Assertions and matchers in Deno - Mini Project: Build & Apply

Using Assertions and Matchers in Deno Testing

📖 Scenario: You are creating simple tests for a calculator module in Deno. You want to check if your functions return the correct results using assertions and matchers.

🎯 Goal: Build a Deno test file that uses assertions and matchers to verify calculator functions work correctly.

📋 What You'll Learn

Create a calculator object with add and subtract functions

Set up a test configuration variable for a tolerance value

Write tests using Deno's assertEquals and assertAlmostEquals matchers

Complete the test suite with proper test definitions and imports

💡 Why This Matters

🌍 Real World

Testing code is essential in real projects to catch bugs early and ensure your functions work as expected.

💼 Career

Knowing how to write tests with assertions and matchers is a key skill for software developers to maintain code quality.

1

Create the calculator object with add and subtract functions

Create a constant called calculator that is an object with two functions: add which takes a and b and returns their sum, and subtract which takes a and b and returns their difference.

Deno

# Your code here

Hint

Use an object literal with arrow functions for add and subtract.

2

Add a tolerance variable for approximate equality

Create a constant called tolerance and set it to 0.001 to use for approximate comparisons in tests.

Deno

const calculator = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b
};

# Your code here

Hint

Just create a constant with the exact name and value.

3

Write tests using Deno assertions and matchers

Import assertEquals and assertAlmostEquals from "https://deno.land/std@0.203.0/testing/asserts.ts". Then write two tests: one named "add test" that checks calculator.add(2, 3) equals 5 using assertEquals, and another named "subtract test" that checks calculator.subtract(5, 2.999) is approximately equal to 2.001 using assertAlmostEquals with the tolerance.

Deno

const calculator = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b
};

const tolerance = 0.001;

# Your code here

Hint

Use Deno.test to define tests and call the correct assertion functions with the exact arguments.

4

Complete the test file with export and proper structure

Ensure the test file is complete by exporting nothing (default for Deno tests) and that all tests are defined at the top level. Confirm the import statement is at the top and no extra code is outside the tests.

Deno

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

const calculator = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b
};

const tolerance = 0.001;

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

Deno.test("subtract test", () => {
  assertAlmostEquals(calculator.subtract(5, 2.999), 2.001, tolerance);
});

# Your code here

Hint

No extra exports or code needed. Just keep the test file clean and structured.

Practice

1. What is the main purpose of assertions in Deno testing?

easy

A. To create new variables automatically

B. To check if the code behaves as expected

C. To speed up the execution of code

D. To format the output of the program

Solution

1. Step 1: Understand what assertions do

   Assertions are used to verify that the code produces the expected results during tests.

2. Step 2: Identify the purpose in testing

   They help catch errors by comparing actual output with expected output.

3. Final Answer:

   To check if the code behaves as expected -> Option B

4. Quick Check:

   Assertions verify behavior = D [OK]

Hint: Assertions verify expected behavior in tests [OK]

Common Mistakes:

• Thinking assertions format output
• Believing assertions speed up code
• Confusing assertions with variable creation

2. Which of the following is the correct syntax to assert equality in Deno tests?

easy

A. assertEquals(actual, expected);

B. assertEqual(actual, expected);

C. assert.equal(actual, expected);

D. assertEquals(expected, actual);

Solution

1. Step 1: Recall Deno's assertion function names

   Deno provides an assertion function called assertEquals to check equality.

2. Step 2: Check parameter order and spelling

   The correct order is assertEquals(actual, expected), matching assertEquals(actual, expected); exactly.

3. Final Answer:

   assertEquals(actual, expected); -> Option A

4. Quick Check:

   Correct function and order = A [OK]

Hint: Use assertEquals(actual, expected) exactly [OK]

Common Mistakes:

• Swapping actual and expected parameters
• Using assertEqual instead of assertEquals
• Using dot notation incorrectly

3. What will be the result of this Deno test code?

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

const result = 5 + 3;
assertEquals(result, 8);

medium

A. Runtime error because result is undefined

B. Test fails with an assertion error

C. Syntax error due to import statement

D. Test passes without errors

Solution

1. Step 1: Evaluate the calculation

   The variable result is assigned 5 + 3, which equals 8.

2. Step 2: Check the assertion

   The assertion compares result (8) with expected value 8, so it matches and passes.

3. Final Answer:

   Test passes without errors -> Option D

4. Quick Check:

   5 + 3 = 8 matches expected = A [OK]

Hint: Check calculation matches expected value exactly [OK]

Common Mistakes:

• Assuming import causes syntax error
• Thinking assertion fails when values match
• Confusing runtime error with assertion failure

4. Identify the error in this Deno test code:

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

const value = "hello";
assertEquals(value, Hello);

medium

A. Missing quotes around Hello causing a ReferenceError

B. Incorrect import URL for assertEquals

C. assertEquals requires three arguments

D. Variable value is not defined

Solution

1. Step 1: Check the assertion parameters

   The second parameter is Hello without quotes, so JavaScript treats it as a variable.

2. Step 2: Identify the error caused

   Since Hello is not defined, this causes a ReferenceError at runtime.

3. Final Answer:

   Missing quotes around Hello causing a ReferenceError -> Option A

4. Quick Check:

   String literals need quotes = C [OK]

Hint: Always quote string literals in assertions [OK]

Common Mistakes:

• Forgetting quotes around strings
• Assuming import URL is wrong
• Thinking assertEquals needs more arguments

5. You want to test if a function returns an array containing the number 10. Which assertion is best to use in Deno?

hard

A. assert(result == [10]);

B. assertEquals(result, 10);

C. assertEquals(result.includes(10), true);

D. assert(result.contains(10));

Solution

1. Step 1: Understand the goal

   You want to check if the array contains the number 10, not if it equals 10 or a single-element array.

2. Step 2: Evaluate each option

   assertEquals(result.includes(10), true); uses result.includes(10) which returns true if 10 is in the array, then asserts it equals true. This is correct.
   assert(result == [10]); compares array to [10] with == which fails because arrays are reference types.
   assertEquals(result, 10); compares array to number 10, which is incorrect.
   assert(result.contains(10)); uses contains which is not a valid JavaScript method.

3. Final Answer:

   assertEquals(result.includes(10), true); -> Option C

4. Quick Check:

   Use includes() to check array contents = B [OK]

Hint: Use includes() method inside assertEquals for array checks [OK]

Common Mistakes:

• Using == to compare arrays
• Using non-existent contains() method
• Comparing array directly to a number