Angular 2: is not a function but it exist

You need to reference the argument name, not the argument type

 this.courses = courseService.getCourses();
                ^ lower case c

I think it's some kind of bug, because TypeScript recognize me the method but when I delete and type the method again getCourses() in the component, it says that the method is not found, then I go to the Service and start to delete blank lines, and the method Works. I'm currently using Angular 4