JSFiddle - React, Tailwind, and code Playground

by Sergei Sokolov

HTML

<label>Superclient
  <input type="checkbox" id="superclient" checked>
</label>
<br>
<label>Client
  <input type="text" name="client">
</label>
<br>
<label>Tourist
  <input type="text" name="tourist">
</label>

JavaScript

/**
 * для вопроса https://qna.habr.com/q/1024968
 * Как проверить checked при загрузке страницы и убрать задержку?
 */
const input = {
  client: document.querySelector('input[name=client]'),
  tourist: document.querySelector('input[name=tourist]'),
  check: document.getElementById('superclient'),
}

const sync = () => {
  input.tourist.value = input.client.value;
}

const update = () => {
  if (input.check.checked) {
  	sync();
    input.tourist.disabled = true;
  } else {
    input.tourist.disabled = false;
  }
}

input.check.addEventListener('input', update);
input.client.addEventListener('input', update);

update();