The documentation for Parse.User.become() says that a single argument should be passed to become, a session token. Where, officially, does this session token come from for the JavaScript API?
I see no call to associate an arbitrary ID (like session cookie) with the Parse user object, so I assume it is the same session token return by the REST API, not one created by my own session manager.
So far I have only seen two ways to get this: a field user._sessionToken, and a field in the response to the REST API login call. The first is more convenient for me since I do not use the REST API directly so far, but it does have that underbar at the start of the name, indicating danger. However, I do not see explicit documentation of either one.