deno test for testing - Step-by-Step Execution

Concept Flow - deno test for testing

Write test function

↓

Run 'deno test'

↓

Deno finds test functions

↓

Execute each test

↓

Check assertions

↓

Pass

↓

Report success

↓

Fail

↓

Report failure

↓

Show summary

↓

Exit

This flow shows how Deno runs tests: you write test functions, run 'deno test', Deno executes tests, checks assertions, and reports results.

Execution Sample

Deno

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

Deno.test("addition works", () => {
  const result = 2 + 3;
  assertEquals(result, 5);
});

This code defines a simple test that checks if 2 + 3 equals 5 using Deno's test runner.

Execution Table

Step	Action	Evaluation	Result
1	Start 'deno test' command	Find test functions	Found 1 test function
2	Run test 'addition works'	Calculate 2 + 3	Result = 5
3	Assert result equals 5	assertEquals(result, 5)	Pass
4	Report test result	Test passed	Success message shown
5	Finish all tests	No more tests	Summary: 1 passed, 0 failed

💡 All tests executed; all assertions passed; test run ends successfully.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
result	undefined	5	5	5

Key Moments - 2 Insights

Why does the test pass even though we only check one assertion?

Because the assertion assertEquals(result, 5) matches the expected value, so the test passes as shown in execution_table step 3.

What happens if the assertion fails?

If the assertion fails, Deno reports a failure at step 3 and shows an error message instead of a success message in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 2?

A 5

B undefined

C 2 + 3

D null

💡 Hint

Check variable_tracker column 'After Step 2' for 'result'

At which step does Deno check if the test passes or fails?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table step 3 where assertion is evaluated

If the assertion was assertEquals(result, 4), what would change in the execution table?

A Step 3 would show Pass

B Step 3 would show Fail

C Step 2 would change result value

D No change at all

💡 Hint

Assertion result depends on matching expected value, see step 3 in execution_table

Concept Snapshot

Deno.test(name, fn) defines a test.
Run tests with 'deno test' command.
Assertions like assertEquals check results.
Passing assertions show success; failing ones show errors.
Deno reports summary after all tests run.

Full Transcript

This visual execution shows how Deno runs tests. First, you write a test function using Deno.test with a name and a function. Inside, you perform operations and use assertions like assertEquals to check expected results. When you run 'deno test', Deno finds all test functions and runs them one by one. It evaluates the code inside, checks assertions, and reports if each test passes or fails. Finally, it shows a summary of all tests. Variables like 'result' change as the test runs, and assertions determine success. If an assertion fails, Deno stops that test and reports the failure. This process helps you verify your code works as expected.

Practice

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

easy

A. To check if your code works as expected by running tests

B. To deploy your code to a server

C. To format your code automatically

D. To create a new Deno project

Solution

1. Step 1: Understand the role of deno test

   deno test is a command used to run tests written in your code to verify correctness.

2. Step 2: Compare with other options

   Deploying, formatting, or creating projects are not related to testing functionality.

3. Final Answer:

   To check if your code works as expected by running tests -> Option A

4. Quick Check:

   Testing = Check code correctness [OK]

Hint: Remember: testing means checking code correctness [OK]

Common Mistakes:

• Confusing testing with deployment
• Thinking deno test formats code
• Assuming it creates projects

2. Which of the following is the correct way to write a simple test in Deno?

easy

A. deno.test(() => { assertEquals(1 + 1, 2); });

B. test deno { assertEquals(1 + 1, 2); }

C. deno.test = () => { assertEquals(1 + 1, 2); }

D. test(() => assertEquals(1 + 1, 2));

Solution

1. Step 1: Identify correct test syntax in Deno

   The correct syntax uses deno.test as a function with a callback containing assertions.

2. Step 2: Check each option

   deno.test(() => { assertEquals(1 + 1, 2); }); correctly uses deno.test(() => { ... }). Options A, B, and D have syntax errors or incorrect usage.

3. Final Answer:

   deno.test(() => { assertEquals(1 + 1, 2); }); -> Option A

4. Quick Check:

   Correct test syntax = deno.test(() => { assertEquals(1 + 1, 2); }); [OK]

Hint: Use deno.test with a function callback [OK]

Common Mistakes:

• Using incorrect keywords like 'test deno'
• Assigning instead of calling deno.test
• Omitting 'deno.' prefix

3. What will be the output when running this test code with deno test?

import { assertEquals } from "https://deno.land/std@0.177.0/testing/asserts.ts";
deno.test("sum test", () => {
  assertEquals(2 + 2, 4);
});

medium

A. Syntax error due to import statement

B. Test fails with assertion error

C. Test passes with no errors

D. Runtime error: deno.test is not a function

Solution

1. Step 1: Analyze the test code logic

   The test checks if 2 + 2 equals 4 using assertEquals, which is true.

2. Step 2: Consider syntax and runtime

   The import statement is correct, and deno.test is valid in Deno environment.

3. Final Answer:

   Test passes with no errors -> Option C

4. Quick Check:

   Correct assertion = Test passes [OK]

Hint: True assertions mean passing tests [OK]

Common Mistakes:

• Thinking import causes syntax error
• Confusing assertion failure with success
• Assuming deno.test is undefined

4. Identify the error in this test code snippet:

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

medium

A. assertEquals is not imported correctly

B. The assertion will fail because 5 is not equal to 4

C. deno.test is used incorrectly without a name

D. The import URL is missing the version number

Solution

1. Step 1: Check the import statement

   The import URL lacks a version number, which is required for stable imports in Deno.

2. Step 2: Verify other parts

   The assertion will fail logically, but the question asks for the error in the snippet, and missing version is a syntax/runtime error.

3. Final Answer:

   The import URL is missing the version number -> Option D

4. Quick Check:

   Stable imports need version numbers [OK]

Hint: Always include version in import URLs [OK]

Common Mistakes:

• Ignoring missing version in import URL
• Confusing assertion failure with syntax error
• Assuming deno.test needs no name

5. You want to write multiple tests in one file using deno test. Which approach correctly groups tests with descriptive names and runs them all?

hard

A. Write one deno.test with multiple assertions but no names

B. Use multiple deno.test calls each with a unique name and callback function

C. Call deno.test inside a loop without names

D. Use deno.runTests() to run tests automatically

Solution

1. Step 1: Understand how to organize multiple tests

   Each test should have a unique name and a callback function using deno.test.

2. Step 2: Evaluate options

   Use multiple deno.test calls each with a unique name and callback function correctly uses multiple named tests. Write one deno.test with multiple assertions but no names lacks test names, making results unclear. Call deno.test inside a loop without names uses a loop without names, which is not recommended. Use deno.runTests() to run tests automatically is invalid; deno.runTests() does not exist.

3. Final Answer:

   Use multiple deno.test calls each with a unique name and callback function -> Option B

4. Quick Check:

   Multiple named tests = Use multiple deno.test calls each with a unique name and callback function [OK]

Hint: Name each test separately with deno.test [OK]

Common Mistakes:

• Writing unnamed tests
• Using loops without test names
• Assuming deno.runTests() exists