Registers some middleware that will only be added when a new reaction of the given type is added to a message.

// Reacts to new '👍' reactions
bot.reaction('👍', ctx => { ... })
// Reacts to new '👍' or '👎' reactions
bot.reaction(['👍', '👎'], ctx => { ... })

Note that you have to enable message_reaction updates in allowed_updates if you want your bot to receive updates about message reactions.

bot.reaction will trigger if:

• a new emoji reaction is added to a message
• a new custom emoji reaction is added a message

bot.reaction will not trigger if:

• a reaction is removed
• an anonymous reaction count is updated, such as on channel posts
• message_reaction updates are not enabled for your bot