Assertions and matchers in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the assertion module from Deno's standard library.

Deno

import { assertEquals } from "[1]";

Drag options to blanks, or click blank then click option'

Ahttps://deno.land/std/http/server.ts

Bhttps://deno.land/std/fs/mod.ts

Chttps://deno.land/std/testing/asserts.ts

Dhttps://deno.land/std/path/mod.ts

Attempts:

3 left

💡 Hint

Common Mistakes

Importing from unrelated modules like fs or path.

Using incorrect URLs that do not point to asserts.ts.

The assertEquals function is imported from Deno's standard testing asserts module.

2fill in blank

medium

Complete the code to write a test function using Deno's test API.

Deno

Deno.test("example test", () => { [1](1 + 1, 2); });

Drag options to blanks, or click blank then click option'

AassertEquals

BassertNotEquals

CassertThrows

Dassert

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertThrows which is for error checking.

Using assertNotEquals which checks inequality.

assertEquals checks if two values are equal, perfect for this test.

3fill in blank

hard

Fix the error in the test to correctly check that a function throws an error.

Deno

Deno.test("throws error", () => { [1](() => { throw new Error("fail"); }); });

Drag options to blanks, or click blank then click option'

AassertEquals

BassertThrows

Cassert

DassertNotEquals

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertEquals which compares values, not errors.

Using assert which only checks truthiness.

assertThrows is used to check if a function throws an error as expected.

4fill in blank

hard

Fill both blanks to create a test that checks if a string contains a substring.

Deno

Deno.test("string contains", () => {
  const text = "hello world";
  [1](text.includes("world"), [2]);
});

Drag options to blanks, or click blank then click option'

Aassert

Btrue

Cfalse

DassertEquals

Attempts:

3 left

💡 Hint

Common Mistakes

Using assert which only checks truthiness without expected value.

Using false as expected value which is incorrect here.

assertEquals checks if the expression equals the expected value, which is true because 'world' is in the string.

5fill in blank

hard

Fill all three blanks to write a test that verifies an array length and contents.

Deno

Deno.test("array test", () => {
  const arr = [1, 2, 3];
  [1](arr.length, [2]);
  [3](arr.includes(2));
});

Drag options to blanks, or click blank then click option'

AassertEquals

B3

Cassert

DassertNotEquals

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertNotEquals which checks for inequality.

Mixing up the order of expected and actual values.

assertEquals checks the length is 3, and assert checks that 2 is included in the array.

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