Why Deno's built-in testing simplifies development - See It in Action

Why Deno's built-in testing simplifies development

📖 Scenario: You are building a small utility in Deno and want to make sure your code works correctly. Deno has built-in testing features that help you write and run tests easily without extra setup.

🎯 Goal: Create a simple Deno test to check a function's behavior using Deno's built-in testing module.

📋 What You'll Learn

Create a function called add that takes two numbers and returns their sum.

Create a test case using Deno.test to verify add(2, 3) returns 5.

Use assertEquals from std/testing/asserts.ts for the test assertion.

Run the test using Deno's built-in test runner.

💡 Why This Matters

🌍 Real World

Testing code ensures it works correctly before sharing or deploying. Deno's built-in testing makes this easy without extra tools.

💼 Career

Many software jobs require writing tests. Knowing Deno's testing helps you write reliable code quickly and professionally.

1

Create the add function

Write a function called add that takes two parameters named a and b and returns their sum using return a + b.

Deno

// Write the add function here
// Your code here

Hint

Think of add as a simple calculator that adds two numbers.

2

Import assertEquals from Deno's standard testing library

Add an import statement to import assertEquals from https://deno.land/std@0.203.0/testing/asserts.ts.

Deno

function add(a, b) {
  return a + b;
}

// Import assertEquals from Deno's standard library
// Your code here

Hint

Use the exact URL and import syntax to get assertEquals.

3

Write a test case using Deno.test

Write a test case using Deno.test with the name "add function test". Inside the test, call assertEquals(add(2, 3), 5) to check the function works correctly.

Deno

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

function add(a, b) {
  return a + b;
}

// Write a Deno.test named "add function test" below
// Your code here

Hint

Use an arrow function inside Deno.test to run the assertion.

4

Run the test with Deno's test runner

Add a comment explaining how to run the test using the command deno test in the terminal.

Deno

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

function add(a, b) {
  return a + b;
}

Deno.test("add function test", () => {
  assertEquals(add(2, 3), 5);
});

// Add a comment on how to run this test
// Your code here

Hint

Write a simple comment that tells how to run the test using Deno's command line.

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