1. type listType = [string, number][]
    2. type keyByFunction = (tag: [string, number]) => number
    4. const maxBy = (list: listType, keyBy:keyByFunction) => {
    5.   return list.reduce((x, y) => {
    6.     return keyBy(x) > keyBy(y) ? x : y
    7.   })
    8. }
    10. const filterTag:keyByFunction = tag => tag[1]
    12. const getMostFrequent = () => {
    13.   const tags = [...document.querySelectorAll('*')]
    14.     .map(tag => tag.tagName)
    15.     .reduce((acc, cur) => {
    16.       acc[cur] = acc[cur] ? acc[cur] + 1 : 1
    17.       return acc
    18.     }, {})
    19.   return maxBy(Object.entries(tags), filterTag)
    20. }
    22. // Todo: querySelectorAll polyfill
    23. type elType = HTMLElement | Document | Element
    25. const getAllTags = (el: elType = document) => {
    26.   const children = Array.from(el.children)
    27.     .reduce((acc, cur) => [...acc, ...getAllTags(cur)], [])
    28.   return children
    29. }