Guide on how to implement a file upload feature using only JavaScript and the FileReader API

Question

Guide on how to implement a file upload feature using only JavaScript and the FileReader API

Allow me to explain the issue I'm facing. To give you some context, I've been working on a forum web application. Lately, I've been trying to enable users to choose a photo from their local file system during sign up. The idea is that once a user selects an image, it should be read using the FileReader API in client-side JavaScript. Then, the selected image URL should be set as the value of the file input before submitting the form.

It's important to note that my goal is not to directly upload the file as a separate image, but rather to store the data URL in my database. While many suggest using Multer for file uploads, I've already made significant progress and pushed changes to my GitHub repository. Thus, installing new npm packages is not an option at this point.

The main issue I'm encountering is that the value of the input does not change, and the uploaded URL only shows the name of the image file (for example, shark.jfif). Despite searching online, I haven't found a solution yet. Below is my current client-side JavaScript code:

const file = document.querySelector('input[type="file"]').files[0];
const fReader = new FileReader();
const imgName = fReader.readAsDataURL(file);
document.querySelector('input[type="file"]').value = imgName.result;

Ideally, I would like to use an event listener to monitor when the input file value changes. However, I'm unsure about which event would be most suitable. Using the submit event is not ideal since the code execution takes some time, and by the time the form is submitted, only the filename is saved in the database. As of now, the value remains unchanged with my existing code.

javascript forms file filereader

Answer 1

Answer №1

A few years back, I worked on a similar project that eventually evolved into a small JavaScript 'library' known as imagecropper.js. This library offers additional features such as the ability to crop images before uploading them anywhere in the file input -> canvas -> img workflow. For the purpose of this explanation, we will focus solely on skipping the canvas step.

The main concept revolves around fetching the data URL from the FileReader and then setting it as the src attribute of the img tag:

const image = document.getElementById('image');
document.getElementById('fileUpload').addEventListener("change", e => {
  const selectedFile = e.target.files[0];
  const fileReader = new FileReader();
  
  // Reading the image as base64 data
  fileReader.readAsDataURL(selectedFile);
  
  // Once loaded, use the image data
  fileReader.onload = (e) => image.setAttribute('src', e.target.result);
});

<input type="file" id="fileUpload" />
<img id="image" src="" />

Answer 2

A few years back, I worked on a similar project that eventually evolved into a small JavaScript 'library' known as imagecropper.js. This library offers additional features such as the ability to crop images before uploading them anywhere in the file input -> canvas -> img workflow. For the purpose of this explanation, we will focus solely on skipping the canvas step.

The main concept revolves around fetching the data URL from the FileReader and then setting it as the src attribute of the img tag:

const image = document.getElementById('image');
document.getElementById('fileUpload').addEventListener("change", e => {
  const selectedFile = e.target.files[0];
  const fileReader = new FileReader();
  
  // Reading the image as base64 data
  fileReader.readAsDataURL(selectedFile);
  
  // Once loaded, use the image data
  fileReader.onload = (e) => image.setAttribute('src', e.target.result);
});

<input type="file" id="fileUpload" />
<img id="image" src="" />

Guide on how to implement a file upload feature using only JavaScript and the FileReader API

Answer №1

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