There are a range of different methods that can be used, but your guess is probably correct regardless of which method you use: "breaking the image into smaller

There are a range of different methods that can be used, but your guess is probably correct regardless of which method you use: "breaking the image into smaller 'patches' and looping through those to look for chunks of a barcode". I would probably train a classifier (SVM or similar) using some rotation invariant feature, such as DAISY or ORB (BRIEF). I tried something similar for CRF image segmentation. This is mighty impressive in JS whatever method they use, the CV libraries in JS are kind of limited.