Javascript unit test-Give a fake value to the variables initialized by constructor :     I have a class 

Test.ts:-     
          class Test
            	{
            		public property1: string;
            		public property2: boolean;
            		constructor(property1, property2)
            		{
            			this.property1 = property1;//will get value while instantiation
            			this.property2 = property2;//will get value while instantiation
                        this.property3 = somevalue;//want to add fake value here 
            		}
        }
    As given above i want to give a fake value for property3 but property1 and and property2 i will get it from instantiation.How can i achieve this??