check if message starts with the prefix

const possiblePrefixes = ['!', '<@PUT_YOUR_CLIENT_ID_HERE_FOR_THE_MENTION>', 'etc'];
const msg = {
  content: '!mycommand myoptions'
};

let startsWithPrefix = false;

for (let prefix of possiblePrefixes) {
  if (msg.content.startsWith(prefix)) {
    startsWithPrefix = true;
    break;
  }
}

console.log(startsWithPrefix); // prints true if msg.content starts with one of the possiblePrefixes