Bug Tracker

Opened 10 years ago

Closed 10 years ago

#12884 closed bug (duplicate)

Errors thrown on callback handlers can break the original promise

Reported by: Alex Kalderimis <[email protected]…> Owned by:
Priority: undecided Milestone: None
Component: unfiled Version: 1.8.2
Keywords: Cc:
Blocked by: Blocking:


If errors are thrown by callback handlers passed to jQuery.Deferred.then then they will trigger a number of unspecified and dangerous behaviours:

  • Callbacks don't catch errors thrown inside them. This is the most minor issue, but it would make a lot of sense to catch errors and issue a rejection, and this simple change would deal with the consequences below.
  • As it is uncaught, the error will blow the stack in the current context of execution, rather than asynchronously, blowing up co-ordinating code, even when the function itself was never called explicitly. This is surprising, but not completely unreasonable; it should be specified in documentation.
  • The call to jQuery.Deferred.then will not return a usable value. It should be part of the contract that then will always return a null safe value implementing the promise interface, even if it is just a rejected promise that reports its own failure to be created. This also points to an inconsistency in interface, in that a error thrown in a callback applied to an unresolved promise will return a new promise, but it the original promise was already resolved, then we will get this different and undesired behaviour.
  • Most seriously the original promise is now broken, and will no longer respond to callbacks passed to then at all. This completely breaks the contract, whereby a promise's callbacks do not affect each other and a promise once resolved or rejected will always respond to new callbacks.

A demonstration of this set of issues is available at: http://jsfiddle.net/3geX6/2/

This whole issue could be resolved by having then (which is meant to return a sensible value, so this applies to then more so than to done, fail or always) catch its errors and return a rejected promise containing the error.

Change History (2)

comment:1 Changed 10 years ago by anonymous

Sorry; pasted the wrong version of the jsfiddle example - this one is better:


comment:2 Changed 10 years ago by dmethvin

Resolution: duplicate
Status: newclosed

Duplicate of #11193.
See also #11292.

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