Manuel Guilbault's Blog

Building an image files picker with Aurelia

This post was initially published on Aurelia’s official blog.

In my book Learning Aurelia, you can see, among other things, how to build an image file picker component, supporting drag and drop and showing a preview of the selected image.

In this post, we’ll use the techniques described in the book to build a multi-select image file picker, also supporting drag and drop, with a gallery-style preview feature.

Picking files

Let’s start by creating a custom file-picker element, which will encapsulate an <input type="file"> element:


import {customElement, useView, bindable, bindingMode} from 'aurelia-framework';

export class FilePicker {

  @bindable accept = '';
  @bindable multiple = false;
  @bindable({ defaultBindingMode: bindingMode.twoWay }) files: FileList;

  input: HTMLInputElement;

  filesChanged() {
    if (!this.files) {

  private clearSelection() {
      this.input.type = '';
      this.input.type = 'file';

This view-model declares three bindable properties:

The view-model also declares an input property, to which the template will assign a reference on the <input type="file"> element.

Lastly, since the input’s files property is read-only and the DOM API doesn’t expose a method to clear the input’s file selection (other than calling the reset method on the whole surrounding form), the view-model uses a hack to clear the selected files when an empty value is assigned to the file-picker’s files property: it sets the input’s type to an empty string then resets it back to file.


  <input type="file" accept.bind="accept" multiple.bind="multiple" 
         files.bind="files" ref="input"
         style="visibility: hidden; width: 0; height: 0;">
  <button class="btn btn-primary" click.delegate="">

The file-picker’s template defines an <input type="file"> element, styled so it is invisible and so it occupies no space in the DOM. Its accept, multiple, and files attributes are also properly bound to their corresponding property on the view-model. Lastly, it assigns a reference on the input to the view-model’s input property.

The template also declares a button element, styled using Bootstrap’s classes. Inside it, a default content projection slot, with the Select text as its default content. Additionally, the button’s click event calls the input’s click method. Thanks to this, the browser’s file dialog will show up when the user clicks the button, even though the input element is not visible.

This component basically just replaces the ugly native file picker with a sexier button.

Adding a file drop target

Next, let’s create a custom attribute allowing to transform any element into a file drag and drop target:


import {customAttribute, bindingMode, autoinject} from 'aurelia-framework';

@customAttribute('file-drop-target', bindingMode.twoWay)
export class FileDropTarget {

  value: FileList | (({files: FileList}) => void);
  constructor(private element: Element) {}

  attached() {
    this.element.addEventListener('dragover', this.onDragOver);
    this.element.addEventListener('drop', this.onDrop);
    this.element.addEventListener('dragend', this.onDragEnd);

  private onDragOver = (e) => {

    e.dataTransfer.dropEffect = 'copy';

  private onDrop = (e) => {

    if (typeof this.value === 'function') {
      this.value({ files: e.dataTransfer.files });
    } else {
      this.value = e.dataTransfer.files;

  private onDragEnd = (e) => {

  detached() {
    this.element.removeEventListener('dragend', this.onDragEnd);
    this.element.removeEventListener('drop', this.onDrop);
    this.element.removeEventListener('dragover', this.onDragOver);

The attribute’s target element will be injected in the view-model’s constructor. When the attribute is attached to the DOM, it starts listening for the dragover, drop, and dragend events on the target element. When the attribute is detached from the DOM, the event listeners are removed.

The attribute is bound two way by default, so the file(s) assigned to its value when a user drops them on the target element are assigned back to the bounded property, if any. However, upon files being dropped on the target element, the view-model checks if the value is a function or not. This means that the attribute can be used either with the .bind command, so the dropped files are assigned to the bound expression, or with the .call command, so the bound expression is called and passed the dropped files whenever a drop event occurs.

Chunking an array

In order to display the selected images as a gallery, we’ll use Bootstrap’s grid system. This means we’ll need to break the files array down in chunks, so we can iterate on chunks to render rows, then on each chunk’s files to render columns.

The best way to do this in Aurelia is with a value converter:


import {valueConverter} from 'aurelia-framework';

export class Chunk {
  toView(array: any[], size: number): any[][] {
    let result = [];
    let nbChunks = Math.ceil(array.length / size);
    for (let i = 0; i < nbChunks; ++i) {
      const offset = i * size;
      result.push(array.slice(offset, offset + size));
    return result;

The chunk value converter expects an array and the chunks’ size as its parameter and returns an array of array.

Displaying a Blob object as an image

The last part we’ll need is some way to display a File instance inside an img element. To do this, we’ll leverage the browser’s URL.createObjectURL function, which takes a Blob object as a parameter and returns a special URL leading to this resource. Our custom attribute, which will be used essentially on img elements, will be bound to a Blob object, will generate an object URL from it, and will assign this URL to the img element’s src attribute.

Some of you might think that a value converter would be a better fit for this type of feature, and I would absolutely agree. A value converter could take as an input a Blob object and return the object URL. It could then be used on a binding between an img element’s src attribute and a property containing a Blob object.

However, in this particular case, each object URL must be released after usage in order to prevent memory leaks, and value converters offer no mechanism to be notified when a value is no longer used. On the contrary, HTML behaviors offer a much richer workflow and a wider set of extension points. That’s why we will create a custom attribute instead:


import {customAttribute, inject} from 'aurelia-framework';

export class BlobSrc {

  private objectUrl: string;

  constructor(private element: HTMLImageElement) {}

  private disposeObjectUrl() {
    if (this.objectUrl) {
      this.element.src = '';
      this.objectUrl = null;

  valueChanged(value) {

    if (value instanceof Blob) {
      this.objectUrl = URL.createObjectURL(value);
      this.element.src = this.objectUrl;

  unbind() {

Bringing it all together

Each of the parts we saw up to this point is shown in the book, even though some have been modified to fit the current context.

The last missing piece is the one that brings everything together: an image-files-picker custom element.


  <div class="jumbotron jumbotron-fluid""add(files)">
    <div class="container">
      <div class="text-center">
        <p>You can drop image files anywhere inside this area</p>
      <div class="row" repeat.for="row of files | chunk:3">
        <div class="col-md-4" repeat.for="file of row">
          <div class="card card-inverse">
            <img class="card-img img-fluid"
                alt="Preview for ${ & oneTime}"
            <div class="card-img-overlay">
              <button type="button" class="close" aria-label="Remove" 
                      click.delegate="remove($parent.$index * 3 + $index)">
                <span aria-hidden="true">&times;</span>
              <p class="card-text">
                <small class="text-muted">${ & oneTime}</small>
  <file-picker accept.bind="accept""true" 

The template starts with a jumbotron container, which acts as a file-drop-target. When files are dragged and dropped on this element, the view-model’s add method will be called and passed the dropped files.

Inside this container, the files array is rendered on three columns using the chunk value converter, each file displayed inside a Bootstrap card component. Each card displays the file in an img element using the blob-src attribute, a button whose click event calls the view-model’s remove method, and the file’s name.

Lastly, underneath the image gallery, a file-picker element allows the user to select image files. The selected files are bound to the view-model’s selectedFiles property, then the change event dispatched by the underlying <input type="file"> element and bubbling up the DOM triggers a call to the addSelectedFiles method. The file-picker’s default projection slot is also overwritten with the text Add.


import {customElement, useView, bindable, bindingMode} from 'aurelia-framework';

export class ImageFilesPicker {

  @bindable({ defaultBindingMode: bindingMode.twoWay }) files: File[] = [];

  selectedFiles: FileList;

  add(files: FileList) {
    for (let i = 0; i < files.length; ++i) {
      const file = files.item(i);

  remove(index) {
    this.files.splice(index, 1);

  addSelectedFiles() {
    this.selectedFiles = null;

The view-model declares a files bindable property, which is bound two way by default. This property is expected to initially contain an empty array.

When files are dropped on the drop target element, the add method is called and the dropped files are appended to the files property. When the user selects files using the file-picker, the selected files are assigned back to the selectedFiles property, then the change event handler calls the addSelectedFiles, which appends the selectedFiles to the files property, and finally assigns null to the selectedFiles.

This last step makes sure that the underlying <input type="file"> element has its selection cleared. Without it, if a user tries to add the same file twice in a row, the change event would not be triggered the second time, because the input’s value would not change, so the second file selection would fail from the user’s perspective.

Using the image files picker

Using the image-files-picker element is then pretty simple. We first need to declare a property hosting the array of files on the App view-model:


export class App {
  files: File[] = [];

Next, we simply need to add the custom element in the template of our App component:


  <require from="bootstrap/css/bootstrap.min.css"></require>
  <section class="container">
    <image-files-picker files.bind="files"></image-files-picker>

Of course, the various parts need to be loaded, either using the require statement in the app.html template, or in the resources/index.ts feature’s configure function.

Filtering out non-image files

At this point, a user can select or drop any type of files using our component. Some logic allowing only image files should be somehow added.

A basic filtering logic, using the same syntax as the <input type="file"> element’s accept attribute, is implemented in the complete code sample, which you can find here. A more complete solution, showing error messages to the user, can easily be implemented. I’ll leave this as an exercise to the reader.

Exploiting the selected images

Typically, such a component would be used to first select a bunch of image files, then to upload those files to some remote endpoint. This is pretty easy to do with Aurelia’s Fetch client and the FormData class from the Fetch API.

Here’s an example of a client service used to upload an array of File instances to some remote endpoint:

import {autoinject} from 'aurelia-framework';
import {HttpClient} from 'aurelia-fetch-client';

export class SomeAPI {
  constructor(private http: HttpClient) {}

  uploadFiles(files: File[]): Promise<void> {
    const body = new FormData();
    for (let i = 0; i < files.length; ++i) {
      body.append(`files[${i}]`, files[i]);
    return this.http.fetch('some/url', { method: 'POST', body });

The Mozilla Developer Network has some great doc about the FormData class.


Once again, Aurelia makes things easy. Its various constructs, such as custom attributes, elements, and value converters, help us decompose a problem and solve each of its parts with a generic, reusable solution, and then recombine them together to address our initial, specific problem. Shameless plug alert: this aspect is one of the many topics addressed in Learning Aurelia. You should definitely give it a look!

Update 2018-03-02

Reader @sokratismanolis pointed out in this comment that the code doesn’t work on IE 11 and Edge. I didn’t have time to find out the core of the issue yet, but it seems to be caused by a delegated event listener being fired before a data binding instruction is refreshed. I worked around the issue by removing the delegated event listener and by using the @observable attribute on selectedFiles, as you can see on this branch.

#Aurelia #JS #TypeScript #Web


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