If I don't have John in my phone contacts (and thus I don't have him in my WhatsApp contacts), and then I open WhatsApp main window, will John see me online? I know my contacts will, but will John?
By "see me online" I mean: if he opens the conversation view with me, will he see the "online" label under my name?
Note: Suppose John has my phone number in his contacts.
Being online can't be hidden as long as the person is not blocked. But you can choose who can see your last seen at information at
Whatsapp -> Top right "three dot" menu -> Settings -> Account -> Privacy -> Last seen
By "see me online" I mean: if he opens the conversation view with me, will he see the "online" label under my name?
Yes - he will be able to see that you are Online.
As per the official FAQ of WhatsApp,
There is no way to hide when you are online or typing.
