[3.1.0] Assertion Failed: You attempted to access the length property

[boris-petrov]

We have the following code (inside prototype-extensions.js):

Ember.A.prototype.flatten = Array.prototype.flatten = function () {
    var result = Ember.A();
    this.forEach(function (element) {
        if (Ember.isArray(element)) {
            result.pushObjects(element.flatten());
        } else {
            result.pushObject(element);
        }
    });
    return result;
};

Which, when called somewhere in our code, results in the following assertion error:

Error: Assertion Failed: You attempted to access the `length` property (of <(unknown mixin):ember1475>).
Since Ember 3.1, this is usually fine as you no longer need to use `.get()`
to access computed properties. However, in this case, the object in question
is a special kind of Ember object (a proxy). Therefore, it is still necessary
to use `.get('length')` in this case.

If you encountered this error because of third-party code that you don't control,
there is more information at https://github.com/emberjs/ember.js/issues/16148, and
you can help us improve this error message by telling us more about what happened in
this situation.
    at new EmberError (error.js:40)
    at assert (index.js:146)
    at Object.get (core_object.js:57)
    at Object.isArray (utils.js:71)
    at prototype-extensions.js:28
    at Array.forEach (<anonymous>)
    at Array.Ember.A.flatten.Array.flatten (prototype-extensions.js:27)
    at prototype-extensions.js:29
    at Array.forEach (<anonymous>)
    at Array.Ember.A.flatten.Array.flatten (prototype-extensions.js:27)

Which blows up inside Ember itself, which I guess is a bug.

[rwjblue]

Switching from Ember.isArray to Array.isArray should resolve the issue I believe.

[boris-petrov]

@rwjblue - yes, that fixed the issue in our case, thanks! But I believe that is a bug nonetheless.

[rwjblue]

It should be possible to ensure Ember.isArray does not trigger this error when probing for length.