Async script execution in Selenium Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this async JavaScript execution in Selenium?

Consider the following Selenium Java code snippet that executes an asynchronous JavaScript script. What will be the value of result after execution?

Selenium Java

JavascriptExecutor js = (JavascriptExecutor) driver;
Object result = js.executeAsyncScript(
  "var callback = arguments[arguments.length - 1];" +
  "setTimeout(function() { callback('done'); }, 1000);"
);

A "undefined"

B "done"

C null

D TimeoutException

💡 Hint

Remember that executeAsyncScript waits for the callback to be called before returning.

The async script uses setTimeout to call the callback with 'done' after 1 second. Selenium waits for this callback and returns its argument as the result.

❓ assertion

intermediate

Which assertion correctly verifies async script execution result?

You executed an async script that returns the string "success". Which assertion correctly verifies this in a JUnit test?

Selenium Java

JavascriptExecutor js = (JavascriptExecutor) driver;
Object result = js.executeAsyncScript(
  "var callback = arguments[arguments.length - 1];" +
  "callback('success');"
);

A assertEquals("success", result);

B assertTrue(result == "success");

C assertSame("success", result);

D assertNotNull(result);

💡 Hint

Use the assertion that compares values for equality.

assertEquals compares the expected and actual values correctly. assertTrue with == compares references, which is unreliable for strings.

🔧 Debug

advanced

Why does this async script cause a TimeoutException?

This Selenium Java code throws a TimeoutException. Identify the cause.

Selenium Java

JavascriptExecutor js = (JavascriptExecutor) driver;
Object result = js.executeAsyncScript(
  "var callback = arguments[arguments.length - 1];" +
  "setTimeout(function() { console.log('done'); }, 5000);"
);

A The console.log causes the script to fail.

B The setTimeout delay is too long and causes a timeout error.

C The callback is never called, so Selenium times out waiting.

D executeAsyncScript does not support setTimeout.

💡 Hint

Check if the callback function is invoked in the script.

executeAsyncScript waits for the callback to be called. Here, callback is never called, so Selenium waits until timeout and throws TimeoutException.

❓ framework

advanced

How to set async script timeout in Selenium WebDriver Java?

Which code snippet correctly sets the async script timeout to 5 seconds?

A driver.setAsyncScriptTimeout(5);

B driver.manage().timeouts().pageLoadTimeout(5, TimeUnit.SECONDS);

C driver.manage().timeouts().implicitlyWait(5, TimeUnit.SECONDS);

D driver.manage().timeouts().setScriptTimeout(Duration.ofSeconds(5));

💡 Hint

Use the timeouts() method with setScriptTimeout and Duration.

setScriptTimeout sets the timeout for async scripts. The other options set different timeouts or are invalid.

🧠 Conceptual

expert

What happens if async script callback is called multiple times?

In Selenium Java, if an async script calls the callback function more than once, what is the behavior?

A Selenium returns the value from the first callback call and ignores subsequent calls.

B Selenium throws an exception on the second callback call.

C Selenium waits for all callback calls and returns the last value.

D Selenium concatenates all callback values and returns them.

💡 Hint

Think about how promises or callbacks usually behave in async APIs.

Selenium treats the first callback call as the completion signal and ignores any further calls to the callback.

Practice

1. What is the main purpose of using executeAsyncScript in Selenium Java?

easy

A. To execute JavaScript code only once when the browser starts.

B. To execute synchronous JavaScript code instantly without waiting.

C. To pause the entire Selenium test execution for a fixed time.

D. To run JavaScript code that waits for asynchronous tasks to complete before returning.

Solution

1. Step 1: Understand async JavaScript in Selenium

   Async scripts allow waiting for tasks like AJAX calls or timers to finish before continuing.

2. Step 2: Role of executeAsyncScript

   This method runs JavaScript that signals completion via a callback, so Selenium waits properly.

3. Final Answer:

   To run JavaScript code that waits for asynchronous tasks to complete before returning. -> Option D

4. Quick Check:

   Async script waits for tasks = A [OK]

Hint: Async scripts wait for callback before continuing [OK]

Common Mistakes:

• Confusing async with synchronous script execution
• Thinking it pauses test for fixed time
• Assuming it runs only once at browser start

2. Which of the following is the correct syntax to execute an async script in Selenium Java?

easy

A. driver.executeAsyncScript("var callback = arguments[0]; setTimeout(() => callback(10), 1000);");

B. driver.executeAsyncScript("return 5 + 5;");

C. driver.executeScript("var callback = arguments[0]; callback(10);");

D. driver.executeAsyncScript(() -> { return 10; });

Solution

1. Step 1: Identify async script syntax

   Async scripts require a callback function as the last argument to signal completion.

2. Step 2: Check each option

   driver.executeAsyncScript("var callback = arguments[0]; setTimeout(() => callback(10), 1000);"); correctly defines a callback and calls it after 1 second delay. driver.executeAsyncScript("return 5 + 5;"); lacks callback, C uses executeScript (sync), D uses wrong syntax.

3. Final Answer:

   driver.executeAsyncScript("var callback = arguments[0]; setTimeout(() => callback(10), 1000);"); -> Option A

4. Quick Check:

   Callback with setTimeout = A [OK]

Hint: Async scripts need callback as last argument [OK]

Common Mistakes:

• Not passing or calling the callback function
• Using executeScript instead of executeAsyncScript
• Wrong Java syntax for async scripts

3. What will be the output of the following Selenium Java code snippet?

Object result = driver.executeAsyncScript(
  "var callback = arguments[0]; setTimeout(() => callback('done'), 500);");
System.out.println(result);

medium

A. TimeoutException

B. null

C. done

D. undefined

Solution

1. Step 1: Understand the async script behavior

   The script waits 500ms then calls callback with 'done'. Selenium waits for this callback and returns its value.

2. Step 2: Check what is printed

   The returned value from executeAsyncScript is 'done', so System.out.println prints 'done'.

3. Final Answer:

   done -> Option C

4. Quick Check:

   Callback returns 'done' = C [OK]

Hint: Callback value is returned and printed [OK]

Common Mistakes:

• Expecting null before callback runs
• Confusing async script with synchronous
• Assuming timeout error without delay

4. Identify the error in this Selenium Java async script usage:

driver.executeAsyncScript("setTimeout(() => { return 'done'; }, 1000);");

medium

A. Missing callback argument and not calling it to signal completion.

B. Using arrow function inside executeAsyncScript is not allowed.

C. Timeout value should be zero for async scripts.

D. executeAsyncScript cannot run setTimeout functions.

Solution

1. Step 1: Check async script requirements

   Async scripts must accept a callback argument and call it to signal completion.

2. Step 2: Analyze the given code

   The code uses setTimeout but does not accept or call any callback, so Selenium never knows when to continue.

3. Final Answer:

   Missing callback argument and not calling it to signal completion. -> Option A

4. Quick Check:

   Callback must be called = D [OK]

Hint: Always pass and call callback in async scripts [OK]

Common Mistakes:

• Forgetting to pass callback as argument
• Returning value instead of calling callback
• Thinking arrow functions are disallowed

5. You want to test a web page that loads data dynamically after 3 seconds. Which async script approach correctly waits for the data load before continuing?

driver.executeAsyncScript(
  "var callback = arguments[0];
   var checkData = () => {
     if(document.querySelector('#data').innerText !== '') {
       callback('loaded');
     } else {
       setTimeout(checkData, 500);
     }
   };
   checkData();
");

hard

A. This script will timeout immediately because callback is never called.

B. This script correctly polls for data and calls callback when ready.

C. This script blocks the browser and never calls callback.

D. This script should use executeScript instead of executeAsyncScript.

Solution

1. Step 1: Understand polling in async scripts

   The script repeatedly checks if '#data' element has text, calling callback only when data is loaded.

2. Step 2: Verify callback usage and waiting

   Callback is called only after data is ready, so Selenium waits properly for dynamic content.

3. Final Answer:

   This script correctly polls for data and calls callback when ready. -> Option B

4. Quick Check:

   Polling with callback = B [OK]

Hint: Poll and call callback when ready to wait dynamically [OK]

Common Mistakes:

• Not calling callback after data loads
• Using executeScript which does not wait
• Blocking browser with infinite loops