If I understand your question correctly, it seems like impossible to monitor transactions (in terms of receiving notifications about new blocks and transactions from the blockchain) with eosjs (at least for the current moment).
As taokayan (contributor) pointed out:
There's no direct way to do it [notify 3rd party service] within the current version [of eosio software]. However, you can make a plugin to capture all the blocks & transactions then notify 3rd party service. Or, alternatively, call "get table" from time to time.
Also, chris-allnutt (contributor) said that:
Eosjs doesn't have anything internal to poll nodes and would more than likely be out of scope for its intended usage - here is a link
IMHO, the state of the art solution is the following. If you have a running nodeos instance and it is synchronized with the mainnet you can try using mongo_db_plugin to store all the blocks and then query mongodb to extract transactions you need. The drawback is that you still won't be able to get notifications from the mongo until setting up a replica set of several mongod instances (which consumes a lot of memory).