Writing test functions in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Deno test function?

Consider this Deno test function. What will be printed when you run it with deno test?

Deno

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

test("sum test", () => {
  const result = 2 + 3;
  console.log("Inside test");
  assertEquals(result, 5);
});

A The console prints "Inside test" but the test fails.

B The console prints nothing and the test passes silently.

C The console prints "Inside test" and the test passes.

D The test fails with a syntax error.

💡 Hint

Remember that console.log inside a test function runs when the test runs.

The console.log inside the test function runs during the test execution, so "Inside test" is printed. The assertion passes, so the test passes.

📝 Syntax

intermediate

Which option correctly defines an async test function in Deno?

You want to write an async test function in Deno that awaits a promise. Which option is syntactically correct?

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

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

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

D test(async () => await Promise.resolve())

💡 Hint

Check the syntax for async arrow functions.

Option B correctly uses an async arrow function with parentheses and braces and includes the test name. Option B misses the test name and uses function keyword without name. Option B has invalid async parameter. Option B misses the test name argument which is required.

🔧 Debug

advanced

Why does this Deno test fail with a timeout?

This test never finishes and times out. Why?

test("timeout test", async () => {
  setTimeout(() => {
    console.log("Done");
  }, 1000);
});

A Because the test function does not await the setTimeout callback, so it finishes immediately without waiting.

B Because setTimeout is not supported in Deno test functions.

C Because the test function is missing the async keyword.

D Because console.log inside setTimeout causes the test to fail.

💡 Hint

Think about how async functions and timers work together.

The test function finishes immediately because setTimeout callback is not awaited or wrapped in a promise. The test runner waits for the async function to complete, but the timer callback runs later, causing a timeout.

❓ state_output

advanced

What is the value of count after running this Deno test?

Given this code, what is the final value of count after the test finishes?

let count = 0;

test("increment count", () => {
  count += 1;
});

count += 1;

A 2

B 1

C 0

D The code throws a ReferenceError.

💡 Hint

Remember the order of code execution outside and inside tests.

The code increments count by 1 outside the test, then the test runs and increments count by 1 again, so total is 2.

🧠 Conceptual

expert

Which option best describes Deno's test runner behavior with multiple tests?

You have multiple tests defined with test() in a file. How does Deno's test runner execute them by default?

A Deno runs all tests concurrently in parallel by default.

B Deno runs tests randomly to avoid order dependencies.

C Deno runs only the first test and skips the rest by default.

D Deno runs tests sequentially in the order they are defined by default.

💡 Hint

Think about test isolation and order guarantees.

By default, Deno runs tests sequentially in the order they are defined to avoid race conditions and ensure predictable results.

Practice

1. What is the main purpose of using deno.test in your code?

easy

A. To write test functions that check if code works correctly

B. To run the main application server

C. To format code automatically

D. To deploy code to the cloud

Solution

1. Step 1: Understand the role of deno.test

   deno.test is used to define test functions that verify code behavior.

2. Step 2: Differentiate from other tasks

   Running servers, formatting, or deploying are not related to deno.test.

3. Final Answer:

   To write test functions that check if code works correctly -> Option A

4. Quick Check:

   Test functions = deno.test usage [OK]

Hint: Tests check code correctness using deno.test [OK]

Common Mistakes:

• Confusing test functions with running servers
• Thinking deno.test formats code
• Assuming deno.test deploys code

2. Which of the following is the correct syntax to define a test function in Deno?

easy

A. test(() => { /* test code */ });

B. test.deno(() => { /* test code */ });

C. deno.runTest(() => { /* test code */ });

D. deno.test(() => { /* test code */ });

Solution

1. Step 1: Recall correct Deno test syntax

   The correct way to define a test is deno.test(() => { ... }).

2. Step 2: Identify incorrect options

   Options B, C, and D use wrong method names or missing namespace.

3. Final Answer:

   deno.test(() => { /* test code */ }); -> Option D

4. Quick Check:

   Correct test syntax = deno.test [OK]

Hint: Use deno.test() exactly to define tests [OK]

Common Mistakes:

• Swapping method names like test.deno or deno.runTest
• Omitting 'deno.' prefix
• Using just test() without import or context

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

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
deno.test("sum test", () => {
  const result = 2 + 3;
  assertEquals(result, 5);
});

medium

A. Runtime error: deno.test is undefined

B. Test fails with assertion error

C. Test passes with no errors

D. Syntax error due to import

Solution

1. Step 1: Analyze the test logic

   The test calculates 2 + 3 and checks if it equals 5, which is true.

2. Step 2: Check for syntax or runtime issues

   The import and usage of deno.test and assertEquals are correct.

3. Final Answer:

   Test passes with no errors -> Option C

4. Quick Check:

   Correct assertion means test passes [OK]

Hint: Correct assertEquals means test passes [OK]

Common Mistakes:

• Assuming test fails when assertion is true
• Thinking import causes syntax error
• Confusing runtime errors with correct usage

4. Identify the error in this Deno test code:

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
deno.test("check value", () => {
  const value = 10;
  assertEquals(value, "10");
});

medium

A. Type mismatch causes assertion failure

B. Missing async keyword in test function

C. Incorrect import URL for assertEquals

D. deno.test requires a name and function separately

Solution

1. Step 1: Check assertion parameters

   The test compares number 10 with string "10", which are not equal.

2. Step 2: Confirm other syntax correctness

   Import URL and test function syntax are correct; async is not required here.

3. Final Answer:

   Type mismatch causes assertion failure -> Option A

4. Quick Check:

   assertEquals fails if types differ [OK]

Hint: assertEquals fails if types differ exactly [OK]

Common Mistakes:

• Thinking async is always needed
• Assuming string and number are equal
• Believing import URL is wrong

5. You want to write a test in Deno that checks if a function getUserName(id) returns a non-empty string for valid IDs. Which test function is best to write?

hard

A. deno.test("user name test", () => { const name = getUserName(1); assertEquals(typeof name, "number"); });

B. deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); });

C. deno.test("user name test", () => { const name = getUserName(1); assertEquals(name, ""); });

D. deno.test("user name test", () => { const name = getUserName(1); assertEquals(name, null); });

Solution

1. Step 1: Understand the test goal

   The test should verify the returned name is a non-empty string.

2. Step 2: Evaluate each option

   deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); }); checks if length > 0, which matches the goal. Options A, B, and D check for number type, empty string, or null, which are incorrect.

3. Final Answer:

   deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); }); -> Option B

4. Quick Check:

   Check non-empty string with length > 0 [OK]

Hint: Check string length > 0 to confirm non-empty [OK]

Common Mistakes:

• Checking for empty string instead of non-empty
• Checking wrong type like number or null
• Using assertEquals with wrong expected value