[JAVA] Replace preview by uploading by clicking the image in file_field of Rails

First stage

If you are using bootstrap_form, the following steps are required. If you are using bootstrap_form, this gem will create a nice UI, but in this implementation it gets in the way.

<%= f.file_field :image %>

スクリーンショット 2020-11-06 10.34.09.png

So make sure that the bootstrap style isn't applied.

<%= f.file_field_without_bootstrap :image %>

スクリーンショット 2020-11-06 10.34.59.png

Click image to upload

<%= f.label :image do %>
  <%= image_tag('user.png') %>
  <%= f.file_field_without_bootstrap :image, style: 'display:none' %>
<% end %>

Simply enclose the image and file_field in label and hide file_field with CSS. If you replace the image part with an icon, you can upload by clicking the icon.

Replace the displayed image with the uploaded image

Add an appropriate class name to the image and uploader so that you can get the element from JS.

<%= f.label :image do %>
  <%= image_tag('user.png', class: 'avatar') %>
  <%= f.file_field_without_bootstrap :image, class: 'uploader', style: 'display:none' %>
<% end %>

Encodes the image selected in JavaScript to base64 and replaces it with the original image.

window.addEventListener('load', () => {
    const uploader = document.querySelector('.uploader');
    uploader.addEventListener('change', (e) => {
      const file = uploader.files[0];
      const reader = new FileReader();
      reader.onload = () => {
        const image = reader.result;
        document.querySelector('.avatar').setAttribute('src', image);

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