Mastering TypeScript 3 - Third Edition

Using spies as fakes

Another benefit of Jasmine spies is that they can act as fakes. In other words, instead of calling a real function, the call is temporarily overridden to call a fake function. These fake functions can also return values—which can be very useful for generating small mocking frameworks. Consider the following test:

class ClassToFake { 
    getValue() : number { 
        return 2; 
    } 
} 
describe("using fakes", () => { 
    it("calls fake instead of real function", () => { 
        let classToFake = new ClassToFake(); 
        spyOn(classToFake, 'getValue').and.callFake ( () => { 
            return 5; 
        }); 
        expect(classToFake.getValue()).toBe(5); 
    }); 
}); 

We start with a class named ClassToFake, which has a single function, getValue, which returns 2. Our test then creates an ...