How Can I Get an Image's Data URL in JavaScript Without Redownloading It?

Getting Image Data URL in JavaScript

This question seeks a method to retrieve the data URL of an image that has already been loaded onto a web page using JavaScript, specifically for use in Greasemonkey scripts with Firefox. The objective is to obtain the base64 encoded content of the image without incurring the overhead of downloading it again.

Solution:

1. Create a Canvas Element: Create an empty canvas element with dimensions matching the image to be encoded.
2. Copy Image Data: Use the "drawImage" function to copy the image data onto the canvas element. This step assumes that the image has already been fully loaded.
3. Obtain Data-URL: Utilize the "toDataURL" function to retrieve a data-URL in the desired format, which can be either PNG or JPEG. The provided solution uses a RegExp to extract only the base64 encoded content from the data-URL.

Note:

a. This method only works when the image is hosted on the same domain as the webpage.
b. The resulting image is a re-encoded version, not an exact replica of the original file.
c. The JavaScript code provided assumes a Greasemonkey environment.

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