javascript ui-ux

Showing preview of files before uploading them to a server, specially for images, is a really interesting functionality for users and is quite simple to implement.

It’s possible to show preview even on heavily AJAX-based web pages/applications, as the preview can be done with JavaScript and it does not rely on server processing. If this is your case (web pages/application AJAX-based) you can also upload your files with AJAX.

Here I’ll show you how to implement image preview with JavaScript by following these steps:

1 - Detect when user selects a file using the onchange event;

2 - Check if the selected file is an image;

3 - Load the content of the image in an IMG element using the *FileReader API*;

The FileReader API provides us a set of JavaScript functions to read content of files stored in user’s computer. It’s supported in all major browsers and you can check more details about compatibility here.

You can check this demo of image preview before upload. And here is its code:

<!DOCTYPE html>
		<meta http-equiv="Content-type" content="text/html; charset=utf-8">
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<title>Image preview before upload / Demo</title>
			<img id="image" src="placeholder.png"><br>
			<input id="image_file" name="image_file" type="file" accept="image/*">
			 * For demonstration porpuse, all JavaScript code was incorporated in
			 * the HTML file. But when developing your application, your JavaScript code
			 * should be in a separated file. Check this page for more information:
				var $image = document.querySelector("#image");
				var $imageFile = document.querySelector("#image_file");
				// The onchange event fires when a file is selected in the image_file field
				$imageFile.onchange = function(){
				function checkForBrowserSupport(){
					if(typeof FileReader != "function"){
						alert("Your browser does not support image preview!");
					return true;
				function isImageFile(selectedFile){
						alert("Selected file is not an image!");
						return false;
					return true;
				function showImagePreview(selectedFile){
					if(!checkForBrowserSupport() || !isImageFile(selectedFile)){
					// Update IMG element with the content of the selected
					// file using the FileReader API
					var reader = new FileReader();
					reader.onload = function(e){
						$image.src =;
				function resetImage(){
					$image.src = "placeholder.png";
					$imageFile.value = "";