Testing Pipes in Angular With Jasmin And Karma

Testing your Angular code is a pain, but Jasmin and Karma are here to help. In this article, we'll create a simple pipe and then write tests for it.

What is a Pipe?

Angular Pipes are functions that allow users to transform data from one format to another. A Pipe might be responsible for formatting dates, parsing JSON, or converting currencies.

A pipe is a filtering mechanism used to transform data in an Angular application. Pipes are used to format data for display, to perform calculations, and to change the data in some way.

Pipes are easy to use in Angular. To use a pipe, you simply include the name of the pipe within double curly braces {{ }} and provide the data you want to transform as an input value. For example, if you want to format a date using the pipe Date, you would write{{date | Date}}.

There are many built-in pipes available in Angular, and you can also create your own custom pipes.

Testing Pipes in Angular with Jasmin and Karma

Pipes in Angular can be tested using a variety of methods. However, two of the most popular methods are to use Jasmine and Karma. In this article, we'll take a look at how to test pipes in Angular using Jasmine and Karma.

When testing pipes in Angular, there are a few things to keep in mind. First, it's important to remember that pipes can be used within your template HTML or within your component TypeScript code. For example, you may have a pipe that formats a date value for display in the template. In this case, you would need to test both the input and output values of the pipe to ensure that it is working correctly.

Another thing to keep in mind when testing pipes is that some pipes may require additional dependencies to be injected into the test bed. For example, if you are testing an async pipe, you will need to inject an additional provider such as the NgZone into your test bed. This is because async pipes rely on NgZone to trigger change detection when their input values emit new values.

Finally, it's also worth noting that some pipes may have side-effects that need to be handled when writing tests. For example, if a pipe updates the DOM (e.g. by setting innerHTML), you will need to run your tests inside of a fakeAsync zone so that any DOM updates are batched together and can be checked after the zone has been flushed.

Testing pipes in Angular with Jasmine and Karma is a relatively simple process. First, we need to create a new pipe:

Next, we need to create the file src/app/new-pipe.pipe.ts and add the following code:

import { Pipe, PipeTransform } from '@angular/core'; 
@Pipe({ name: 'newPipe' }) 
export class NewPipePipe implements PipeTransform {
  transform(input: string): string {
    return input.length === 0 ? '' :
      input.replace(/\w\S*/g, (txt => txt[0].toUpperCase() + txt.slice(1).toLowerCase() ));
  }
}

Now that we have our new pipe created, we can write our tests. We'll start by creating the spec file src/app/new-pipe.pipe.spec.ts.

In this file, we will import our newly created pipe:

import { NewPipePipe } from './new-pipe.pipe';
 
describe('NewPipePipe', () => {
  const pipe = new NewPipePipe();
 
  it('should be defined', () => { 
  expect(new NewPipePipe()).toBeDefined(); 
  });
 
  it('transforms "abc" to "Abc"', () => {
    expect(pipe.transform('abc')).toBe('Abc');
  });
 
  it('transforms "abc def" to "Abc Def"', () => {
    expect(pipe.transform('abc def')).toBe('Abc Def');
  });
 
});
 

As you can see, testing a pipe is just like testing anything else in Angular - we use Jasmine to write our tests and then run them using Karma. Overall, testing pipes in Angular with Jasmine and Karma is not too difficult. By following the steps outlined in this article, you should be able to get everything up and running without any problems.