Testing pipes

As we learned in Chapter 4, Enhance Components with Pipes and Directives, a pipe is a TypeScript class that implements the PipeTransform interface. It exposes a transform method that is usually synchronous, which means that it is straightforward to test. Let's create a simple pipe that converts a comma-separated string into a list:

list.pipe.ts

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 really simple. The only thing that we need to do is to instantiate an object of ListPipe and verify the outcome of the transform method with some mock data:

it('should return an array', () =>&...