Add

Basic case

Add sets a new value for a key if and only if that key doesn't already exist

client
    .add('firstName', 'Victor')
    .then(function() {
        console.log('Successfully added the key firstName');
    });

Error if key already exists

If a key already exists and you try to use the add command, Memcached will return an error which Memcache Plus will throw.

// If 'firstName' already exists
client
    .add('firstName', 'Victor')
    .then(function() {
        // This will not get hit because `add` will throw on error
        console.log('Successfully added the key firstName');
    })
    .catch(function(err) {
        // Will print: 'Cannot "add" for key "firstName" because it already exists'
        console.error(err);
    });

Callbacks

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.add('firstName', 'Victor, function(err) {
    console.log('Successfully added the key firstName');
});

And if you try to add to a key that already exists:

// If 'firstName' already exists
client.add('firstName', 'Victor', function(err) {
    // Will print: 'Cannot "add" for key "firstName" because it already exists'
    console.error(err);
});