Possibly unhandled rejection in Angular 1.6

Question

I have a code with AngularJS:

service.doSomething()
  .then(function(result) {
      //do something with the result
  });

In AngularJS 1.5.

Answer 1

This information helped me to track down what (in my case) was creating the promise and not adding an error handler. I found it buried in the discussion of issue #2889 "Possibly unhandled rejection with Angular 1.5.9".

The gist, is, patch $q to cache a stack-trace on creating promises, such that it can be retrieved when the error is triggered.

To do it, insert this code to decorate $q somewhere near the top of your angular app:

// Decorate the $q service when app starts
app.decorator('$q', ["$delegate", function($delegate) {
  // Create a new promise object
  var promise = $delegate.when();

  // Access the `Promise` prototype (nonstandard, but works in Chrome)
  var proto = promise.__proto__;

  // Define a setter for `$$state` that creates a stacktrace 
  // (string) and assigns it as a property of the internal `$$state` object.
  Object.defineProperty(proto, '$$state', {
    enumerable: true,
    set: function(val) {
      val.stack = new Error().stack;
      this._$$state = val;
    },
    get: function() {
      return this._$$state;
    }
  });

  return $delegate;
}]);

Then search the angular code for the message "possibly unhandled rejection" and put a breakpoint on that line. When the breakpoint is reached, print out the value of toCheck.stack on the console, and you'll see something like this:

>> toCheck.stack
"set@http://localhost:8000/js/dual-site.js:18:19
Promise@http://localhost:8000/js/angular.js:17008:22
then@http://localhost:8000/js/angular.js:17016:20
catch@http://localhost:8000/js/angular.js:17026:14
SyncStrategy.prototype.send@http://localhost:8000/js/angular-state-machine.js:436:24
StateMachine/this.send@http://localhost:8000/js/angular-state-machine.js:235:16

The offending code is the frame calling angular's catch/then functions.