What are the "spec.ts" files generated by Angular CLI for?

I am using Angular CLI to generate and serve projects. It seems to work well – though for my little learning projects, it produces more than I need – but that's to be expected.

I've noticed that it generates spec.ts for each Angular element in a project (Component, Service, Pipe, etc). I've searched around but have not found an explanation of what these files are for.

Are these build files which are normally hidden when using tsc? I wondered because I wanted to change the name of a poorly named Component I'd created and discovered that the name was also referenced in these spec.ts files.

import {
  beforeEach,
  beforeEachProviders,
  describe,
  expect,
  it,
  inject,
} from '@angular/core/testing';
import { ComponentFixture, TestComponentBuilder } from '@angular/compiler/testing';
import { Component } from '@angular/core';
import { By } from '@angular/platform-browser';
import { PovLevelComponent } from './pov-level.component';

describe('Component: PovLevel', () => {
  let builder: TestComponentBuilder;

  beforeEachProviders(() => [PovLevelComponent]);
  beforeEach(inject([TestComponentBuilder], function (tcb: TestComponentBuilder) {
    builder = tcb;
  }));

  it('should inject the component', inject([PovLevelComponent],
      (component: PovLevelComponent) => {
    expect(component).toBeTruthy();
  }));

  it('should create the component', inject([], () => {
    return builder.createAsync(PovLevelComponentTestController)
      .then((fixture: ComponentFixture) => {
        let query = fixture.debugElement.query(By.directive(PovLevelComponent));
        expect(query).toBeTruthy();
        expect(query.componentInstance).toBeTruthy();
      });
  }));
});

@Component({
  selector: 'test',
  template: `
    
  `,
  directives: [PovLevelComponent]
})
class PovLevelComponentTestController {
}

The spec files are unit tests for your source files. The convention for Angular applications is to have a .spec.ts file for each .ts file. They are run using the Jasmine javascript test framework through the Karma test runner (https://karma-runner.github.io/) when you use the ng test command.

You can use this for some further reading:

https://angular.io/guide/testing

Stack Overflow Q#37502809, score: 376