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Handling Asynchronous Code

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In this section, we will explore how to handle asynchronous code in Playwright using TypeScript. Asynchronous programming is crucial in modern web development, especially when dealing with operations like network requests, file I/O, and timers. Playwright, being a browser automation tool, heavily relies on asynchronous operations to interact with web pages.

Key Concepts

  1. Asynchronous Programming:

    • Allows your program to perform other operations while waiting for an asynchronous task to complete.
    • Commonly used with operations that take time to complete, such as fetching data from a server.

  2. Promises:

    • A promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value.
    • Promises have three states: pending, fulfilled, and rejected.

  3. Async/Await:

    • Syntactic sugar built on top of promises, making asynchronous code look and behave more like synchronous code.
    • async functions always return a promise.
    • await pauses the execution of the async function, waiting for the promise to resolve.

Practical Example

Let's look at a simple example of using async/await in a Playwright script written in TypeScript.

import { chromium, Browser, Page } from 'playwright';

async function run() {
  // Launch a new browser instance
  const browser: Browser = await chromium.launch();
  // Create a new page
  const page: Page = await browser.newPage();
  
  // Navigate to a website
  await page.goto('https://example.com');
  
  // Wait for a specific element to be visible
  await page.waitForSelector('h1');
  
  // Extract the text content of the element
  const headingText: string = await page.textContent('h1');
  console.log(`Heading text: ${headingText}`);
  
  // Close the browser
  await browser.close();
}

run().catch(error => console.error(error));

Explanation

  • Importing Playwright: We import necessary modules from Playwright to interact with the browser.
  • Async Function: The run function is declared as async, allowing us to use await within it.
  • Launching Browser: We use await to wait for the browser to launch before proceeding.
  • Navigating and Interacting: We navigate to a URL and wait for an element to appear using await.
  • Handling Errors: We catch any errors that occur during the execution of the run function.

Exercises

Exercise 1: Fetch and Log Page Title

Write a Playwright script that navigates to a webpage and logs the page title.

Solution:

import { chromium } from 'playwright';

async function logPageTitle() {
  const browser = await chromium.launch();
  const page = await browser.newPage();
  await page.goto('https://example.com');
  
  const title = await page.title();
  console.log(`Page title: ${title}`);
  
  await browser.close();
}

logPageTitle().catch(error => console.error(error));

Exercise 2: Wait for Element and Log Text

Modify the previous script to wait for a paragraph element and log its text content.

Solution:

import { chromium } from 'playwright';

async function logParagraphText() {
  const browser = await chromium.launch();
  const page = await browser.newPage();
  await page.goto('https://example.com');
  
  await page.waitForSelector('p');
  const paragraphText = await page.textContent('p');
  console.log(`Paragraph text: ${paragraphText}`);
  
  await browser.close();
}

logParagraphText().catch(error => console.error(error));

Common Mistakes and Tips

  • Forgetting await: Ensure you use await with asynchronous operations to avoid unexpected behavior.
  • Error Handling: Always handle errors using try/catch blocks or .catch() to prevent unhandled promise rejections.
  • Understanding Promise States: Be aware of the promise states to better manage asynchronous flows.

Conclusion

In this section, we covered the basics of handling asynchronous code in Playwright using TypeScript. We explored promises, async/await syntax, and provided practical examples and exercises to solidify your understanding. Mastering asynchronous programming is essential for writing efficient and effective Playwright scripts. In the next module, we will delve into advanced Playwright features, building on the foundation we've established here.

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