JavaScript animated GIF detection!

detectanimation.js

function isAnimatedGif(src, cb) {
	var request = new XMLHttpRequest();
	request.open('GET', src, true);
	request.responseType = 'arraybuffer';
	request.addEventListener('load', function () {
		var arr = new Uint8Array(request.response),
			i, len, length = arr.length, frames = 0;
			
		// make sure it's a gif (GIF8)
		if (arr[0] !== 0x47 || arr[1] !== 0x49 || 
			arr[2] !== 0x46 || arr[3] !== 0x38)
		{
			cb(false);
			return;
		}
		
		//ported from php http://www.php.net/manual/en/function.imagecreatefromgif.php#104473
		//an animated gif contains multiple "frames", with each frame having a 
		//header made up of:
		// * a static 4-byte sequence (\x00\x21\xF9\x04)
		// * 4 variable bytes
		// * a static 2-byte sequence (\x00\x2C) (some variants may use \x00\x21 ?)
		// We read through the file til we reach the end of the file, or we've found 
		// at least 2 frame headers
		for (i=0, len = length - 9; i < len, frames < 2; ++i) {
			if (arr[i] === 0x00 && arr[i+1] === 0x21 &&
				arr[i+2] === 0xF9 && arr[i+3] === 0x04 &&
				arr[i+8] === 0x00 && 
				(arr[i+9] === 0x2C || arr[i+9] === 0x21))
			{
				frames++;
			}
		}
		
		// if frame count > 1, it's animated
		cb(frames > 1);
	});
	request.send();
}

looks like it's never satisfying the for conditions if the GIF is not animated.
can't understand why

It is because multiple comma-separated conditions in a for loop like this

for (i=0, len = length - 9; i < len, frames < 2; ++i)

will evaluate both checks, but use only the last one for determining whether to run the loop again. So unless more than one frame is found, i will just grow and frames < 2 never satisfy.

In this case, better to use || (or have an inner loop)

Implemented @olapeter's suggestion here, only using && since the logic is inverted between the comment and the if statement.

Also updated to support variable length extension control blocks.

N.B. the img.replace(/\x00\x21\xF9\x04\x00[\x2c\x21]/g solution suffers the same problem as the original code here -- if the ECB isn't 4 bytes this regex won't find the frame (see the wikipedia article examples).

great rubust solution from @marckubischta ! thanks for that.
anyone able to come up with detecting animation for webp image format?