promise-examples.js

// 2 problems for the code below:
// (1) use somethingAsync(function(err, result)) format that could swallowed up the error in the nested call.
// (2) nested async call as a whole can be flattened out
firstThingAsync().then(function(result) {  
    // handle success but any error coming from secondThingAsync() will be swallowed
    secondThingAsync().then(function(result2) {
        // do something with result1 and result2
    });
},
function(err) {  
    // handle error
});
 
// avoid somethingAsync(function(err, result)) format and use catch for error handling after then()
somethingAsync()  
  .then(function(result) { 
    // handle success 
  })
  .catch(function(err) {
    // handle error
  });
 
// to avoid nested promise you can use promise.all(list) that will have all async calls run in parallel
Promise.all([firstThingAsync, secondThingAsync])  
  .then(function(results) {
    // do something with result1 and result2
    // available as results[0] and results[1] respectively
  })
  .catch(function(err) { /* ... */ });

// if you need the result from first call for the 2nd call, you can do the following
firstThingAsync()  
  .then(function(result1) {
    return Promise.all([result1, secondThingAsync(result1)]); 
  })
  .then(function(results) {
    // do something with results array: results[0], results[1]
  })
  .catch(function(err){ /* ... */ });