Promise returns a locked object inside lightning component when locker service is enabled

Question

I am trying to use javascript promises to perform multi-step asynchronous operations in lightning components. Unfortuantely I am stuck with the problem where the promise behaves incorrectly within $A.getCallback context. Here is a simplified app and js controller to demostrate the problem.

I have a simple app definition with a string attribute that defaults to "superman".

<aura:application >
    <aura:handler name="init" value="{!this}" action="{!c.init}"/>

    <aura:attribute name="myatt" type="String" required="false" default="superman" access="private"  />
</aura:application>

Associated controller.js code uses Promises:

({
init : function( cmp, evt, h) {

    Promise.resolve(1).then($A.getCallback(function(val) {
            console.log('val %o', val);
            return new Promise($A.getCallback(function(done, err){
                setTimeout($A.getCallback(function(){
                    // console.log('now!');
                    done('yo ' + cmp.get('v.myatt'));
                }), 2000);
            }));
            // return 'yo ' + cmp.get('v.myatt');
    })).then($A.getCallback(function(res){
        console.log('Result: %o', res);
    }));
}
})

When locker service is disabled, the console log outputs an expected output (notice that Result appears after 2 seconds):

​val 1
Result: "yo superman"

However, when Locker Service is enabled, the Result becomes javascript Object with no useful attributes that is returned immediatly:

val 1
Result: Object
(When object is expanded)
-> $then$: function ()
-> __proto__: Object

I would really like to utilise the power of promises as it's a nightmare to use callbacks all the time. Any ideas?

Original question on sf dev forum, posting here as received no answer so far: https://developer.salesforce.com/forums/ForumsMain?id=9060G000000XebSQAS

Answer 1

This is fixed internally. I've tried out your repro myself and see it working on my local. The fix is scheduled for the next major release - Spring '17.

Edit (Nov 15): We've decided to backport the fix to an upcoming patch. You can expect the fix to hit production the first couple days of next month (December).