Получить полное слово, которое курсор касается (или внутри) JavaScript

const inp = document.querySelector('input');
document.onselectionchange =
  inp.onselect = inp.onclick = // Firefox doesn't fire selectionchange in <input>
    (evt) => {
  const text = inp.value;
  const start_index = inp.selectionStart;
  const end_index = inp.selectionEnd;
  const previous_space_index = text.lastIndexOf( " ", start_index - 1 );
  const next_space_index = text.indexOf( " ", end_index );
  const begin = previous_space_index < 0 ? 0 : previous_space_index + 1;
  const end = next_space_index < 0 ? text.length : next_space_index;
  const between_spaces = text.substring( begin, end );

  console.log( between_spaces );
};

<input type="text" value="this is a sentence">

const [$show, $input] = document.querySelectorAll('span,textarea')
const getWord = (s, pos) => {
  const n = s.substring(pos).match(/^[a-zA-Z0-9-_]+/)
  const p = s.substring(0, pos).match(/[a-zA-Z0-9-_]+$/)
  // if you really only want the word if you click at start or between
  // but not at end instead use if (!n) return
  if(!p && !n) return ''
  return (p || '') + (n || '')
}
const showWord = e => $show.innerText = getWord(e.target.value, e.target.selectionStart)
$input.addEventListener('click', showWord)
$input.addEventListener('input', showWord)

textarea{
  width: 500px;
  height: 500px;
}

<span id="inputshow"></span><br/>
<textarea>In computer science, an x-fast trie is a data structure for storing integers from a bounded domain. It supports exact and predecessor or successor queries in time O(log log M), using O(n log M) space, where n is the number of stored values and M is the maximum value in the domain. The structure was proposed by Dan Willard in 1982,[1] along with the more complicated y-fast trie, as a way to improve the space usage of van Emde Boas trees, while retaining the O(log log M) query time. </textarea>

let input = document.querySelector('input');

input.addEventListener('click', function() {
    // We only need the character offset here
    // Assume all non-word character(s) is a pipe
    let test = this.value.replace(/\W/g, '|');
    let caret = this.selectionStart;
    // Get the last word part length before caret
    let indexBeforeCaret = test.substring(0, caret).split('|').pop().length;
    // Get the first word part length after caret
    let indexAfterCaret = test.substring(caret).split('|')[0].length;
    // let word = this.value.substring(caret - indexBeforeCaret, caret + indexAfterCaret);
    // console.log(word);
    // Expand current caret position to the whole word
    this.selectionStart = caret - indexBeforeCaret;
    this.selectionEnd = caret + indexAfterCaret;
}, false);

<input type="text" value="this is another sentence!">