In my last post, How to Use HTML5 File Drag & Drop , we discovered how to use the HTML5 File API, implement drag and drop events, and retrieve file information. Today, we’ll attempt to load files on the client using JavaScript.

Why Open Local Files in JavaScript?

Uploading files from an HTML form is clunky. People often use it when transferring multi-megabyte photographs from their camera to a web server. Assuming they locate the file, the upload could take several minutes only to find it was the wrong photo, an unsupported format, or a larger file size than permitted. Until now, developers had to rely on Flash or other plugins to provide a better user experience. Pre-processing in JavaScript offers a number of benefits:

• Local file processing is fast.
• Files can be analyzed to ensure they’re the correct format and lower than a specific size.
• Files such as images can be previewed before upload.
• It’s possible to crop or resize an image on a canvas element then upload the resulting file.

The FileReader object

FileReader forms part of the W3C File API and offers four methods to asynchronously load data from a file referenced in a File object:

• .readAsText(File f, [encoding]): reads File f into a string. UTF-8 encoding is assumed, but the optional encoding parameter can specify a different format.
• .readAsDataURL(File f): reads File f into an encoded data URL
• .readAsBinaryString(File f): reads File f as a binary string
• .readAsArrayBuffer(File f): reads File f as an ArrayBuffer object.

In the following code we’ll use the first two methods to load and display text and image files.

Opening Files Asynchronously in JavaScript

Here’s our original ParseFile() function which is passed a File object when it is selected or dropped onto the #filedrag element:

// output file information
function ParseFile(file) {

	Output(
		"<p>File information: <strong>" + file.name +
		"</strong> type: <strong>" + file.type +
		"</strong> size: <strong>" + file.size +
		"</strong> bytes</p>"
	);
	
}

Following the status update, we’ll check if we have a text file (text/plain, text/html, text/css, etc.), load it using the FileReader.readAsText() method and display the result (after escaping characters):

	// display text
	if (file.type.indexOf("text") == 0) {
		var reader = new FileReader();
		reader.onload = function(e) {
			Output(
				"<p><strong>" + file.name + ":</strong></p><pre class="brush:php;toolbar:false">" + 
				e.target.result.replace(/</g, "&lt;").replace(/>/g, "&gt;") +
				"

" ); } reader.readAsText(file); } Similarly, we can check whether we have an image file (image/jpeg, image/gif, image/png etc.), load it into a data URL using the FileReader.readAsDataURL() method, and pass the result to the src attribute of an img tag:

	// display an image
	if (file.type.indexOf("image") == 0) {
		var reader = new FileReader();
		reader.onload = function(e) {
			Output(
				"<p><strong>" + file.name + ":</strong><br />" +
				'<img src="' + e.target.result + '" /></p>'
			);
		}
		reader.readAsDataURL(file);
	}

Please view the demonstration page in Firefox, Chrome, or Opera (no drag & drop support). You can also download the files to examine the code. While this is useful, ultimately, we’ll need to upload our files to a web server. Stay tuned for How to Asynchronously Upload Files Using HTML5 and Ajax…

Frequently Asked Questions (FAQs) about HTML5 and JavaScript Open Dropped Files

What is the purpose of using HTML5 and JavaScript to open dropped files?

HTML5 and JavaScript are used to open dropped files to enhance the user experience on a website. This feature allows users to drag and drop files from their local system onto a web page. Once dropped, the files can be read and processed using JavaScript. This can be particularly useful for tasks such as uploading files, reading file content, or processing images or documents within a web application.

How does the drag and drop feature work in HTML5?

The drag and drop feature in HTML5 is facilitated by a set of events and properties. When a user selects a file on their system and drags it over a web page, the ‘dragover’ event is triggered. If the file is dropped, the ‘drop’ event is triggered. The dataTransfer property, which is part of these events, holds the file data and can be used to access and process the file.

How can I read the content of a dropped file using JavaScript?

JavaScript provides the FileReader API to read the content of files. Once a file is dropped onto a web page, it can be accessed through the dataTransfer property. A new FileReader object can be created and the readAsText or readAsDataURL method can be used to read the file content. The result can be accessed in the ‘load’ event of the FileReader object.

Can I open multiple dropped files at once?

Yes, you can open multiple dropped files at once. The dataTransfer property provides a ‘files’ property which is a FileList object. This object represents a list of all the files dropped. You can loop through this list to access and process each file individually.

What types of files can I open using HTML5 and JavaScript?

You can open any type of file using HTML5 and JavaScript. However, how you process the file will depend on its type. For example, text files can be read as text, while image files can be read as data URLs and displayed using an img element.

Is it safe to open dropped files using HTML5 and JavaScript?

Opening dropped files using HTML5 and JavaScript is generally safe as the files are only read and processed on the client side. However, if the file data is sent to a server, it should be properly validated and sanitized to prevent security issues.

Can I use HTML5 and JavaScript to open dropped files on all browsers?

Most modern browsers support the HTML5 drag and drop feature and the JavaScript FileReader API. However, there may be differences in how these features are implemented. Therefore, it’s important to test your code on different browsers to ensure compatibility.

How can I display the content of a dropped file on a web page?

The content of a dropped file can be displayed on a web page using JavaScript. For example, if the file is an image, it can be read as a data URL and set as the src attribute of an img element. If the file is a text file, its content can be inserted into a text element.

Can I restrict the types of files that can be dropped?

While you can’t directly restrict the types of files that can be dropped, you can check the type of a dropped file using the ‘type’ property of the File object. If the file type doesn’t match the allowed types, you can display an error message and ignore the file.

Can I drag and drop files from different folders?

Yes, you can drag and drop files from different folders. The drag and drop feature doesn’t depend on the location of the files. As long as the files can be accessed by the user, they can be dragged and dropped onto a web page.

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