CODE EXAMPLE FOR JAVASCRIPT

messageReactionAdd

Update 2022. 
You may need to add intents for GUILD_MESSAGES and GUILD_MESSAGE_REACTIONS with cache messages to make it work.

const client = new Client({
    intents: [Intents.FLAGS.GUILDS, Intents.FLAGS.GUILD_MESSAGES, Intents.FLAGS.GUILD_MESSAGE_REACTIONS],
    partials: ['MESSAGE', 'CHANNEL', 'REACTION'],
});