Added example of mocking and spying on constructor mocks

Author: NielsSteensma

jest.config.js

@@ -0,0 +1,4 @@
+module.exports = {
+    preset: 'ts-jest',
+    testEnvironment: 'node',
+  };

package-lock.json

@@ -24,6 +24,7 @@
     "sequelize": "^5.8.7",
     "sequelize-typescript": "^1.0.0-alpha.9",
     "ts-jest": "^24.0.2",
-    "ts-node": "^8.0.3"
+    "ts-node": "^8.0.3",
+    "typescript": "^3.3.4000"
   }
 }

package.json

@@ -0,0 +1,10 @@
+import Cat from "../models/cat.model";
+
+export class CatService{
+    public createANewCatByConstructor(name: string, age: number){
+        const cat = new Cat({
+            name: name,
+            age: age
+        })
+    }
+}

src/services/cat-service.ts

@@ -0,0 +1,27 @@
+import Cat from "../src/models/cat.model";
+import { CatService } from "../src/services/cat-service";
+
+// The jest.mock method mocks the whole cat model. 
+// You will always need to mock the whole model, otherwise you get errors on the initialization of the model.
+jest.mock("../src/models/cat.model");
+
+beforeEach(() => {
+    jest.resetAllMocks();
+});
+
+// This test shows how the constructor can be mocked, and how to spy on passed parameters.
+describe("given a new instance of the cat model", () => {
+    it("sets garfield as the name of the cat", () => {
+        new CatService().createANewCatByConstructor("garfield", 12);
+        expect(Cat).toBeCalledWith(expect.objectContaining({
+            name: "garfield",
+        }));
+    });
+
+    it("sets 12 as the age of the cat", () => {
+        new CatService().createANewCatByConstructor("garfield", 12);
+        expect(Cat).toBeCalledWith(expect.objectContaining({
+            age: 12,
+        }));
+    });
+});