.tapCatch

.tapCatch is a convenience method for reacting to errors without handling them with promises - similar to finally but only called on rejections. Useful for logging errors.

It comes in two variants. - A tapCatch-all variant similar to .catch block. This variant is compatible with native promises. - A filtered variant (like other non-JS languages typically have) that lets you only handle specific errors. This variant is usually preferable.

tapCatch all

.tapCatch(function(any value) handler) -> Promise

Like .finally that is not called for fulfillments.

getUser().tapCatch(function(err) {
    return logErrorToDatabase(err);
}).then(function(user) {
    //user is the user from getUser(), not logErrorToDatabase()
});

Common case includes adding logging to an existing promise chain:

Rate Limiting

Promise.
  try(logIn).
  then(respondWithSuccess).
  tapCatch(countFailuresForRateLimitingPurposes).
  catch(respondWithError);

Circuit Breakers

Promise.
  try(makeRequest).
  then(respondWithSuccess).
  tapCatch(adjustCircuitBreakerState).
  catch(respondWithError);

Logging

Promise.
  try(doAThing).
  tapCatch(logErrorsRelatedToThatThing).
  then(respondWithSuccess).
  catch(respondWithError);

Note: in browsers it is necessary to call .tapCatch with console.log.bind(console) because console methods can not be called as stand-alone functions.

Filtered tapCatch

.tapCatch(
    class ErrorClass|function(any error),
    function(any error) handler
) -> Promise

.tapCatch(
    class ErrorClass|function(any error),
    function(any error) handler
) -> Promise

This is an extension to .tapCatch to filter exceptions similarly to languages like Java or C#. Instead of manually checking instanceof or .name === "SomeError", you may specify a number of error constructors which are eligible for this tapCatch handler. The tapCatch handler that is first met that has eligible constructors specified, is the one that will be called.

Usage examples include:

Rate Limiting

Promise.
  try(logIn).
  then(respondWithSuccess).
  tapCatch(InvalidCredentialsError, countFailuresForRateLimitingPurposes).
  catch(respondWithError);

Circuit Breakers

Promise.
  try(makeRequest).
  then(respondWithSuccess).
  tapCatch(RequestError, adjustCircuitBreakerState).
  catch(respondWithError);

Logging

Promise.
  try(doAThing).
  tapCatch(logErrorsRelatedToThatThing).
  then(respondWithSuccess).
  catch(respondWithError);