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 3-byte sequence (\x00\x21\xF9
		// * one byte indicating the length of the header (usually \x04)
		// * variable length header (usually 4 bytes)
		// * a static 2-byte sequence (\x00\x2C) (some variants may use \x00\x21 ?)
		// We read through the file as long as we haven't reached the end of the file
		// and we haven't yet found at least 2 frame headers
		for (i=0, len = length - 3; i < len && frames < 2; ++i) {
		  if (arr[i] === 0x00 && arr[i+1] === 0x21 && arr[i+2] === 0xF9)            
		  {
			blocklength = arr[i+3];
			afterblock = i + 4 + blocklength;
			if (afterblock + 1 < length && 
			  arr[afterblock] === 0x00 &&
			  (arr[afterblock+1] === 0x2C || arr[afterblock+1] === 0x21)) 
			{
			  frames++;
			}
		  }
		}
		
		// if frame count > 1, it's animated
		cb(frames > 1);
	});
	request.send();
}

Missing variable declarations for blocklength and afterblock?