Testing Pipes

You can test pipes without the Angular testing utilities.

If you'd like to experiment with the application that this guide describes, run it in your browser or download and run it locally.

Testing the TitleCasePipe

A pipe class has one method, transform, that manipulates the input value into a transformed output value. The transform implementation rarely interacts with the DOM. Most pipes have no dependence on Angular other than the @Pipe metadata and an interface.

Consider a TitleCasePipe that capitalizes the first letter of each word. Here's an implementation with a regular expression.

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({name: 'titlecase', pure: true})
/** Transform to Title Case: uppercase the first letter of the words in a string. */
export class TitleCasePipe implements PipeTransform {
  transform(input: string): string {
    return input.length === 0 ? '' :
      input.replace(/\w\S*/g, (txt => txt[0].toUpperCase() + txt.slice(1).toLowerCase() ));

Anything that uses a regular expression is worth testing thoroughly. Use simple Jasmine to explore the expected cases and the edge cases.

describe('TitleCasePipe', () => {
  // This pipe is a pure, stateless function so no need for BeforeEach
  const pipe = new TitleCasePipe();

  it('transforms "abc" to "Abc"', () => {

  it('transforms "abc def" to "Abc Def"', () => {
    expect(pipe.transform('abc def')).toBe('Abc Def');

  // ... more tests ...

Writing DOM tests to support a pipe test

These are tests of the pipe in isolation. They can't tell if the TitleCasePipe is working properly as applied in the application components.

Consider adding component tests such as this one:

it('should convert hero name to Title Case', () => {
  // get the name's input and display elements from the DOM
  const hostElement: HTMLElement = fixture.nativeElement;
  const nameInput: HTMLInputElement = hostElement.querySelector('input')!;
  const nameDisplay: HTMLElement = hostElement.querySelector('span')!;

  // simulate user entering a new name into the input box
  nameInput.value = 'quick BROWN  fOx';

  // Dispatch a DOM event so that Angular learns of input value change.
  nameInput.dispatchEvent(new Event('input'));

  // Tell Angular to update the display binding through the title pipe

  expect(nameDisplay.textContent).toBe('Quick Brown  Fox');
Last reviewed on Mon Feb 28 2022

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