Custom JS in Rails 6

I had a JavaScript class that I wanted to make use of in my Rails view. Sounded like a 5 minute job, but I spent a lot of time figuring out what the right approach is. Here's what I ended up with and you can tell me if it's a good or bad idea.

Users can upload an image as part of an edit form on my site and I want them to preview the image before hitting save. This is the JavaScript that's responsible for loading the preview image:

class ImagePreview {
  constructor(inputButton) {
    this.inputButton = inputButton;
    this.imageElement = this.findImageElement(inputButton);

  // The file input button has a data attribute that contains the id of the corresponding image preview element
  // This method is responsible for finding the image preview element based on that data attribute
  findImageElement() {
    let imageElementId = this.inputButton.dataset["imagePreviewId"]
    return document.getElementById(imageElementId);

  loadImage() {
    let file = this.inputButton.files[0];
    this.imageElement.src = URL.createObjectURL(file);

    this.imageElement.onload = function() {

  freeMemory() {

I wanted to call this JavaScript in my view like so:

  f.file_field :image,
    accept: "image/png,image/gif,image/jpeg",
    onchange: "new ImagePreview(",
    data: { image_preview_id: "TheIdOfThePreviewImage" }

I first just shoved the JS inside a <script></script> tag on the page it was needed but that didn't feel great. By doing that I avoided all the good stuff that Rails Webpacker offers.

With a lot of trial and error, this is the solution I eventually came up with that makes use of webpack.

I added an image_preview.js file inside the packs/ directory (inside was a copy and paste of the JavaScript class above):

tree app/javascript
├── channels
│   ├── consumer.js
│   └── index.js
└── packs
    ├── application.js
    └── image_preview.js

I updated the class to export the object by default:

  export default class ImagePreview {

I then imported the code inside my application.js file and attached the ImagePreview object to the window. This is what my application.js file looked like afterwards:


import ImagePreview from "./image_preview"
window.ImagePreview = ImagePreview

Here it is in action:

I had a really confusing time trying to figure this out and I'm still not sure if this approach is correct. It works and I think that's enough for my use-case, but if you know of a better way please let me know!

Previous post: Awk Notes

Next post: On Measuring Engineers