jQuery Promise with evented resolution/rejection conditions.
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Schrödinger's cat would be the ultimate Maybe, man.

Let me introduce the highly respectable Maybe. It is an enhanced $.Deferred interface, featuring a simple way to specify when and why it should be resolved and/or rejected. It really tries to be a more expressive, less all-around alternative to jQuery.when (If you want to know about Deferred or refresh your knowledge, head to this review and the doc).

This custom interface relies on a fully evented interface for the promise object associated to the deferrable. When using a Maybe, one is expected to specify evented conditions for success and failure. Those conditions, if and when matched, will trigger the deferrable's resolution or rejection callbacks accordingly.

This feature is made possible by exposing a custom Promise to the outside world that accepts doIf and failIf helpers to specify the aforementioned evented conditions.

By default, a Maybe will wait for an event to occur, managing a so-called "infinite grace-period". One can specify a finite timeout instead, in which case the Maybe will wait until this delay is over to succeed. In both cases, the Maybe can be either early-resolved (doIf) or early-rejected (failIf).


Say you are the admin of a real-time, collaborative webapp. You want to enforce a setting to all users, but you would be kind enough to make this a proposal and let them discard it. That means, displaying a notification to them, asking whether they want to apply the change. There would be a 10s delay before actually performing the update, because 10s is enough time for boorish users to refuse your kind proposal by clicking on this doomed "Cancel" button.

Using jquery.maybe, you would simply write:

$('.my_notification').maybe({wait: 10000})
                     .failIf('click', '.cancel')

where success,failure and hide are callbacks of your choice. The first two will be called when the action is respectively resolved or rejected; hide will always be called, no matter what.

failIf() is used to attach a condition for rejection. In the example, we're listening for a click on $('.cancel', '.my_notification') during the 10 s lifetime of the timeout. If such an event occurs, the action is rejected and success never fires; instead, failure then hide do fire. If no click event is catched while in the grace-period, the action ends up being resolved: success then hide are triggered, whereas failure never shows up.

Available options for maybe()

  • wait - in milliseconds, the delay before resolving in success (default: -1). When wait is set to -1, the action is pending for ever and must be resolved explicitly, so one is expected to provide resolution conditions with doIf / failIf. If wait is set to a positive value, it specifies a timeout before which the Maybe may be (haha!) resolved/rejected; otherwise, it will be automagically resolved in success when the time is over.
  • clear - whether to forget about the promise once resolved/rejected (default: true). See the note below.

Custom methods available on the exposed Promise

  • failIf - specifies a condition for rejecting the action. The signature matches the flat arguments' flavour of $.on():
$('.foo').maybe().failIf('my.event', '.my_element');
  • doIf - specifies a condition for resolving the action. Same pattern as failIf.

Apart from that, this is a regular Promise as defined by jQuery!

Note that, when initializing the plugin, one may either chain methods:

$('.my_notification').maybe(// options).done(// cb).fail(// cb);

or not:

$('.my_notification').maybe(// options);
$('.my_notification').done(// callback);
$('.my_notification').fail(// callback);

When using the second form, it is important to understand that the exposed promise is memoized by jquery.maybe until the action is either resolved or rejected, so in-between calls to maybe() on the same element will return the singleton promise, which hopefully leads to the same result as the chaining method.

On clear

In our example, once resolved or rejected, using legacy Deferred's behavior, the notification's state would be settled. That is, the exposed promise would be either resolved or rejected, and would remain so forever. When using jquery.maybe with a non-unique element, such as the .my_notification in the previous example (there could be several notifications displayed at once!), it could turn really annoying. I think this is the typical use-case, therefore by default, jquery.maybe will clear the "binding" between the element and its now-settled promise/maybe. A new call to maybe() on this element will simply generate a new, viviv folk, replacing the staled one. If this is not what you want, you can pass {clear: false} in the options to leave the promise in peace and stare at it dead until you die too.


MIT, see LICENSE.txt.