Deferreds and Promises should use a prototype

[[email protected]…]

Deferreds and Promises should have a prototype so plugins can extend them and add functions. I guess using a prototype would even make jQuery more lightweight because functions like then or always, … don't need to be redefined for each new Deferred. As an example: imagine a function that triggers if the Deferred isn't resolved after a given timeout (e.g. to show a loading text only after something loads for more than 1s already) ​http://jsfiddle.net/kDYPy/1/

Other adventages would be being able to check if an object is a deferred/promise using instanceof

this is similar to ​http://bugs.jquery.com/ticket/9937 i guess note: ​http://bugs.jquery.com/ticket/9937#comment:4 says "'the behaviour is exactly as expected and changing it here will break stuff' No-one is proposing to move methods to Deferred.prototype, if that was your concern." Well I'm proposing exactly that, but my guess is, that it wouldn't break anything.

[dmethvin]

I think we intentionally designed the promise so that it could augment an existing object rather than inherit from a prototype.

Putting out the jaubourg-shaped signal light for further discussion.

[jaubourg]

Well, as long as the idea is that those methods would only be on the Deferred object and NOT on the Promise, we could have a Deferred use the prototype of $.Deferred. You would never be able to override default methods because they're on the object though.

As for determining if an object is a promise, we don't really have that in jQuery, you rather test if an object is "observable through a promise". So you test for the presence of a method called promise and call it to get the actual promise.

If you want to add methods on promises, then create a new type of object and call the promise method of a Deferred passing an instance of that object.

Example:

function MyPromise() {
}

MyPromise.prototype = {
  myAlways: function( fn ) {
    return this.done( fn ).fail( fn );
  }
};

var myPromise = Deferred().promise( new MyPromise() );

myPromise.myAlways( fn1 ).myAlways( fn2 );

myPromise instanceof MyPromise; // true

This works because promise and deferred methods chain on their context.

[fastfasterfastest]

As for determining if an object is a promise, we don't really have that in jQuery, you rather test if an object is "observable through a promise". So you test for the presence of a method called promise and call it to get the actual promise.

I believe the presence of the promise method on promise objects is currently not documented - it is only documented that the promise method exists on deferred objects. So I filed a documentation bug for this at ​https://github.com/jquery/api.jquery.com/issues/375

[jaubourg]

I believe the presence of the promise method on promise objects is currently not documented - it is only documented that the promise method exists on deferred objects. So I filed a documentation bug for this at ​https://github.com/jquery/api.jquery.com/issues/375

Nice catch. I didn't realize that.