Spying on properties
Properties are more complicated than functions. In Jasmine, you can do anything with a property spy that you can do with a function spy, but you may need to use different syntax.
Use spyOnProperty
to create either a getter or setter spy.
it("allows you to create spies for either type", function() {
spyOnProperty(someObject, "myValue", "get").and.returnValue(30);
spyOnProperty(someObject, "myValue", "set").and.callThrough();
});
Changing the value of an existing spy is more difficult than with a function, because you cannot refer to a property without calling its getter
method. To get around this, you can save a reference to the spy for later changes.
beforeEach(function() {
this.propertySpy = spyOnProperty(someObject, "myValue", "get").and.returnValue(1);
});
it("lets you change the spy strategy later", function() {
this.propertySpy.and.returnValue(3);
expect(someObject.myValue).toEqual(3);
});
If saving a reference to the spy is awkward, you can also access it from anywhere in your test using
Object.getOwnPropertyDescriptor
.
beforeEach(function() {
spyOnProperty(someObject, "myValue", "get").and.returnValue(1);
});
it("lets you change the spy strategy later", function() {
Object.getOwnPropertyDescriptor(someObject, "myValue").get.and.returnValue(3);
expect(someObject.myValue).toEqual(3);
});
You can create a spy object with several properties on it quickly by passing an array or hash of properties as
a third argument to createSpyObj
. In this case you won’t have a reference to the created spies, so if you
need to change their spy strategies later, you will have to use the Object.getOwnPropertyDescriptor
approach.
it("creates a spy object with properties", function() {
let obj = createSpyObj("myObject", {}, { x: 3, y: 4 });
expect(obj.x).toEqual(3);
Object.getOwnPropertyDescriptor(obj, "x").get.and.returnValue(7);
expect(obj.x).toEqual(7);
});