Writing test functions in Deno

Introduction

Testing helps you check if your code works correctly. Writing test functions lets you catch mistakes early and keep your code reliable.

When you want to make sure a new feature works as expected.

Before changing code to avoid breaking existing parts.

To automatically check your code after updates.

When sharing code with others to prove it works.

To learn how your code behaves with different inputs.

Syntax

Deno

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

deno.test("test name", () => {
  // test code here
  assertEquals(actual, expected);
});

Use Deno.test to define a test function with a name and a callback.

Use assertions like assertEquals to check if values match.

Examples

This test checks if adding 2 and 3 equals 5.

Deno

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

Deno.test("adds numbers", () => {
  const sum = 2 + 3;
  assertEquals(sum, 5);
});

This test checks if the greet function returns the right message.

Deno

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

function greet(name: string) {
  return `Hello, ${name}!`;
}

Deno.test("greet returns correct message", () => {
  assertEquals(greet("Alice"), "Hello, Alice!");
});

Sample Program

This program defines a multiply function and two tests. One test checks multiplying two positive numbers. The other checks multiplying by zero.

Deno

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

function multiply(a: number, b: number): number {
  return a * b;
}

Deno.test("multiply two positive numbers", () => {
  assertEquals(multiply(4, 5), 20);
});

Deno.test("multiply by zero", () => {
  assertEquals(multiply(7, 0), 0);
});

Important Notes

Run tests in Deno by using the command deno test in your terminal.

Test names should be clear to understand what they check.

You can add more assertions like assert, assertThrows for different checks.

Summary

Test functions help verify your code works as expected.

Use Deno.test with assertions like assertEquals to write tests.

Run tests often to catch errors early and keep code reliable.

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