Tildes Formatting Toolbar - Userscript adds text formatting buttons and keyboard shortcuts

// ==UserScript==
// @name        Format Hotkeys for Tildes
// @version     16-August-2022
// @author      Jurid
// @description Format Hotkeys for tildes.net
// @icon        https://tildes.net/favicon-16x16.png
// @match       *://tildes.net/*
// ==/UserScript==

var ctrl = {                                  // mnemonic
  "Digit1": ["## ", ""],                      // 1+
  "Digit2": ["### ", ""],
  "Digit3": ["#### ", ""],
  "Digit4": ["##### ", ""],
  "Digit5": ["###### ", ""],
  "Digit6": ["<sup>", "</sup>"],              // ^
  "Digit8": ["0. ", ""],                      // *
  "Minus": ["<sub>", "</sub>"],               // _
  "Equal": ["<summary>", "</summary>"],       // +
  "KeyB": ["**"],                             // bold
  "KeyD": ["<details>", "</details>"],        // details
  "KeyH": ["\n---\n", ""],                    // horizontal
  "KeyI": ["_"],                              // italic
  "KeyL": ["[", "]( \"\")"],                  // link
  "KeyM": ["`"],                              // monospace
  "KeyS": ["~~"],                             // strikethrough

  "shift": {
    "KeyD": ["<details open>", "</details>"], // details opened
    "KeyI": ["<ins>", "</ins>"],              // insert
    "KeyM": ["```\n", "\n```"],               // monospace
    "KeyS": ["<small>", "</small>"],          // small
  }
}

function initiation() {
  [].forEach.call(document.querySelectorAll("textarea"), element =>
    element.addEventListener("keydown", combination, true)
  )
} initiation()

function combination(event) {
  for (let cmb of [
    [event.ctrlKey && event.shiftKey && ctrl.shift[event.code], ctrl.shift[event.code]],
    [event.ctrlKey && ctrl[event.code], ctrl[event.code]],
  ]) {
    if (cmb[0]) {
      event.preventDefault()
      modification(...cmb[1])
      break
    }
  }
}

function modification(open, close = open) {
  let field = document.activeElement
  let before = field.value.substring(0, field.selectionStart) + open
  let select = field.value.substring(field.selectionStart, field.selectionEnd)
  let after = close + field.value.substring(field.selectionEnd, field.value.length)

  document.execCommand("insertText", false, open + select + close)

  if ([open, close].every(tag => ctrl.KeyL.includes(tag)) && select.length) {
    field.selectionStart = field.value.length - after.length + 2
    field.selectionEnd = field.selectionStart
  } else {
    field.selectionStart = before.length
    field.selectionEnd = field.value.length - after.length
  }
}

new MutationObserver(() => initiation())
  .observe(document.body, { childList: true, subtree: true })