Blob URLs Not Revoked, Causing Memory Leaks

Introduction

In JavaScript, Blob URLs (also known as object URLs) are temporary URLs created by the browser to reference Blob or File objects. These URLs are used to access binary data in web applications without needing to store them in files or databases. However, if Blob URLs are not properly revoked, they remain in memory and can cause memory leaks, leading to increased memory consumption and degraded application performance.

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Understanding Blob URLs

Blob URLs are created using the URL.createObjectURL(blob) method, which returns a unique URL referencing a Blob or File object. This URL can then be used as a source for images, videos, downloads, and other elements requiring binary data.

Example of Blob URL Creation

const blob = new Blob(["Hello, Blob!"], { type: "text/plain" });
const blobURL = URL.createObjectURL(blob);
console.log(blobURL); // Example output: blob:http://example.com/abcd-efgh-ijkl

In this example:

• A Blob object is created with the text "Hello, Blob!"
• URL.createObjectURL(blob) generates a Blob URL that allows accessing the Blob data

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Common Uses of Blob URLs

1. Displaying Images Dynamically

const img = document.createElement("img");
const blob = new Blob(["binary image data"], { type: "image/png" });
img.src = URL.createObjectURL(blob);
document.body.appendChild(img);

2. Creating a Downloadable File

const blob = new Blob(["Hello, world!"], { type: "text/plain" });
const link = document.createElement("a");
link.href = URL.createObjectURL(blob);
link.download = "hello.txt";
document.body.appendChild(link);
link.click();
document.body.removeChild(link);

3. Using Blob URLs in Web Workers

const workerBlob = new Blob(["onmessage = function(e) { postMessage('Hello from Worker'); };"], { type: "application/javascript" });
const worker = new Worker(URL.createObjectURL(workerBlob));

worker.onmessage = function(event) {
  console.log(event.data);
};

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The Problem: Memory Leaks from Unrevoked Blob URLs

Each time URL.createObjectURL(blob) is called, a new Blob URL is created and stored in memory. If this URL is not revoked when no longer needed, the memory associated with the Blob remains allocated, even if the original JavaScript reference to the Blob is lost.

Example of a Memory Leak

function createBlobURL() {
    const blob = new Blob(["Leaking memory!"], { type: "text/plain" });
    return URL.createObjectURL(blob);
}

setInterval(() => {
    const url = createBlobURL();
    console.log(url);
}, 1000);

This function continuously generates new Blob URLs every second, but never revokes them, leading to a memory leak.

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How to Prevent Blob URL Memory Leaks

1. Use URL.revokeObjectURL() When Done

To prevent memory leaks, you must revoke the Blob URL once it is no longer needed.

Example: Revoking a Blob URL After Use

const blob = new Blob(["Clean memory!"], { type: "text/plain" });
const url = URL.createObjectURL(blob);

setTimeout(() => {
    console.log("Revoking URL:", url);
    URL.revokeObjectURL(url);
}, 5000); // URL revoked after 5 seconds

Once revoked, the URL becomes invalid, and the memory is released.

2. Revoke Blob URLs for Images

If using a Blob URL as an image source, revoke it once the image has loaded:

const img = document.createElement("img");
const blob = new Blob(["binary data"], { type: "image/png" });
const url = URL.createObjectURL(blob);

img.onload = () => {
    URL.revokeObjectURL(url); // Release memory after the image loads
};

img.src = url;
document.body.appendChild(img);

3. Revoke Blob URLs After Download

If using Blob URLs for downloads, revoke them after the download completes:

const blob = new Blob(["Download this!"], { type: "text/plain" });
const url = URL.createObjectURL(blob);
const link = document.createElement("a");

link.href = url;
link.download = "file.txt";
document.body.appendChild(link);
link.click();
document.body.removeChild(link);

URL.revokeObjectURL(url); // Cleanup after download

4. Automatically Revoke in Web Workers

For Web Workers using Blob URLs, ensure that you revoke them after creating the worker:

const workerBlob = new Blob(["onmessage = function(e) { postMessage('Hello'); };"], { type: "application/javascript" });
const workerURL = URL.createObjectURL(workerBlob);
const worker = new Worker(workerURL);

worker.onmessage = (event) => {
    console.log(event.data);
    URL.revokeObjectURL(workerURL); // Clean up once worker is created
};

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Checking for Blob URL Leaks

Using Chrome DevTools

1. Open Chrome DevTools (F12 or Ctrl+Shift+I)
2. Go to the Memory tab
3. Take a heap snapshot before creating Blob URLs
4. Run your script and take another snapshot
5. Search for Blob in the heap snapshot to check for memory leaks

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