In the almost 20 years since Kent Beck’s Test Driven Development, software developers have taken more and more responsibility for writing automated tests for their applications. Whether you practice full TDD or write tests after the fact, having a robust automated test suite for your project is incredibly freeing. As Martin Fowler said, “Legacy code is untested code.” It’s risky to change untested code, especially if it was developed by someone else (or even by you months ago). People tend to work around untested code and avoid making changes to it. Having a robust suite of automated tests gives you the confidence to refactor your code and improve it, even while adding new features.

Angular is built from the ground up to be testable. Angular makes it easy to architect your application as a collection of loosely coupled components, and the TypeScript-based dependency injection makes it easy to mock dependencies. Unit tests in Angular can exercise your entire component, including the template and DOM. In the coming chapters, you’ll learn how to write effective unit tests for Angular.

In chapter 2, we’ll start by covering the basics of writing unit tests with Jasmine. If you’ve written unit tests in JavaScript and are familiar with ES6 classes, most of this chapter will be familiar to you.

Chapter 3 introduces the fundamental building blocks of Angular applications—components—and how to test them. If you're familiar with JavaScript testing but not testing Angular applications, this is a good place to start.

We discuss testing directives in chapter 4. Directives in Angular are different from AngularJS directives, and we explain those differences. Testing directives is a good example of how to write DOM-focused unit tests in Angular.

In Angular, pipes are simple functions that transform data in templates. Chapter 5 covers how to test them and, along the way, introduces some functional programming concepts.

Similar to AngularJS, you share data in Angular throughout your app via singleton objects called services. Chapter 6 explores testing services, including using dependency injection to provide mocks and handling asynchronous behavior with promises and RxJS observables.

Finally, the Angular router allows your application to switch between different views. Chapter 7 dives deep into using the router and testing its configuration.