Testing pipes

As we learned in Chapter 5, Enrich Applications Using Pipes and Directives, a pipe is a TypeScript class that implements the PipeTransform interface. It exposes a transform method, which is usually synchronous, which means it is straightforward to test. The list.pipe.ts file contains a pipe that converts a comma-separated string into a list:

import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
  name: 'list'
})
export class ListPipe implements PipeTransform {
  transform(value: string): string[] {
    return value.split(',');
  }
}

Writing a test for it is simple. The only thing that we need to do is to instantiate an instance of ListPipe and verify the outcome of the transform method with some mock data:

import { ListPipe } from './list.pipe';
describe('ListPipe', () => {
  it('create an instance', () => {
    const pipe = new ListPipe();
    expect(pipe).toBeTruthy...