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What is an Object URL in JavaScript?

Published in JavaScript URLs 4 mins read

An object URL in JavaScript is a string that represents a File or Blob object. It acts as a temporary URL, created by the browser, which can be used to access the data contained within the object. This URL allows you to use File or Blob data as a source for elements like <img>, <video>, <iframe>, or even in web workers without needing to upload the data to a server.

Understanding Object URLs

Here's a breakdown of object URLs and their usage:

  • Purpose: Object URLs provide a way to refer to data stored within File or Blob objects. These objects often contain binary data, such as images, videos, or audio, which can't be directly represented as a standard URL.

  • Creation: You create an object URL using the URL.createObjectURL() method. This method takes a File or Blob object as input and returns a unique string representing the URL.

    const blob = new Blob(['This is some text.'], { type: 'text/plain' });
const url = URL.createObjectURL(blob);
console.log(url); // Output: blob:http://example.com/d0e8b0c8-9b3a-4a4d-a0a7-6e3e5f6b7c8d (example)

  • Usage: The generated URL can then be used as the src attribute of an <img> or <video> tag, or in any other context where a URL is expected.

    <img id="myImage" src="" alt="Image">
<script>
  const blob = new Blob([''], { type: 'image/png' }); // Replace with actual image data
  const url = URL.createObjectURL(blob);
  document.getElementById('myImage').src = url;
</script>

  • Lifetime: Object URLs are temporary and are managed by the browser. They persist as long as the document that created them is still loaded.

  • Memory Management: It's crucial to release object URLs when they are no longer needed using the URL.revokeObjectURL() method. Failing to do so can lead to memory leaks.

    // After the image is loaded or no longer needed:
URL.revokeObjectURL(url);

Benefits of Using Object URLs

  • Client-Side Processing: Avoids the need to upload data to a server for simple operations like displaying an image preview before submitting a form.

  • Efficiency: Allows direct access to the underlying data without copying it, improving performance.

  • Security: Object URLs are scoped to the creating document, offering a degree of security. The data is not exposed beyond the current context unless explicitly shared.

Example: Displaying an Image Preview

Here's a complete example demonstrating how to display an image preview using an object URL:

<!DOCTYPE html>
<html>
<head>
  <title>Image Preview</title>
</head>
<body>
  <input type="file" id="imageInput" accept="image/*">
  <img id="imagePreview" src="#" alt="Image Preview" style="max-width: 200px;">

  <script>
    const imageInput = document.getElementById('imageInput');
    const imagePreview = document.getElementById('imagePreview');

    imageInput.addEventListener('change', function(event) {
      const file = event.target.files[0];

      if (file) {
        const url = URL.createObjectURL(file);
        imagePreview.src = url;

        // Revoke the URL when the image is no longer needed (e.g., on form submission)
        imagePreview.onload = function() {
          URL.revokeObjectURL(url);
        };
      }
    });
  </script>
</body>
</html>

This code allows the user to select an image file, and it displays a preview of the image using an object URL. The URL.revokeObjectURL() method is called after the image is loaded to free up resources.

In summary, object URLs are a powerful tool in JavaScript for working with File and Blob objects, allowing you to manipulate and display data efficiently within the browser. Remember to always revoke the URLs to prevent memory leaks.

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