Async test patterns in Deno

Introduction

Async test patterns help you check code that works with things happening over time, like waiting for data. They make sure your program works right even when things take a moment.

Testing a function that fetches data from the internet.

Checking if a timer or delay works as expected.

Verifying code that reads files asynchronously.

Ensuring a database query returns the correct result.

Testing event-driven code that uses promises or async/await.

Syntax

Deno

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

deno.test("test name", async () => {
  const result = await asyncFunction();
  assertEquals(result, expectedValue);
});

Use async keyword before the test function to handle asynchronous code.

Use await inside the test to wait for promises to resolve.

Examples

This test waits for a promise that resolves to 42 and checks the value.

Deno

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

deno.test("async test with await", async () => {
  const data = await Promise.resolve(42);
  assertEquals(data, 42);
});

This test waits 100 milliseconds before checking a simple true value.

Deno

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

deno.test("async test with delay", async () => {
  function delay(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
  }
  await delay(100);
  assertEquals(true, true);
});

This test calls an async function and checks its returned string.

Deno

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

async function fetchData() {
  return "hello";
}

deno.test("test async function fetchData", async () => {
  const result = await fetchData();
  assertEquals(result, "hello");
});

Sample Program

This test waits for a number returned after a short delay and checks if it equals 7.

Deno

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

// Simulate async data fetch
async function getNumberAfterDelay() {
  return new Promise(resolve => {
    setTimeout(() => resolve(7), 50);
  });
}

deno.test("async test pattern example", async () => {
  const number = await getNumberAfterDelay();
  assertEquals(number, 7);
});

Important Notes

Always mark your test function as async when testing async code.

Use await to pause the test until the async operation finishes.

Deno's test runner shows ok when the test passes successfully.

Summary

Async test patterns let you test code that works with delays or waiting.

Use async and await in your test functions.

Deno's test runner handles async tests smoothly and shows clear results.

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