Writing test functions in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a test function in Deno?

A test function in Deno checks if a piece of code works as expected by running it and verifying the results automatically.

beginner

How do you define a test function in Deno?

You use the built-in Deno.test() function, passing a name and a function that contains the test code.

beginner

What is the role of assertions inside a Deno test function?

Assertions check if conditions are true during the test. If an assertion fails, the test fails, showing what went wrong.

beginner

How can you run tests in a Deno project?

You run tests by using the command deno test in the terminal, which finds and runs all test functions.

beginner

Why is it helpful to give a descriptive name to a test function in Deno?

A descriptive name helps you and others understand what the test checks, making it easier to find and fix problems.

Which function is used to create a test in Deno?

A test()

B Deno.test()

C runTest()

D checkTest()

Deno provides the built-in function Deno.test() to define test functions.

What happens if an assertion inside a Deno test fails?

A The test fails and shows an error

B The test passes anyway

C The test is skipped

D The test runs again automatically

If an assertion fails, Deno marks the test as failed and shows the error details.

How do you run all tests in a Deno project?

A deno test

B deno execute tests

C deno start tests

D deno run tests

The command deno test runs all test functions found in the project.

What should you include inside a Deno test function?

A Code to start the server

B User interface code

C Database connection code

D Assertions to check code behavior

Test functions contain assertions to verify that code behaves as expected.

Why is naming your test functions clearly important?

A To reduce code size

B To make tests run faster

C To help understand what each test checks

D To avoid writing assertions

Clear names help you and others quickly understand the purpose of each test.

Explain how to write a basic test function in Deno and what it should include.

Think about the steps from defining to running a test.

Describe why assertions are important in Deno test functions and what happens when they fail.

Focus on the role of assertions in verifying code.

Practice

1. What is the main purpose of using deno.test in your code?

easy

A. To write test functions that check if code works correctly

B. To run the main application server

C. To format code automatically

D. To deploy code to the cloud

Solution

1. Step 1: Understand the role of deno.test

   deno.test is used to define test functions that verify code behavior.

2. Step 2: Differentiate from other tasks

   Running servers, formatting, or deploying are not related to deno.test.

3. Final Answer:

   To write test functions that check if code works correctly -> Option A

4. Quick Check:

   Test functions = deno.test usage [OK]

Hint: Tests check code correctness using deno.test [OK]

Common Mistakes:

• Confusing test functions with running servers
• Thinking deno.test formats code
• Assuming deno.test deploys code

2. Which of the following is the correct syntax to define a test function in Deno?

easy

A. test(() => { /* test code */ });

B. test.deno(() => { /* test code */ });

C. deno.runTest(() => { /* test code */ });

D. deno.test(() => { /* test code */ });

Solution

1. Step 1: Recall correct Deno test syntax

   The correct way to define a test is deno.test(() => { ... }).

2. Step 2: Identify incorrect options

   Options B, C, and D use wrong method names or missing namespace.

3. Final Answer:

   deno.test(() => { /* test code */ }); -> Option D

4. Quick Check:

   Correct test syntax = deno.test [OK]

Hint: Use deno.test() exactly to define tests [OK]

Common Mistakes:

• Swapping method names like test.deno or deno.runTest
• Omitting 'deno.' prefix
• Using just test() without import or context

3. What will be the output when running this test code in Deno?

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
deno.test("sum test", () => {
  const result = 2 + 3;
  assertEquals(result, 5);
});

medium

A. Runtime error: deno.test is undefined

B. Test fails with assertion error

C. Test passes with no errors

D. Syntax error due to import

Solution

1. Step 1: Analyze the test logic

   The test calculates 2 + 3 and checks if it equals 5, which is true.

2. Step 2: Check for syntax or runtime issues

   The import and usage of deno.test and assertEquals are correct.

3. Final Answer:

   Test passes with no errors -> Option C

4. Quick Check:

   Correct assertion means test passes [OK]

Hint: Correct assertEquals means test passes [OK]

Common Mistakes:

• Assuming test fails when assertion is true
• Thinking import causes syntax error
• Confusing runtime errors with correct usage

4. Identify the error in this Deno test code:

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
deno.test("check value", () => {
  const value = 10;
  assertEquals(value, "10");
});

medium

A. Type mismatch causes assertion failure

B. Missing async keyword in test function

C. Incorrect import URL for assertEquals

D. deno.test requires a name and function separately

Solution

1. Step 1: Check assertion parameters

   The test compares number 10 with string "10", which are not equal.

2. Step 2: Confirm other syntax correctness

   Import URL and test function syntax are correct; async is not required here.

3. Final Answer:

   Type mismatch causes assertion failure -> Option A

4. Quick Check:

   assertEquals fails if types differ [OK]

Hint: assertEquals fails if types differ exactly [OK]

Common Mistakes:

• Thinking async is always needed
• Assuming string and number are equal
• Believing import URL is wrong

5. You want to write a test in Deno that checks if a function getUserName(id) returns a non-empty string for valid IDs. Which test function is best to write?

hard

A. deno.test("user name test", () => { const name = getUserName(1); assertEquals(typeof name, "number"); });

B. deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); });

C. deno.test("user name test", () => { const name = getUserName(1); assertEquals(name, ""); });

D. deno.test("user name test", () => { const name = getUserName(1); assertEquals(name, null); });

Solution

1. Step 1: Understand the test goal

   The test should verify the returned name is a non-empty string.

2. Step 2: Evaluate each option

   deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); }); checks if length > 0, which matches the goal. Options A, B, and D check for number type, empty string, or null, which are incorrect.

3. Final Answer:

   deno.test("user name test", () => { const name = getUserName(1); assert(name.length > 0); }); -> Option B

4. Quick Check:

   Check non-empty string with length > 0 [OK]

Hint: Check string length > 0 to confirm non-empty [OK]

Common Mistakes:

• Checking for empty string instead of non-empty
• Checking wrong type like number or null
• Using assertEquals with wrong expected value