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The then method returns a promise, which in this example, I’m assigning to outputPromise.

var outputPromise = getInputPromise()
   .then(
     function (input) {
     },
     function (reason) {
     }
    );

The outputPromise variable becomes a new promise for the return value of either handler. Since a function can only either return a value or throw an exception, only one handler will ever be called and it will be responsible for resolving outputPromise.

• If you return a value in a handler, outputPromise will get fulfilled.

• If you throw an exception in a handler, outputPromise will get rejected.

• If you return a promise in a handler, outputPromise will “become” that promise. Being able to become a new promise is useful for managing delays, combining results, or recovering from errors.

If the getInputPromise() promise gets rejected and you omit the rejection handler, the error will go to outputPromise:

var outputPromise = getInputPromise()
  .then(function (value) {
  });

If the input promise gets fulfilled and you omit the fulfillment handler, the value will go to outputPromise:

var outputPromise = getInputPromise()
  .then(null, function (error) {
  });

Q promises provide a fail shorthand for then when you are only interested in handling the error:

var outputPromise = getInputPromise()
  .fail(function (error) {
  });

If you are writing JavaScript for modern engines only or using CoffeeScript, you may use catch instead of fail.

Promises also have a fin function that is like a finally clause.The final handler gets called, with no arguments, when the promise returned by getInputPromise() either returns a value or throws an error. The value returned or error thrown by getInputPromise() passes directly to outputPromise unless the final handler fails, and may be delayed if the final handler returns a promise.

var outputPromise = getInputPromise()
  .fin(function () {
    // close files, database connections, stop servers, conclude tests
  });

• If the handler returns a value, the value is ignored
• If the handler throws an error, the error passes to outputPromise
• If the handler returns a promise, outputPromise gets postponed. The eventual value or error has the same effect as an immediate return value or thrown error: a value would be ignored, an error would be forwarded.

If you are writing JavaScript for modern engines only or using CoffeeScript, you may use finally instead of fin.