Async test patterns in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async test function in Deno.

Deno

Deno.test("example test", async () => { await [1]; });

Drag options to blanks, or click blank then click option'

APromise.resolve()

BsetTimeout()

Cconsole.log()

Dfetch()

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-Promise functions like setTimeout without wrapping in Promise.

Not marking the test function as async.

The test function must await a Promise to handle async code properly. Promise.resolve() returns a resolved Promise immediately, making it suitable here.

2fill in blank

medium

Complete the code to test an async function that returns a value.

Deno

Deno.test("async value test", async () => { const result = await getValue(); if (result !== [1]) throw new Error("Wrong value"); });

Drag options to blanks, or click blank then click option'

A"success"

B"done!"

C"done"

D"ok"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong expected string causes the test to fail.

Not awaiting the async function before checking the result.

The test expects the async function getValue() to return the string "success". The blank must match this expected value.

3fill in blank

hard

Fix the error in the async test by completing the code.

Deno

Deno.test("error test", async () => { try { await asyncFail(); } catch ([1]) { return; } throw new Error("Did not throw"); });

Drag options to blanks, or click blank then click option'

Aerror

Berr

Ce

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the catch parentheses empty causes syntax errors.

Using invalid variable names.

The catch block needs a variable name to capture the thrown error. "e" is a common and valid choice here.

4fill in blank

hard

Fill both blanks to create a test that waits for a promise and asserts its result.

Deno

Deno.test("promise test", async () => { const data = await fetchData(); if (data.status [1] 200) throw new Error("Status not OK"); if (data.body [2] "") throw new Error("Empty body"); });

Drag options to blanks, or click blank then click option'

A!==

B===

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality instead of inequality for status or inequality instead of equality for body causes wrong test logic.

Mixing up strict and loose equality operators.

The first condition checks if status is not equal to 200 using "!==". The second checks if body is equal to empty string using "===".

5fill in blank

hard

Fill all three blanks to create a test that mocks an async function and checks the call count.

Deno

const mockFn = [1](() => Promise.resolve(42)); Deno.test("mock call count", async () => { await mockFn(); if (mockFn.[2] !== [3]) throw new Error("Wrong call count"); });

Drag options to blanks, or click blank then click option'

Ajest.fn

Bcalls.length

C1

Dsinon.stub

Attempts:

3 left

💡 Hint

Common Mistakes

Using Jest syntax in Deno environment causes errors.

Checking call count incorrectly or forgetting to await the async call.

Using Sinon, sinon.stub creates the mock function. The call count is checked by mockFn.calls.length, expecting 1 call.

Practice

1. What is the main reason to use async and await in Deno test functions?

easy

A. To automatically retry failed tests

B. To make tests run faster by skipping delays

C. To avoid writing any assertions in tests

D. To handle code that involves waiting or delays smoothly

Solution

1. Step 1: Understand async/await purpose

   Async/await lets code wait for operations like fetching data or timers before continuing.

2. Step 2: Connect async/await to testing

   In tests, async/await ensures the test waits for the code under test to finish before checking results.

3. Final Answer:

   To handle code that involves waiting or delays smoothly -> Option D

4. Quick Check:

   Async tests wait properly = A [OK]

Hint: Async tests wait for promises to resolve before asserting [OK]

Common Mistakes:

• Thinking async makes tests faster
• Skipping await and causing false positives
• Confusing async with retry logic

2. Which of the following is the correct way to declare an async test in Deno?

easy

A. Deno.test(() => { await someAsyncFunc(); });

B. Deno.test(async () => { await someAsyncFunc(); });

C. async Deno.test(() => { someAsyncFunc(); });

D. Deno.test(async function someAsyncFunc() {});

Solution

1. Step 1: Check async function syntax in test

   The test function must be declared async to use await inside it.

2. Step 2: Validate each option

   Deno.test(async () => { await someAsyncFunc(); }); correctly declares an async arrow function with await inside. Deno.test(() => { await someAsyncFunc(); }); misses async keyword. async Deno.test(() => { someAsyncFunc(); }); has invalid syntax. Deno.test(async function someAsyncFunc() {}); passes an async function but without calling it properly.

3. Final Answer:

   Deno.test(async () => { await someAsyncFunc(); }); -> Option B

4. Quick Check:

   Async arrow function with await = A [OK]

Hint: Async test functions need async keyword before () => [OK]

Common Mistakes:

• Omitting async keyword before arrow function
• Trying to await outside async function
• Incorrect async function syntax in test

3. What will be the output when running this Deno test?

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";

Deno.test("delayed test", async () => {
  const result = await new Promise((resolve) => setTimeout(() => resolve(42), 100));
  assertEquals(result, 42);
});

medium

A. Test passes after 100ms delay

B. Test fails immediately with assertion error

C. SyntaxError due to incorrect async usage

D. Test times out and fails

Solution

1. Step 1: Analyze the async test code

   The test waits 100ms using a Promise with setTimeout, then resolves with 42.

2. Step 2: Check assertion correctness

   The test asserts that the resolved value is 42, which matches the expected value.

3. Final Answer:

   Test passes after 100ms delay -> Option A

4. Quick Check:

   Awaited promise resolves correctly = D [OK]

Hint: Await promises to ensure test waits for async results [OK]

Common Mistakes:

• Expecting immediate test pass without await
• Confusing assertion values
• Assuming syntax error without checking code

4. Identify the error in this Deno async test code:

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";

Deno.test("broken test", () => {
  const result = await Promise.resolve(10);
  assertEquals(result, 10);
});

medium

A. assertEquals is not imported correctly

B. Promise.resolve cannot be awaited

C. Missing async keyword before the test function

D. Test name is invalid

Solution

1. Step 1: Check async usage in test function

   The test function uses await but is not declared async, which causes a syntax error.

2. Step 2: Verify other parts

   assertEquals is imported correctly, Promise.resolve can be awaited, and test name is valid.

3. Final Answer:

   Missing async keyword before the test function -> Option C

4. Quick Check:

   Await requires async function = B [OK]

Hint: Add async before test function to use await inside [OK]

Common Mistakes:

• Forgetting async keyword with await
• Assuming Promise.resolve can't be awaited
• Misreading import statements

5. You want to test a function fetchData() that returns a promise resolving to an object. Which pattern correctly tests it with Deno's async test and handles errors gracefully?

hard

A. Deno.test("fetchData test", async () => {
try {
const data = await fetchData();
assert(data !== null);
} catch {
throw new Error("fetchData failed");
}
});

B. Deno.test("fetchData test", () => {
fetchData().then(data => assert(data !== null));
});

C. Deno.test("fetchData test", async () => {
const data = await fetchData();
assert(data !== null);
});

D. Deno.test("fetchData test", async () => {
fetchData().catch(() => {});
assert(true);
});

Solution

1. Step 1: Understand async test with error handling

   Using async/await with try/catch allows catching errors and failing the test properly.

2. Step 2: Evaluate each option

   Deno.test("fetchData test", async () => { const data = await fetchData(); assert(data !== null); }); lacks error handling, so test may silently fail. Deno.test("fetchData test", () => { fetchData().then(data => assert(data !== null)); }); is not async and may finish before promise resolves. Deno.test("fetchData test", async () => { try { const data = await fetchData(); assert(data !== null); } catch { throw new Error("fetchData failed"); } }); uses async/await with try/catch correctly. Deno.test("fetchData test", async () => { fetchData().catch(() => {}); assert(true); }); ignores errors and asserts true regardless.

3. Final Answer:

   Deno.test("fetchData test", async () => { try { const data = await fetchData(); assert(data !== null); } catch { throw new Error("fetchData failed"); } }); -> Option A

4. Quick Check:

   Async test with try/catch for errors = C [OK]

Hint: Use async/await with try/catch to catch errors in tests [OK]

Common Mistakes:

• Not awaiting promises in tests
• Ignoring errors causing false passes
• Using then() without returning promise in test