<h1 id="foo"></h1>
<h2 class="bar"></h2>
<h2 class="bar"></h2>

const $ = function(e) {
  const c = e[0];
  const len = e.split(' ').length;
  if (c === '#' && len === 1) {
    return document.getElementById(e.slice(1));
  } else {
    return document.querySelector(e);
  }
}
const $$ = function(e) {
  return document.querySelectorAll(e);
};
$('#foo').textContent = 'Hello';
$$('.bar').forEach(function(x) {
  x.textContent = 'world!'
});