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Catch the JavaScript Error the Test Missed

advanced Python 15 min

A checkout test passes while the page throws an uncaught TypeError. Use WebDriver BiDi to make the test fail on JavaScript errors.

The Scenario

A recent release broke the analytics call that fires after a coupon is applied. The total still updates, so the test passes, but every real user’s browser logs an uncaught TypeError and the analytics team has lost a week of data.

Your Task

Enable WebDriver BiDi and make the test (and, through a fixture, every test) fail when the page throws an uncaught JavaScript error, with the error text in the failure message.

What Good Looks Like

  • BiDi enabled via options, not by hand-rolling a WebSocket.
  • A reusable fixture that collects errors and asserts after the test body.
  • The assertion message lists the errors so triage does not need a rerun.

Your task

A checkout test passes while the page throws an uncaught TypeError. Use WebDriver BiDi to make the test fail on JavaScript errors.

Problem Code
@pytest.fixture
def driver():
    d = webdriver.Chrome()
    yield d
    d.quit()

def test_apply_coupon(driver):
    driver.get("https://app.example.com/checkout")
    driver.find_element(By.NAME, "promoCode").send_keys("SAVE10")
    driver.find_element(By.ID, "apply").click()
    WebDriverWait(driver, 10).until(
        EC.text_to_be_present_in_element((By.CSS_SELECTOR, "[data-testid='order-total']"), "$89.00"))
    # Passes. Meanwhile the console shows:
    # Uncaught TypeError: Cannot read properties of undefined (reading 'analytics')

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Solution

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