discord.js how to make a reaction roles command code example

Example: reaction role by id discord.js

const Discord = require('discord.js');
const client = new Discord.Client({ partials: ['MESSAGE', 'CHANNEL', 'REACTION'] }); //partials arent really needed but I woudld reccomend using them because not every reaction is stored in the cache (it's new in v12)
const prefix = "-";

client.on('messageReactionAdd', async (reaction, user) => {
    if (reaction.partial) { //this whole section just checks if the reaction is partial
        try {
            await reaction.fetch(); //fetches reaction because not every reaction is stored in the cache
        } catch (error) {
            console.error('Fetching message failed: ', error);
            return;
        }
    }
    if (!user.bot) {
        if (reaction.emoji.id == yourEmojID) { //if the user reacted with the right emoji

            const role = reaction.message.guild.roles.cache.find(r => r.id === yourRoleID); //finds role you want to assign (you could also user .name instead of .id)

            const { guild } = reaction.message //store the guild of the reaction in variable

            const member = guild.members.cache.find(member => member.id === user.id); //find the member who reacted (because user and member are seperate things)

            member.roles.add(role); //assign selected role to member

        }
    }
})