Why use `URL.createObjectURL(blob)` instead of `image.src`?

Why Use URL.createObjectURL(blob) Over image.src for Dynamic Images?

Explore the technical advantages and use cases of URL.createObjectURL(blob) for displaying local or generated images, comparing it against directly assigning to image.src.

When working with dynamic image content in web applications, developers often face a choice: directly assign a data: URL or a remote URL to an image.src attribute, or leverage URL.createObjectURL(blob) for Blob or File objects. While image.src is straightforward for external resources, URL.createObjectURL() offers significant advantages, particularly when dealing with client-side generated images, local files, or large data streams. This article delves into the reasons why URL.createObjectURL(blob) is often the superior choice for these scenarios, focusing on performance, memory management, and security.

Understanding Blobs and Object URLs

A Blob object represents a file-like immutable raw data. It can be created from various sources, including JavaScript arrays, other Blobs, and ArrayBuffer objects. File objects are a specific type of Blob, representing files from the user's system (e.g., via an <input type="file"> element or drag-and-drop). URL.createObjectURL(blob) generates a unique, pseudo-URL that points to the data contained within the Blob or File object. This URL is a string like blob:http://example.com/some-guid, and it can be used anywhere a regular URL is expected, such as in an <img> tag's src attribute, a <a> tag's href, or a CSS url() function. The browser manages the underlying data, making it accessible via this temporary URL.

Lifecycle of an Object URL

Advantages of URL.createObjectURL(blob)

Using object URLs provides several key benefits over other methods, especially for large or sensitive data. These benefits include improved performance, efficient memory management, and enhanced security.

const fileInput = document.getElementById('imageUpload');
const imageElement = document.getElementById('myImage');

fileInput.addEventListener('change', (event) => {
  const file = event.target.files[0];
  if (file) {
    const objectURL = URL.createObjectURL(file);
    imageElement.src = objectURL;
    // Don't forget to revoke the URL when it's no longer needed!
    // URL.revokeObjectURL(objectURL);
  }
});

Example of displaying a user-selected image using URL.createObjectURL.

Performance and Memory Efficiency

When you assign a large data: URL to image.src, the entire base64-encoded string is stored in the DOM, which can be memory-intensive and slow down rendering, especially for multiple large images. URL.createObjectURL(), on the other hand, creates a reference to the Blob data. The browser can then efficiently manage this data in memory without needing to store the entire content within the DOM. This is crucial for applications that handle user-uploaded files, generated images (e.g., from a <canvas>), or large data blobs. Once the object URL is no longer needed, URL.revokeObjectURL() should be called to release the memory, preventing memory leaks.

Security Implications

Object URLs are created within the context of the current document's origin. This means that an object URL from http://example.com cannot be directly accessed by a script from http://another.com, adhering to the Same-Origin Policy. This provides a level of security by preventing cross-origin access to local or generated data. In contrast, embedding data: URLs directly can sometimes expose sensitive data in the HTML source, although this risk is generally mitigated by the same-origin policy for script access. The transient nature of object URLs, coupled with the ability to explicitly revoke them, offers better control over the data's lifecycle and accessibility.

Consider a scenario where a user uploads a large image file. Using URL.createObjectURL() allows the browser to efficiently display this image without converting it to a potentially enormous base64 string, which would bloat the DOM and consume more memory. This method is also ideal for displaying images generated on the client-side, such as those drawn onto a <canvas> element, before sending them to a server or allowing a user to download them.

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

// Draw something on the canvas
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, 100, 100);

canvas.toBlob((blob) => {
  const imgElement = document.createElement('img');
  const objectURL = URL.createObjectURL(blob);
  imgElement.src = objectURL;
  document.body.appendChild(imgElement);
  
  // Revoke after a short delay or when the image is loaded
  imgElement.onload = () => {
    URL.revokeObjectURL(objectURL);
  };
}, 'image/png');

Generating an image from a canvas and displaying it using an object URL.