Assertions and matchers in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of assertions in Deno testing?

Assertions check if a condition is true during tests. If false, the test fails. They help confirm code works as expected.

beginner

Name a common matcher function in Deno's testing library.

assertEquals() checks if two values are equal. It is often used to compare expected and actual results.

intermediate

How does assertThrows() help in testing?

assertThrows() verifies that a function throws an error when expected. It ensures error handling works correctly.

beginner

What happens if an assertion fails during a Deno test run?

The test stops and is marked as failed. This signals that the code did not behave as expected.

intermediate

Explain the difference between assert() and assertEquals() in Deno.

assert() checks if a value is truthy (true or truthy value). assertEquals() compares two values for exact equality.

Which Deno assertion checks if two values are exactly equal?

A assert()

B assertThrows()

C assertEquals()

D assertNotEquals()

assertEquals() compares two values for exact equality in Deno tests.

What does assertThrows() verify in a test?

A That a function throws an error

B That a value is truthy

C That two values are equal

D That a function returns true

assertThrows() checks if a function throws an error as expected.

If an assertion fails in Deno, what happens?

A The test passes anyway

B The test retries automatically

C The test is skipped

D The test fails and stops

A failed assertion causes the test to fail and stop running.

Which assertion would you use to check if a value is truthy?

A assert()

B assertThrows()

C assertEquals()

D assertNotEquals()

assert() checks if a value is truthy (true or truthy).

What is the main role of matchers in Deno testing?

A To run tests in parallel

B To compare values or check conditions

C To format test output

D To install dependencies

Matchers compare values or check conditions to verify test results.

Describe how you would use assertions and matchers to test a function that should throw an error when given bad input.

Think about how to confirm your code handles mistakes safely.

Explain the difference between assert() and assertEquals() in Deno testing and when to use each.

One checks truthiness, the other checks exact equality.

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