I'm trying to build an app with comment system and wanna know if
is there any safe way to use the mongolab api without share my key with everyone?
if not what you guys recommend to use instead of mongolab api key with ajax.
Unfortunately there is no way to use the mLab Data API with Ajax and keep the API Key safe. We recommend implementing your own RESTful API server on top of your mLab database that can act as a gatekeeper to the database. Your client application would then make Ajax requests to the API server.
Although this tutorial uses Angular instead of Ajax, here is a good example of the architecture I've described: https://devcenter.heroku.com/articles/mean-apps-restful-api.