Why Deno's built-in testing simplifies development - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import Deno's testing module.

Deno

import { [1] } from "https://deno.land/std/testing/asserts.ts";

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

AassertEquals

BassertTrue

CassertThrows

DassertNotEquals

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing an assertion function that does not check equality.

Forgetting to import from the correct URL.

The assertEquals function is commonly used to check if two values are equal in Deno tests.

2fill in blank

medium

Complete the code to define a test case using Deno's built-in test function.

Deno

Deno.[1]("simple addition test", () => {
  const result = 2 + 3;
  assertEquals(result, 5);
});

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

Acheck

Btest

CrunTest

Dassert

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent function like runTest.

Confusing assertion functions with test definition functions.

Deno uses the test function to define test cases.

3fill in blank

hard

Fix the error in the test function to properly handle async code.

Deno

Deno.test("async test", async () => {
  const data = await fetch("https://example.com");
  const text = await data.text();
  [1](text.length > 0);
});

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

AassertTrue

BassertEquals

Cassert

DassertThrows

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertEquals when comparing to a boolean.

Using assertThrows which is for error checking.

For checking a condition that should be true, assertTrue is the correct assertion.

4fill in blank

hard

Fill both blanks to create a test that expects an error to be thrown.

Deno

Deno.test("error test", () => {
  [1](() => {
    throw new Error("fail");
  }, [2]);
});

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

AassertThrows

BassertEquals

CError

DassertTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertEquals instead of assertThrows.

Not specifying the error type correctly.

assertThrows checks if a function throws an error, and Error specifies the expected error type.

5fill in blank

hard

Fill all three blanks to create a test that checks if a string includes a substring.

Deno

Deno.test("substring test", () => {
  const message = "Hello Deno!";
  [1](message.[2]("Deno"), [3]);
});

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

AassertTrue

Bincludes

Ctrue

DassertEquals

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertTrue instead of assertEquals.

Using wrong string methods like indexOf without comparison.

assertEquals compares values, includes checks substring presence, and true is the expected boolean.

Practice

1. What is one main advantage of Deno's built-in testing feature?

easy

A. You don't need to install extra testing libraries.

B. It requires complex setup before running tests.

C. Tests must be written in a separate language.

D. It only works with web browsers.

Solution

1. Step 1: Understand Deno's testing setup

   Deno includes testing tools by default, so no extra installs are needed.

2. Step 2: Compare options

   The options describing separate languages, complex setup, or browser-only use are incorrect for Deno's built-in testing.

3. Final Answer:

   You don't need to install extra testing libraries. -> Option A

4. Quick Check:

   Built-in testing = No extra installs [OK]

Hint: Remember: Deno has testing tools ready out-of-the-box [OK]

Common Mistakes:

• Thinking you must install Jest or Mocha separately
• Assuming tests run only in browsers
• Believing tests require a different language

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

easy

A. Deno.test('example', () => { /* code */ })

B. deno.test('example', () => { /* code */ })

C. test('example', () => { /* code */ })

D. Test('example', () => { /* code */ })

Solution

1. Step 1: Recall Deno's test syntax

   Deno's built-in testing uses the global object Deno with the method test.

2. Step 2: Check options

   Only Deno.test('example', () => { /* code */ }) uses the correct case and syntax. deno.test('example', () => { /* code */ }) uses lowercase 'deno', which is invalid. test('example', () => { /* code */ }) omits the Deno prefix. Test('example', () => { /* code */ }) uses wrong casing.

3. Final Answer:

   Deno.test('example', () => { /* code */ }) -> Option A

4. Quick Check:

   Correct syntax = Deno.test [OK]

Hint: Use capital D in Deno.test for correct syntax [OK]

Common Mistakes:

• Using lowercase 'deno' instead of 'Deno'
• Omitting the 'Deno.' prefix
• Using 'Test' with uppercase T

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

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

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

medium

A. Test fails with an assertion error.

B. Test passes with no errors.

C. Syntax error due to import statement.

D. Runtime error: assertEquals not defined.

Solution

1. Step 1: Analyze the test code

   The test imports assertEquals correctly and checks if 2 + 3 equals 5.

2. Step 2: Evaluate the assertion

   Since 2 + 3 is 5, assertEquals(result, 5) passes without error.

3. Final Answer:

   Test passes with no errors. -> Option B

4. Quick Check:

   Correct assertion = Test passes [OK]

Hint: Check if assertion matches expected value exactly [OK]

Common Mistakes:

• Assuming import causes error without checking URL
• Thinking 2 + 3 is not equal to 5
• Confusing assertEquals with other assertion functions

4. Identify the error in this Deno test code:

Deno.test("fail test", () => {
  const value = 10;
  assertEquals(value, 5);
});

medium

A. No error; test will pass.

B. Incorrect test name syntax.

C. Using arrow function instead of regular function.

D. Missing import of assertEquals function.

Solution

1. Step 1: Check for imports

   The code uses assertEquals but does not import it from Deno's standard library.

2. Step 2: Understand impact

   Without importing, assertEquals is undefined, causing a runtime error.

3. Final Answer:

   Missing import of assertEquals function. -> Option D

4. Quick Check:

   assertEquals must be imported [OK]

Hint: Always import assertion functions before use [OK]

Common Mistakes:

• Assuming assertEquals is globally available
• Thinking arrow functions cause errors here
• Ignoring missing import errors

5. You want to write multiple tests in Deno that share setup code. Which approach best uses Deno's built-in testing to simplify this?

hard

A. Repeat the setup code inside every Deno.test block manually.

B. Use a third-party testing library to manage setup and teardown.

C. Write a helper function for setup and call it inside each Deno.test block.

D. Put all tests inside one big Deno.test block without separation.

Solution

1. Step 1: Understand Deno's testing features

   Deno.test allows multiple test blocks but does not have built-in setup/teardown hooks like some libraries.

2. Step 2: Choose best practice for shared setup

   Writing a helper function for setup and calling it inside each test keeps code clean and avoids repetition.

3. Step 3: Evaluate other options

   Repeat the setup code inside every Deno.test block manually. repeats code, which is inefficient. Use a third-party testing library to manage setup and teardown. adds complexity by using external libraries. Put all tests inside one big Deno.test block without separation. mixes tests, reducing clarity.

4. Final Answer:

   Write a helper function for setup and call it inside each Deno.test block. -> Option C

5. Quick Check:

   Helper function for setup = clean, reusable tests [OK]

Hint: Use helper functions to share setup code in tests [OK]

Common Mistakes:

• Copy-pasting setup code in every test
• Relying on external libraries unnecessarily
• Combining all tests in one block