Async test patterns in Deno - Mini Project: Build & Apply

Async Test Patterns in Deno

📖 Scenario: You are building a simple utility function that fetches user data asynchronously. You want to write tests for this function using Deno's built-in test runner to ensure it works correctly.

🎯 Goal: Write asynchronous tests in Deno to verify the behavior of an async function that fetches user data.

📋 What You'll Learn

Create an async function called fetchUser that returns a user object after a delay.

Create a test configuration variable called expectedUserId with the value 42.

Write an async test using Deno.test that calls fetchUser and checks the user ID matches expectedUserId.

Add a final test that checks the user name is exactly "Alice".

💡 Why This Matters

🌍 Real World

Async tests are essential when working with APIs, databases, or any code that waits for data. This project shows how to test such code in Deno.

💼 Career

Many backend and full-stack jobs require writing reliable async tests to ensure code correctness and prevent bugs in asynchronous workflows.

1

Create the async function fetchUser

Create an async function called fetchUser that returns an object { id: 42, name: "Alice" } after a 100ms delay using Promise and setTimeout.

Deno

# Your code here

Hint

Use new Promise and setTimeout inside the async function to simulate delay.

2

Add the test configuration variable expectedUserId

Add a constant variable called expectedUserId and set it to 42.

Deno

async function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 42, name: "Alice" });
        }, 100);
    });
}

# Your code here

Hint

Use const expectedUserId = 42; to create the variable.

3

Write an async test to check the user ID

Write a test using Deno.test named "fetchUser returns correct id" that is an async function. Inside it, call fetchUser() and use assertEquals to check the returned user's id equals expectedUserId. Import assertEquals from "https://deno.land/std@0.203.0/testing/asserts.ts".

Deno

async function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 42, name: "Alice" });
        }, 100);
    });
}

const expectedUserId = 42;

import { assertEquals } from "https://deno.land/std@0.203.0/testing/asserts.ts";

// Write async test for user id here
# Your code here

Hint

Use Deno.test with an async arrow function and await fetchUser().

4

Add a test to check the user name

Add another test using Deno.test named "fetchUser returns correct name" that is async. Inside it, call fetchUser() and use assertEquals to check the returned user's name is exactly "Alice".

Deno

async function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 42, name: "Alice" });
        }, 100);
    });
}

const expectedUserId = 42;

import { assertEquals } from "https://deno.land/std@0.203.0/testing/asserts.ts";

Deno.test("fetchUser returns correct id", async () => {
    const user = await fetchUser();
    assertEquals(user.id, expectedUserId);
});

// Add test for user name here
# Your code here

Hint

Write a second Deno.test with an async function and check user.name.

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