wrapper.getElementsByTagName('*'), 0)
    .filter(function (el) {
      return el.parentNode === wrapper &&
        el.tagName.match(/H[1-6]/) &&
        el.id
    })
  var l = 2
  var toc = document.createElement('ul')
  toc.innerHTML = els.map(function (el) {
    var i = el.tagName.charAt(1)
    var out = ''
    while (i > l) {
      out += '<ul>'
      l++
    }
    while (i < l) {
      out += '</ul>'
      l--
    }
    out += '<li><a href=\'#' + el.id + '\'>' +
      (el.innerText || el.text || el.innerHTML) +
      '</a>'
    return out
  }).join('\n')
  toc.id = 'toc'
  document.body.appendChild(toc)
})()