The test coverage percentage is decreased when update Angular to 13

Question

I've migrated the project from the older version into v13, and I found that the test coverage percentage has decreased.

So, I have looked at the test coverage results for both v12 and v13. Seems like the test did not cover the method if the method never got called with callThrough which is correct IMO.

Test Coverage Angular V12 Test Coverage Angular V13

Even that, I still need to know the root cause of this problem. Do anyone has clue or suggestion on this approach?

Note:

I've also tried to change the target in tsconfig.json to use es2015 and seems like I got the same result with test result in Angular v13

tsconfig.json (v12, change target to es2015)

{
  "compileOnSave": false,
  "compilerOptions": {
    "baseUrl": "./",
    "outDir": "./dist/out-tsc",
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "sourceMap": true,
    "declaration": false,
    "downlevelIteration": true,
    "experimentalDecorators": true,
    "moduleResolution": "node",
    "importHelpers": true,
    "target": "es2015",
    "module": "es2020",
    "lib": [
      "es2018",
      "dom"
    ]
  },
  "angularCompilerOptions": {
    "enableI18nLegacyMessageIdFormat": false,
    "strictInjectionParameters": true,
    "strictInputAccessModifiers": true,
    "strictTemplates": true
  }
}

Test Coverage Angular V12, change target to es2015

So I think it might be related to the reason of the coverage change.

---

Project's Code:

package.json

{
  "name": "test-angular-app",
  "version": "0.0.0",
  "scripts": {
    "ng": "ng",
    "start": "ng serve",
    "build": "ng build",
    "watch": "ng build --watch --configuration development",
    "test": "ng test",
    "test:coverage": "rm -rf coverage/ && ng test --code-coverage --watch=false --browsers=ChromeHeadless"
  },
  "private": true,
  "dependencies": {
    "@angular/animations": "~13.4.0",
    "@angular/common": "~13.4.0",
    "@angular/compiler": "~13.4.0",
    "@angular/core": "~13.4.0",
    "@angular/forms": "~13.4.0",
    "@angular/platform-browser": "~13.4.0",
    "@angular/platform-browser-dynamic": "~13.4.0",
    "@angular/router": "~13.4.0",
    "rxjs": "~6.6.0",
    "tslib": "^2.3.0",
    "zone.js": "~0.11.4"
  },
  "devDependencies": {
    "@angular-devkit/build-angular": "~13.3.11",
    "@angular/cli": "~13.3.11",
    "@angular/compiler-cli": "~13.4.0",
    "@types/jasmine": "~3.8.0",
    "@types/node": "^12.11.1",
    "jasmine-core": "~3.8.0",
    "karma": "~6.3.0",
    "karma-chrome-launcher": "~3.1.0",
    "karma-coverage": "~2.0.3",
    "karma-jasmine": "~4.0.0",
    "karma-jasmine-html-reporter": "~1.7.0",
    "typescript": "~4.4.2"
  }
}

tsconfig.json

{
  "compileOnSave": false,
  "compilerOptions": {
    "baseUrl": "./",
    "outDir": "./dist/out-tsc",
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "sourceMap": true,
    "declaration": false,
    "downlevelIteration": true,
    "experimentalDecorators": true,
    "moduleResolution": "node",
    "importHelpers": true,
    "target": "es5",
    "module": "es2020",
    "lib": [
      "es2018",
      "dom"
    ]
  },
  "angularCompilerOptions": {
    "enableI18nLegacyMessageIdFormat": false,
    "strictInjectionParameters": true,
    "strictInputAccessModifiers": true,
    "strictTemplates": true
  }
}

app.component.spec.ts

import { TestBed } from '@angular/core/testing';
import { RouterTestingModule } from '@angular/router/testing';
import { AppComponent } from './app.component';

describe('AppComponent', () => {
  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  });

  it('should create the app', () => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.componentInstance;
    expect(app).toBeTruthy();
  });

  it(`should have as title 'testAngularApp'`, () => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.componentInstance;
    expect(app.title).toEqual('testAngularApp');
  });

  it('should render title', () => {
    const fixture = TestBed.createComponent(AppComponent);
    fixture.detectChanges();
    const compiled = fixture.nativeElement as HTMLElement;
    expect(compiled.querySelector('.content span')?.textContent).toContain('testAngularApp app is running!');
  });
});

app.component.ts

import { Component } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent {
  title = 'testAngularApp';

  testFunc() {
    console.log(0);
  }
}