Add ko.isPureComputed function and tests

spec/dependentObservableBehaviors.js

@@ -15,6 +15,11 @@ describe('Dependent Observable', function() {
         expect(ko.isComputed(instance)).toEqual(true);
     });
 
+    it('Should advertise that instances are not pure computed', function () {
+        var instance = ko.computed(function () { });
+        expect(ko.isPureComputed(instance)).toEqual(false);
+    });
+
     it('Should advertise that instances cannot have values written to them', function () {
         var instance = ko.computed(function () { });
         expect(ko.isWriteableObservable(instance)).toEqual(false);

spec/observableBehaviors.js

@@ -10,6 +10,11 @@ describe('Observable', function() {
         expect(ko.isObservable(instance)).toEqual(true);
     });
 
+    it('Should advertise that instances are not pure computed', function () {
+        var instance = ko.observable();
+        expect(ko.isPureComputed(instance)).toEqual(false);
+    });
+
     it('Should be able to write values to it', function () {
         var instance = new ko.observable();
         instance(123);

spec/pureComputedBehaviors.js

@@ -5,6 +5,11 @@ describe('Pure Computed', function() {
         expect(ko.isComputed(computed)).toEqual(true);
     });
 
+    it('Should advertise that instances are pure computed', function () {
+        var instance = ko.pureComputed(function () { });
+        expect(ko.isPureComputed(instance)).toEqual(true);
+    });
+
     it('Should require an evaluator function as constructor param', function () {
         expect(function () { ko.pureComputed(); }).toThrow();
     });

src/subscribables/dependentObservable.js

@@ -440,9 +440,15 @@ ko.isComputed = function (instance) {
     return ko.hasPrototype(instance, ko.computed);
 };
 
+ko.isPureComputed = function (instance) {
+    return ko.hasPrototype(instance, ko.computed) &&
+        instance.getVersion === pureComputedOverrides.getVersion;
+}
+
 ko.exportSymbol('computed', ko.computed);
 ko.exportSymbol('dependentObservable', ko.computed);    // export ko.dependentObservable for backwards compatibility (1.x)
 ko.exportSymbol('isComputed', ko.isComputed);
+ko.exportSymbol('isPureComputed', ko.isPureComputed);
 ko.exportSymbol('computed.fn', computedFn);
 ko.exportProperty(computedFn, 'peek', computedFn.peek);
 ko.exportProperty(computedFn, 'dispose', computedFn.dispose);