Why Assertions and matchers in Deno? - Purpose & Use Cases

The Big Idea

What if your code could tell you exactly when and why it breaks, without guesswork?

The Scenario

Imagine you write code and want to check if it works correctly by manually printing values and reading logs every time.

You have to guess if the output is right or not, and fix bugs by trial and error.

The Problem

Manually checking code results is slow and tiring.

You can easily miss mistakes or misunderstand what went wrong.

It's hard to keep track of many checks and repeat them after every change.

The Solution

Assertions and matchers let you write clear checks that automatically verify your code's behavior.

They tell you exactly what failed and why, so you can fix bugs faster.

This makes testing reliable, repeatable, and easy to understand.

Before vs After

✗ Before

if (result !== expected) { console.log('Oops, something is wrong'); }

✓ After

assertEquals(result, expected);

What It Enables

You can confidently change code knowing tests will catch mistakes instantly.

Real Life Example

When building a calculator app, assertions check if adding or subtracting numbers always gives the right answer, so users never get wrong results.

Key Takeaways

Manual checks are slow and error-prone.

Assertions and matchers automate and clarify testing.

They help catch bugs early and improve code quality.

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