JSFiddle - React, Tailwind, and code Playground

HTML

<div class="result"></div>

JavaScript

var astring = "I'm a string not too long but neither so short";

var position = 13;

var newstring = astring.substring(position);

var finalstring = newstring.split(" ");

var wrapped = '<b>'+finalstring[0]+'</b>';

var finalwrapped = astring.substring(0,position)+wrapped+astring.substring(position+finalstring[0].length);

$('.result').html(finalwrapped);