propagators no longer run unless all their input cells have values

propagator.js

@@ -19,16 +19,23 @@ function Propagator(func,input,output) {
       this.output = output;
 
       function applyFuncToInputs() {
-          var inputValues = self.input.map((cell) => cell.getContents())
+          var inputs = self.input.map((cell) => cell.getContents())
+          var output = {"value" : undefined, "usable" : false}
 
-          return func.apply(null,inputValues);
+          inputs.fullyPopulated = inputs.every((value) => typeof value !== "undefined")
+
+          if(inputs.fullyPopulated) {
+              output.value = func.apply(null,inputs);
+              output.usable = true;
+          }
 
+          return output;
       }
 
       this.propagate = function propagate() {
 
           var outputValues = applyFuncToInputs();
-          self.output.update(outputValues, self)
+          if (outputValues.usable) self.output.update(outputValues.value, self)
       }
 
       this.input.forEach( (cell) => cell.addListener(self.propagate) ) 

test/test-propagator.js

@@ -48,6 +48,25 @@ describe("The propagator constructor",function() {
         expect(mockOutputCell.getContents()).to.equal(addValuesTogether(10,1,1))
     }) 
 
+    it("should not update an output cell unless all of its input cells have values",function(){
+        var mockInputCell1 = Propagator.makeCell()
+        var mockInputCell2 = Propagator.makeCell()
+        var mockInputCell3 = Propagator.makeCell()
+        var mockOutputCell = Propagator.makeCell()
+        var addValuesTogether = ((x,y,z) => x+y+z)
+
+        var testPropagator = new Propagator(addValuesTogether,[mockInputCell1,mockInputCell2,mockInputCell3],mockOutputCell);
+
+        mockInputCell1.update(1)
+        expect(mockOutputCell.getContents()).to.be.undefined;
+
+        mockInputCell2.update(10)
+        expect(mockOutputCell.getContents()).to.be.undefined;
+
+        mockInputCell3.update(100)
+        expect(mockOutputCell.getContents()).to.equal(addValuesTogether(1,10,100))
+    })
+
     it("should throw an error when the constructor is called with no arguments",function(){
 
         expect( () => new Propagator() ).to.throw(Error)