Flush causes all items to expire. It does not free up or flush memory.
client
    .flush()
    .then(function() {
        console.log('Successfully cleared all data');
    });
You can add a delay in seconds before the flush is executed.
client
    .flush(1)
    .then(function() {
        console.log('Successfully cleared all data');
    });
Memcache Plus will always return a Promise, but it can also take a traditional callback for any of its methods so it can work just like most of the other Memcache modules out there. For example:
client.flush(function() {
    console.log('Successfully cleared all data');
});