Async test patterns in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this async test using Deno's testing framework?

Consider this Deno test code snippet. What will be printed when the test runs?

Deno

import { assertEquals } from "https://deno.land/std@0.177.0/testing/asserts.ts";

deno.test("async addition test", async () => {
  const result = await Promise.resolve(5 + 3);
  console.log("Result is", result);
  assertEquals(result, 8);
});

A SyntaxError due to missing await

B Result is Promise { 8 }

C No output, test silently passes

D Result is 8

💡 Hint

Remember that await unwraps the Promise before logging.

The await keyword unwraps the Promise, so the console.log prints the resolved value 8. The test passes and prints 'Result is 8'.

❓ state_output

intermediate

What is the value of variable 'count' after this async test runs?

Look at this Deno test. What will be the final value of 'count' after the test completes?

Deno

let count = 0;

deno.test("increment count asynchronously", async () => {
  await new Promise((resolve) => setTimeout(resolve, 50));
  count += 1;
});

A undefined

B 1

C 0

D Throws ReferenceError

💡 Hint

The test waits before incrementing count.

The test awaits a 50ms delay, then increments count by 1. After the test finishes, count is 1.

🔧 Debug

advanced

Which option causes a test timeout error in Deno?

Given these test snippets, which one will cause Deno to report a timeout error?

A deno.test("timeout test", async () => { await new Promise(() => {}); });

B deno.test("error test", async () => { throw new Error("fail"); });

C deno.test("sync test", () => { return 42; });

D deno.test("quick test", async () => { await Promise.resolve(); });

💡 Hint

Look for a Promise that never resolves or rejects.

Option A awaits a Promise that never resolves, causing the test to hang and eventually timeout.

📝 Syntax

advanced

Which option contains a syntax error in async test declaration?

Identify the option with invalid syntax for an async test in Deno.

A deno.test("async test", async function() { await Promise.resolve(); });

B deno.test("async test", async () => { await Promise.resolve(); });

C deno.test(async "async test", () => { await Promise.resolve(); });

D deno.test("async test", () => { return Promise.resolve(); });

💡 Hint

Check the order of parameters and keywords.

Option C places the async keyword before the test name string, which is invalid syntax.

🧠 Conceptual

expert

Which option best explains why 'await' is necessary in Deno async tests?

Why must you use 'await' inside async Deno tests when calling asynchronous code?

A To pause the test until the Promise resolves, ensuring test assertions run after async code completes.

B To make the test run faster by skipping asynchronous operations.

C To automatically retry the test if the Promise rejects.

D To convert synchronous code into asynchronous code.

💡 Hint

Think about test timing and when assertions run.

Using await pauses the test until the async operation finishes, so assertions check the correct state.

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