Assertions and matchers in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the output of this Deno test assertion

What will be the result when running this Deno test code snippet?

Deno

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

Deno.test("Check sum", () => {
  const result = 2 + 2;
  assertEquals(result, 4);
});

A Test passes without errors

B Test fails with an AssertionError

C SyntaxError due to missing import

D Runtime error: result is undefined

💡 Hint

Check what assertEquals compares and the values used.

The assertion compares result which is 4, to the expected value 4. Since they are equal, the test passes without errors.

❓ component_behavior

intermediate

What happens if assertThrows is used incorrectly?

Consider this Deno test code. What is the outcome when running it?

Deno

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

Deno.test("Throw test", () => {
  assertThrows(() => {
    const x = 5 + 5;
  });
});

A Test fails with AssertionError because no error was thrown

B Test passes because the function runs without error

C SyntaxError due to missing error type in assertThrows

D Runtime error: x is not defined

💡 Hint

assertThrows expects the function to throw an error.

The function inside assertThrows does not throw any error. Therefore, assertThrows fails with an AssertionError.

❓ state_output

advanced

What is the output of this test using assertMatch?

Given this test code, what will be the test result?

Deno

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

Deno.test("Regex match test", () => {
  const message = "Hello Deno!";
  assertMatch(message, /^Hello/);
});

A Test fails because message does not match regex

B Test fails because regex is invalid

C Test passes because message starts with 'Hello'

D Runtime error: assertMatch is not a function

💡 Hint

Check if the string matches the regular expression pattern.

The string "Hello Deno!" starts with "Hello", so the regex /^Hello/ matches and the test passes.

🔧 Debug

advanced

Why does this test fail with TypeError?

Analyze this test code and identify why it fails with a TypeError.

Deno

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

Deno.test("Type error test", () => {
  const data = null;
  assertEquals(data.length, 0);
});

A SyntaxError due to missing semicolon

B AssertionError because data.length is not 0

C Test passes because null is treated as empty

D TypeError because null has no property 'length'

💡 Hint

Check the type of data and what properties it has.

Accessing length on null causes a TypeError because null has no properties.

🧠 Conceptual

expert

Which assertion correctly tests that a function throws a specific error message?

You want to test that calling failFunc() throws an error with message "Invalid input". Which assertion code is correct?

Deno

function failFunc() {
  throw new Error("Invalid input");
}

A assertThrows(() => failFunc(), Error, "Invalid input");

B assertThrows(() => failFunc(), Error, /Invalid input/);

C assertThrows(() => failFunc(), /Invalid input/);

D assertThrows(() => failFunc(), "Invalid input");

💡 Hint

Check the signature of assertThrows for matching error messages.

In Deno, assertThrows accepts a function, an optional error constructor (e.g. Error), and a regex to match the error message. Option B correctly uses the constructor and regex to verify both the error type and message.

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