Create custom command permission checks using Typescript decorators.

checks.ts

// Decorator check example, similar to discord.py
// Requires Typescript with  "experimentalDecorators": true and "emitDecoratorMetadata": true in tsconfig
// Also requires the "reflect-metadata" module

// in src/util/checks.ts

import { Command, CommandoMessage } from 'discord.js-commando';

export function customCheck(check: (message: CommandoMessage, args: object) => Promise<boolean> | boolean, reason?: string) {
    return function <T extends ProxyCommand>(target: T, key: string, descriptor: PropertyDescriptor): PropertyDescriptor {
        const method = descriptor.value;
        
        descriptor.value = async function(...args: any[]) {
            const message: CommandoMessage = args[0];
            const commandArgs = args[1];
            
            // Check if the conditions are right for the command
            if (await check(message, commandArgs)) {
                // Run the actual command
                return method.apply(this, args);
            }
          
            // Check failed, do whatever you want
            
            // For example send a nice failure message, telling user why they can't use this command	
            // const embed = new MessageEmbed()	
            //    .setTitle('ERROR')	
            //    .setDescription(reason)	
            //    .setColor('DARK_RED');	
            //  message.channel.send({ embed });
            
            // Emit the error with the custom reason
            // In theory you could do whatever you wanted with the error here
            message.client.emit('commandBlocked', message, reason || 'customCheck');
        };
        
        return descriptor;
    };
}

export const bro = customCheck((message: CommandMessage, args: object) => {
  return message.author.username.includes('bro');
}, 'You are not a bro!');

// Supports asynchronous or synchronous functions!
export const registered = customCheck(async (message: CommandMessage, args: object) => {
  // A fake database lookup example
  const author = await message.client.db.find(message.author.id);

  return author != null;
}, 'This command requires you have registered yourself with the bot!');

raffle.ts

// in src/commands/raffle.ts

import { Command, CommandoMessage } from 'discord.js';

import { registered, bro } from '../util/checks';

class RaffleCommand extends Command {
    @bro
    @registered
    async run(message: CommandMessage) {
        return message.channel.send('Hello from raffle command!');
    }
}