promises

index.js

// nested
callA().then(function () {                                                                                                                                      
    return callB().then(function () {                                                                                                                           
        return callC()                                                                                                                                          
    });                                                                                                                                                         
});                                                                                                                                                             
                                                                                                                                                                
// equivalent                                                                                                                                                                
callA().then(function () {                                                                                                                                      
    return callB();                                                                                                                                             
}).then(function () {                                                                                                                                           
    return callC();                                                                                                                                             
}) 

callA().then(callB).then(callC)