once()

instance.once(eventName, handler);

// Run handler only on first send
instance.once('pre:send', (event, instance) => {
    console.log('First message being sent');
    event.message.message.content = event.message.message.content.toUpperCase();
});

// Initialize something once when chat is ready
instance.once('chat:ready', (event, instance) => {
    console.log('Performing one-time initialization');
    instance.updateCustomHeaderItems([/* ... */]);
});