Promise returns undefined

Can someone please explain why am I getting 'undefined'?

```
bot.postMessageToChannel('test-bot-channel', 'Hi',params).always(function(data) {
            console.log('success? -->'+data); //output: success? -->undefined
 }); 
```

The message was posted to the channel successfully. 

Edit:  when I change the method to postMessage it works, data is populated.