JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="text">The quick brown fox jumps over the lazy dog.</textarea>

JavaScript

const ta = document.querySelector('#text')

const count = textarea => {
	let counts = { }
	textarea.value.toLowerCase().split('').sort().forEach(x => counts[x] = (counts[x] || 0) + 1)
  return counts
}

const pre = document.createElement('pre')
pre.innerText = JSON.stringify(count(ta), null, 4)
ta.parentNode.insertBefore(pre, ta.nextNode)