Registers some middleware for certain chat types only. For example, you can use this method to only receive updates from private chats. The four chat types are "channel", "supergroup", "group", and "private". This is especially useful when combined with other filtering logic. For example, this is how can you respond to /start commands only from private chats:

bot.chatType("private").command("start", ctx => { ... })

Naturally, you can also use this method on its own.

// Private chats only
bot.chatType("private", ctx => { ... });
// Channels only
bot.chatType("channel", ctx => { ... });

You can pass an array of chat types if you want your middleware to run for any of several provided chat types.

// Groups and supergroups only
bot.chatType(["group", "supergroup"], ctx => { ... });

Remember also that you can access the chat type via ctx.chat.type.