Mulit-Key Combination Detection

<script lang="ts">
  import { cloneDeep } from 'lodash';
  import { onDestroy } from 'svelte';
 
  function isLetter(letter: string): boolean {
    if (letter.length != 1) return false;
    return letter.match(/[a-zA-Z]/) ? true : false;
  }
 
  function isShortcut(letters: string[]): boolean {
    if (letters.length <= 1 || letters.length > 3) return false;
    return letters.filter((letter) => isLetter(letter)).length == 1;
  }
 
  let keyCombination = '';
  let curKeyCombinationArr: string[] = [];
  let keyCombMemo: string[] = [];
  const keyDownMap = new Map();
 
  const updatePressedKeys = () => {
    console.log('update');
 
    curKeyCombinationArr = [];
    keyDownMap.forEach((value, key, map) => {
      if (value) curKeyCombinationArr.push(key);
    });
    if (isShortcut(curKeyCombinationArr)) {
      keyCombMemo = cloneDeep(curKeyCombinationArr);
    }
  };
 
  function onKeyDown(e: KeyboardEvent) {
    keyDownMap.set(e.key, true);
    updatePressedKeys();
  }
  function onKeyUp(e: KeyboardEvent) {
    keyDownMap.delete(e.key);
    updatePressedKeys();
  }
 
  document.addEventListener('keydown', onKeyDown);
  document.addEventListener('keyup', onKeyUp);
  $: keyCombination = keyCombMemo.join('+');
 
  onDestroy(() => {
    document.removeEventListener('keydown', onKeyDown);
    document.removeEventListener('keyup', onKeyUp);
  });
</script>
 
<div>
  <p>Hotkey Select</p>
  <p>{keyCombination}</p>
</div>