sutty/app/javascript/etc/image_preview.js

document.addEventListener('turbolinks:load', () => {
  document.querySelectorAll('input[type=file]').forEach(file => {
    if (!file.dataset.preview) return

    file.addEventListener('change', event => {
      if (file.files.length === 0) return

      document.querySelector('#' + file.dataset.preview).src = window.URL.createObjectURL(file.files[0])
    })
  })
})