This guide will teach you how to set up an image uploader and display a preview on the screen using plain HTML, CSS & JavaScript.
Project Setup
Create a directory on your computer (e.g. file-upload) and three files inside:
- index.html
- styles.css
- scripts.js
HTML Template
Starting with HTML, we'll create a base template and link it with two other files:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<!-- Font awesome for fa icons -->
<script src="https://use.fontawesome.com/fe459689b4.js"></script>
<!-- Including styles.css file -->
<link rel="stylesheet" type="text/css" href="./styles.css" />
<title>File upload with preview</title>
</head>
<body>
<!-- Including scripts.js file -->
<script src="./scripts.js"></script>
</body>
</html>
I've also included a script for the Font Awesome icons that we'll use for UI/UX.
Setting up file uploader
Now we add the HTML code for file upload within the two <body></body> tags:
<div class="profile-picture">
<h1 class="upload-icon">
<i class="fa fa-plus fa-2x" aria-hidden="true"></i>
</h1>
<input
class="file-uploader"
type="file"
onchange="upload()"
accept="image/*"
/>
</div>
- The
<div class="profile-picture">is our main container that the user will interact with and it will serve as a display for the uploaded image. - The
<h1 class="upload-icon">is a heading for the icon that will appear over the image.
Looks pretty terrible, but we'll get to it shortly.
Peeking into the input
- Input
type="file"is what tells HTML to change this input from its default behavior (textbox) to a button that can upload a file from your machine. - The
onchange="upload()"is anonchangeevent on the element that will trigger when the file is selected and invoke the functionupload()that we'll create in the JavaScript file. - The
accept="image/*"tag tells the app to look exclusively for image-type files when browsing through the files on your machine. Other file types won't appear in the window.
Styling the uploader
The downside of the default HTML <input/> element has limited styling options. To make the file upload button look like the one in the cover picture we'll mask the <input/> with the <div class="profile-picture"> covering it.
The app user will interact with the upload input by clicking on the <div class="profile-picture">. To do that we'll give our container a style of position: relative, while the <h1> tag and the <input/> tag will have a style of position: absolute.
Inside the styles.css file, we'll style each element.
Profile-picture container styles
.profile-picture {
opacity: 0.75;
height: 250px;
width: 250px;
position: relative;
overflow: hidden;
/* default image */
background: url('https://qph.cf2.quoracdn.net/main-qimg-f32f85d21d59a5540948c3bfbce52e68');
background-position: center;
background-repeat: no-repeat;
background-size: cover;
box-shadow: 0 8px 6px -6px black;
}
File-uploader input styles
.file-uploader {
/* make it invisible */
opacity: 0;
/* make it take the full height and width of the parent container */
height: 100%;
width: 100%;
cursor: pointer;
/* make it absolute */
position: absolute;
top: 0%;
left: 0%;
}
Upload-icon heading styles
We'll position the Font Awesome icon in the middle of the container and make it invisible by default.
.upload-icon {
position: absolute;
top: 45%;
left: 50%;
transform: translate(-50%, -50%);
/* initial icon state */
opacity: 0;
transition: opacity 0.3s ease;
color: #ccc;
-webkit-text-stroke-width: 2px;
-webkit-text-stroke-color: #bbb;
}
The icon will appear only after the user hovers over the container:
/* toggle icon state */
.profile-picture:hover .upload-icon {
opacity: 1;
}
Looks way better! Now onto adding the functionality.
Scripting the upload
Inside the script.js file, we'll create the upload() function that will be triggered by the <input onchange="upload"> event. Inside we're going to pick up the image from the <input class="file-uploader" element in the HTML:
function upload() {
const fileUploadInput = document.querySelector('.file-uploader');
We can also validate the input based on the selected file type, file size, etc.
const fileUploadInput = document.querySelector('.file-uploader');
// using index [0] to take the first file from the array
const image = fileUploadInput.files[0];
// check if the file selected is not an image file
if (!image.type.includes('image')) {
return alert('Only images are allowed!');
}
// check if size (in bytes) exceeds 10 MB
if (image.size > 10_000_000) {
return alert('Maximum upload size is 10MB!');
}
Lastly, we'll make use of the built-in FileReader class to convert the file we've uploaded (that is in Blob format) to a base64-encoded data URL representing the image.
const fileReader = new FileReader();
fileReader.readAsDataURL(image);
When the fileReader finishes processing the Blob image, it will trigger the onload event on the instance. Inside we'll use the base-64 URL as a background image of our container:
const fileReader = new FileReader();
fileReader.readAsDataURL(image);
fileReader.onload = (fileReaderEvent) => {
const profilePicture = document.querySelector('.profile-picture');
profilePicture.style.backgroundImage = `url(${fileReaderEvent.target.result})`;
}
Demo
Photo by Helena Lopes from Pexels
The next logical step is to upload the Blog image to a cloud, like a Cloudinary or an AWS S3 bucket, which is out of the scope of this article.
You can find the full code on the Github.
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