郵便番号
REST API

document.getElementById('postcode')
  .addEventListener('keyup', function(e) {

  if (!e.key.match(/[0-9]/)) return;

  const postcode = e.target.value;
  if (postcode.length !== 7) return;

  const url =
    `https://postcode.teraren.com/postcodes/${postcode}.json`;

  fetch(url)
    .then(response => response.json())
    .then(json => {
      document.getElementById('prefecture').value
        = json.prefecture;
      document.getElementById('city').value
        = json.city;
      document.getElementById('suburb').value
        = json.suburb;
    })
    .catch(error => {
      console.error(error);
      ['prefecture', 'city', 'suburb'].forEach(id => {
        document.getElementById(id).value = '';
      });
    });
});

<form>
  <input type="text" id="postcode"
    placeholder="1600023" maxlength="7" autofocus>
  <input type="text" id="prefecture"
    placeholder="東京都" disabled>
  <input type="text" id="city"
    placeholder="新宿区" disabled>
  <input type="text" id="suburb"
    placeholder="西新宿" disabled>
</form>