canReflectPromise(promise)

If promise is an instance of a Promise type (either provided natively by the platform or by a polyfill library), the prototype of the Promise type is decorated with can-symbol symbols for @@can.onKeyValue, @@can.offKeyValue, @@can.getKeyValue, @@can.onValue, and @@can.offValue, which allow it to be used like an observable Map-like in reflection.

Then promise and all future instances of promise.constructor will be initialized on first access to have the following keys available through @@can.getKeyValue:

• "state" -- one of "pending", "resolved", or "rejected"
• "isPending" -- true if the promise neither resolved nor rejected, false otherwise.
• "isResolved" -- true if the promise is resolved, false otherwise.
• "isRejected" -- true if the promise is rejected, false otherwise.
• "value" -- the resolved value of the promise if resolved, otherwise undefined.
• "reason" -- the rejected value of the promise if rejected, otherwise undefined.

Note that, due to native Promises' lack of inspection, it is impossible to tell whether a Promise is resolved or rejected without using .then(). If the Promise is Promises/A+ compliant, then at the time the Promise is initialized by can-reflect-promise, isPending will always be true, even if the Promise has already resolved, and will remain so during synchronous execution. On the next tick, the state will update to resolved/rejected if the Promise has already updated its state. The best strategy for ensuring a proper read of a resolved value or rejected reason is to listen for the change in state or value with @@can.onKeyValue immediately after creating a Promise.

In the cases where promise is a plain object (e.g. if it is a jQuery.Deferred), the symbols will be applied to promise itself, not to the proto.

var p = new Promise(function(resolve) {
    setTimeout(resolve.bind(null, "a"), 10);
});

canReflectPromise(p);

canReflect.getKeyValue(p, "isPending"); // -> true

setTimeout(function() {
    canReflect.getKeyValue(p, "isResolved"); // -> true
    canReflect.getKeyValue(p, "value"); // -> "a"
}, 20);