E2E testing with Playwright in NextJS - Performance & Optimization

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Practice

1. What is the main purpose of E2E testing in a Next.js application using Playwright?

easy

A. To check only the backend API responses

B. To verify CSS styles and layout only

C. To test individual React components in isolation

D. To test the entire user experience from start to finish

Solution

1. Step 1: Understand E2E testing scope

   E2E (end-to-end) testing simulates real user actions across the whole app, not just parts.

2. Step 2: Identify Playwright's role

   Playwright automates browser actions to test full user flows, like clicking and filling forms.

3. Final Answer:

   To test the entire user experience from start to finish -> Option D

4. Quick Check:

   E2E testing = full user experience [OK]

Hint: E2E tests cover full user flows, not just parts [OK]

Common Mistakes:

• Confusing E2E with unit or integration tests
• Thinking E2E tests only check backend APIs
• Assuming E2E tests verify only UI styles

2. Which of the following is the correct syntax to launch a browser and open a new page in Playwright for Next.js E2E testing?

easy

A. const browser = await chromium.launch(); const page = await browser.newPage();

B. const browser = await chromium.launch(); const page = browser.newPage();

C. const browser = playwright.launchChromium(); const page = browser.newPage();

D. const browser = await playwright.chromium.launch(); const page = await browser.newPage();

Solution

1. Step 1: Recall Playwright import and usage

   Playwright's browser types like chromium are imported directly, so you use chromium.launch().

2. Step 2: Check async usage and method names

   launch() and newPage() are async and must be awaited; syntax must match Playwright docs.

3. Final Answer:

   const browser = await chromium.launch(); const page = await browser.newPage(); -> Option A

4. Quick Check:

   Use 'await chromium.launch()' and 'await browser.newPage()' [OK]

Hint: Use 'await chromium.launch()' and 'await browser.newPage()' [OK]

Common Mistakes:

• Using 'playwright.chromium.launch()' instead of 'chromium.launch()'
• Missing 'await' keyword for async calls
• Calling 'newPage()' directly on Playwright instead of browser

3. Given this Playwright test snippet for a Next.js app:

await page.goto('http://localhost:3000');
await page.click('button#submit');
const text = await page.textContent('div.result');
console.log(text);

What will be logged if the button triggers showing 'Success' inside div.result?

medium

A. Error: selector not found

B. undefined

C. 'Success'

D. '' (empty string)

Solution

1. Step 1: Understand the test flow

   The test navigates to the page, clicks the submit button, then reads text from 'div.result'.

2. Step 2: Analyze expected page behavior

   Clicking the button triggers showing 'Success' inside 'div.result', so textContent returns 'Success'.

3. Final Answer:

   'Success' -> Option C

4. Quick Check:

   textContent('div.result') = 'Success' after click [OK]

Hint: Click triggers text update; textContent reads updated text [OK]

Common Mistakes:

• Assuming textContent returns undefined if element exists
• Expecting error without checking if selector exists
• Thinking textContent returns empty string if no text

4. You wrote this Playwright test for a Next.js app:

await page.goto('http://localhost:3000');
await page.click('button.submit');
await expect(page.locator('div.result')).toHaveText('Success');

But the test fails with timeout error. What is the most likely cause?

medium

A. The page.goto URL is incorrect

B. The selector 'button.submit' does not match any element

C. The assertion syntax is invalid

D. The test forgot to import 'expect' from Playwright

Solution

1. Step 1: Analyze failure reason

   Timeout usually means the expected text never appeared in 'div.result'.

2. Step 2: Check selector correctness

   If 'button.submit' selector is wrong (e.g., class is 'submit-btn'), click never happens, so text never changes.

3. Final Answer:

   The selector 'button.submit' does not match any element -> Option B

4. Quick Check:

   Wrong selector = no click = assertion timeout [OK]

Hint: Check selectors carefully; wrong selector blocks actions [OK]

Common Mistakes:

• Assuming assertion syntax is wrong without checking selectors
• Ignoring that click may fail silently if selector is wrong
• Not verifying page URL correctness

5. You want to write a Playwright test for a Next.js app that fills a login form and verifies the welcome message. Which sequence of steps is correct?

hard

A. Go to login page, fill username and password, click submit, wait for welcome message, assert text

B. Click submit button, fill username and password, go to login page, assert welcome message

C. Fill username and password, assert welcome message, click submit, go to login page

D. Assert welcome message, go to login page, fill username and password, click submit

Solution

1. Step 1: Understand logical user flow

   User must first visit login page, then fill form fields, then submit.

2. Step 2: Verify post-submit behavior

   After submitting, wait for welcome message to appear, then assert its text.

3. Final Answer:

   Go to login page, fill username and password, click submit, wait for welcome message, assert text -> Option A

4. Quick Check:

   Follow user steps in order: visit, fill, submit, check [OK]

Hint: Follow real user steps: open page, fill, submit, then check [OK]

Common Mistakes:

• Trying to assert before submitting the form
• Clicking submit before filling fields
• Checking messages before navigating to page