JSFiddle - React, Tailwind, and code Playground

HTML

<form class="form">
 <input type="text" name="username" />
 <input type="text" name="password" />
 <input type="submit" value="Войти" disabled />
</form>

JavaScript

function setSubmitState(e) {
	const inputs = e.path[1].querySelectorAll('input[type="text"]');
  const btn = e.path[1].querySelector('input[type="submit"]');
  let isFilled = false;
  let filledInputs = [...inputs].map(inp => Boolean(inp.value.length >= 5))
	
  isFilled = filledInputs.indexOf(false) === -1 ? true : false
  
  if(isFilled)
  	btn.removeAttribute('disabled')
  else
  	btn.setAttribute('disabled', true)
}

const form = document.querySelector('.form');

form.addEventListener('keyup', e => setSubmitState(e))