Ngx Page Object Model

ngx-page-object-model — Angular component testing

This project uses the ngx-page-object-model library. Every Angular component unit test must follow the Page Object Model (POM) pattern described here. The library ships its own documentation, which you should consult on demand — see the routing table below.

The core principle

A component's public API is its template + inputs + outputs. Methods, properties, signals, computed values, and internal state are implementation detail. A test drives the component the way a real user would: through the DOM. You assert on what the user would see, not on what the class holds.

This is the same reasoning behind testing a service through its public methods, not its private ones. Tests that couple to internals break on harmless refactors and silently miss real bugs in the template — because the template was never exercised.

Forbidden patterns

The following are unconditional rules, regardless of access modifier (private, protected, or public):

• Never call a method on the component instance:
  • component.someMethod()
  • page.fixture.componentInstance.someMethod()
  • page.component.someMethod()
• Never read or write a property on the component instance:
  • component.someSignal()
  • component.formGroup
  • page.fixture.componentInstance.someProp = 'x'
• Never use type casts to bypass access modifiers:
  • (component as any).someMethod()
  • (page.fixture.componentInstance as any)['someProperty']
  • (page.component as any).whatever

If you find yourself reaching for as any to access a component member, stop. That cast is the signal that you are about to bypass the public API. The right move is to step back and ask: what DOM interaction would a real user perform to trigger this code path, and what rendered output would they see as a result? There is essentially always a way through the template — and for the one case where there isn't (extracting a FormControl bound via [formControl]), the library provides a dedicated DOM-based extractor; see "Working with forms" below.

Why so absolute? Because every shortcut here is a test that doesn't test the template. Setting a public modifier "just for testing" is even worse — it advertises that the contract can be reached around, and the next developer will do exactly that.

Canonical test structure

Every component test in this project should look roughly like this:

import { TestBed } from '@angular/core/testing';
import { DebugHtmlElement, PageObjectModel } from 'ngx-page-object-model';
import { MyComponent } from './my.component';

describe(MyComponent.name, () => {
  class Page extends PageObjectModel<MyComponent> {
    // element-access methods — one per DOM element the tests need
    submitButton(): DebugHtmlElement<HTMLButtonElement> {
      return this.getDebugElementByTestId('submit-button');
    }
    statusText(): DebugHtmlElement<HTMLSpanElement> {
      return this.getDebugElementByTestId('status-text');
    }

    // action methods — what a user would do
    clickSubmit(): void {
      this.submitButton().nativeElement.click();
    }
  }

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [MyComponent],
    }).compileComponents();
  });

  // setup() returns everything tests need; nothing mutable is shared at module scope
  const setup = (config: { initialName?: string } = {}) => {
    const page = new Page(TestBed.createComponent(MyComponent));
    page.fixture.componentRef.setInput('name', config.initialName ?? 'world');
    return { page };
  };

  it('greets the user when submit is clicked', async () => {
    const { page } = setup();
    page.clickSubmit();
    await page.fixture.whenStable();

    expect(page.statusText().nativeElement.textContent).toContain('Hello, world');
  });
});

Five non-obvious things this template encodes:

1. Page is a class, not a bag of helpers. Element-access methods return DebugHtmlElement<HTMLSomething> so call sites get type-safe nativeElement.
2. Use getDebugElementByTestId as the default lookup. CSS selectors are a fallback. The library throws descriptive errors (Element with selector "..." was not found.) instead of returning null — no null-checking required.
3. setup() is a function, not beforeEach. Only TestBed configuration goes in beforeEach; all mutable per-test state is created in setup() and returned. This prevents order-dependent flakiness from accidentally leaked state. Immutable shared fixtures (constant input data, sample objects) still belong inside the describe block, never at module scope, which pollutes the file's namespace.
4. Inputs go through componentRef.setInput(), never by writing to a class property.
5. Assertions are on the DOM (text content, attribute presence, element existence), never on signals or component properties.

When to read which doc

The full library documentation ships inside the package. Read only the doc(s) relevant to the current task — don't pre-load all of them.

Base path: node_modules/ngx-page-object-model/docs/ (If you're working inside this library's own repo, the same files live at website/docs/.)

When in doubt, start with reference/page-object-methods.md — it's the fastest lookup — and branch out from there.

Working with forms (the most common pitfall)

When a test needs a FormControl or FormGroup that lives inside the component, the temptation is to write (component as any).formGroup. Don't. The library provides DOM-based extractors specifically for this:

import { getFormControlOfDebugElement, getFormGroupOfDebugElement } from 'ngx-page-object-model';

const input = page.someInputElement();                // a DebugHtmlElement
const control = getFormControlOfDebugElement(input);  // the FormControl bound to [formControl]

const formEl = page.myForm();                         // a DebugHtmlElement<HTMLFormElement>
const group = getFormGroupOfDebugElement(formEl);     // the FormGroup bound to [formGroup]

This extracts the control through the DOM, which means the test simultaneously verifies that the binding works. Reaching into the class would only verify that the constructor ran. For typed variants and the getFormGroupOfDebugElement<{ field: FormControl<string> }>(...) form, read advanced/testing-form-controls.md.

Verifying inputs passed to a child component

To assert what the component under test passes down to a child, prefer the rendered DOM. When the value isn't observable there (an icon name, a color, a flag with no visible text), read it from the child's public input via the child's debug element — that input is the child's public API, not the component-under-test's internals:

const emblem = page.getDebugElementByDirective(StatusEmblemComponent).componentInstance;
expect(emblem.color()).toBe('warn'); // input bound by the parent

This stays forbidden for the component under test's own instance — see "Forbidden patterns".

Other utilities the library exports

• typeInElement(element, value) — simulates user typing by setting .value and dispatching an input event. Use this instead of assigning to element.value directly; the latter doesn't notify Angular's form machinery.
• tickAsync() — Promise-based replacement for fakeAsync's tick(), intended for zoneless apps. Use in async tests where you'd otherwise reach for fakeAsync.

Element-absence and optional assert

Library lookup methods (getDebugElementByTestId, getDebugElementByCss, getDebugElementByDirective) accept an optional assert parameter that defaults to true — meaning they throw a descriptive error if the element is missing. When writing a Page Object method for an element that may legitimately be absent, forward assert through, don't hardcode false:

// Good — assertion still fires by default; tests opt out when they expect absence.
optionalBanner(assert = true): DebugHtmlElement<HTMLDivElement> {
  return this.getDebugElementByTestId('banner', assert);
}

// Then in the one test that expects it to be missing:
expect(page.optionalBanner(false)).toBeFalsy();

Hardcoding assert = false permanently disables the descriptive error for every caller and turns a clear "element not found" message into a vague "cannot read properties of undefined" downstream. See getting-started/testing-absent-elements.md for the full pattern.

When you slip up

If you're mid-test and notice yourself typing (component as any) or componentInstance.something, the skill has failed to land. Re-read "Forbidden patterns" and ask the user-action question: what would a real user click, type, or see to make this happen? If the genuine answer is "there is no user-visible interaction" — e.g. you're testing a ControlValueAccessor and need the bound FormControl — that's precisely what getFormControlOfDebugElement is for. There is essentially always a way through the public API; reaching past it is almost always a sign the test is about to lie.

Source: FrancescoBorzi/ngx-page-object-model — distributed by TomeVault.