Executing asynchronous methods in a Playwright implementation: A guide on constructor assignment

My current implementation uses a Page Object Model to handle browser specification. However, I encountered an issue where assigning a new context from the browser and then assigning it to the page does not work as expected due to the asynchronous nature of the method. This makes it impossible to create the object without initializing it with a method after its creation.

I am wondering if it is possible to assign an async value in the constructor or if there is a better pattern to follow?

export class AppPage {
  readonly browser: Browser;
  readonly page: Page;
  readonly context: BrowserContext;

  public currentPage: Page;


  constructor(browser: Browser) {
    this.browser = browser;
    // It is not feasible to assign in the constructor as these are async methods
    // this.context = await this.browser.newContext();
    // this.page = await context.newPage();
  }

  async initialize() {
    const context = await this.browser.newContext();
    const page = await context.newPage();
    // An extra method call is required for initialization
    this.currentPage = page;
  }

  async goto() {
    await this.currentPage.goto('https://playwright.dev');
  }
}

The current implementation requires initializing the page assignment.

test('test using context with page object model', async ({ browser }) => {
  const app = new AppPage(browser);
  await app.initialize();

  await app.goto();
});

Check out the live sample on Stackblitz.

Answer №1

To effectively handle this situation, one method is to have the constructor store a promise for the initialization process and then ensure that every other method awaits this promise before executing. Consequently, instead of having context and currentPage as properties of the instance, they become the resolved values of the aforementioned promise. Although they can be retrieved from the promise multiple times, it must always be done asynchronously.

export class AppPage {
  readonly browser: Browser;

  constructor(browser: Browser) {
    this.browser = browser;
    this.ready = this.initialize();
  }

  async initialize() {
    const context = await this.browser.newContext();
    const page = await context.newPage();
    return {context, page};
  }

  async goto() {
    const {page} = await this.ready;
    await page.goto('https://playwright.dev');
  }

  async close() {
    const {context} = await this.ready;
    await context.close();
  }
}

The test script no longer needs to include the call to initialize:

test('test using context with page object model', async ({ browser }) => {
  const app = new AppPage(browser);
  await app.goto();
});

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