Writing test functions in Deno - Performance & Optimization

Performance: Writing test functions

MEDIUM IMPACT

This affects page load speed and interaction responsiveness by controlling when and how test code runs during development or CI.

Running test code in the same runtime as production code

Deno

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

if (import.meta.main) {
  Deno.test("example test", () => {
    const result = expensiveCalculation();
    assertEquals(result, 42);
  });
}

function expensiveCalculation() {
  for (let i = 0; i < 1e8; i++) {}
  return 42;
}

Tests run only when explicitly executed, avoiding blocking production runtime.

📈 Performance Gainnon-blocking main runtime, tests run separately

Running test code in the same runtime as production code

Deno

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

Deno.test("example test", () => {
  const result = expensiveCalculation();
  assertEquals(result, 42);
});

function expensiveCalculation() {
  // heavy CPU work
  for (let i = 0; i < 1e8; i++) {}
  return 42;
}

Running heavy test code inline blocks the event loop and delays page rendering or server response.

📉 Performance Costblocks rendering for 100+ ms depending on CPU

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Inline heavy test code	0	0	Blocks event loop delaying paint	[X] Bad
Tests run separately (import.meta.main)	0	0	No blocking, paint proceeds normally	[OK] Good

Rendering Pipeline

Test functions run in the JavaScript runtime and can block the event loop if executed inline, delaying rendering and interaction.

→Script Execution

→Event Loop

⚠️ BottleneckBlocking synchronous test code delays event loop and rendering

Optimization Tips

1Run test functions separately from production code to avoid blocking.

2Use import.meta.main to conditionally run tests only when intended.

3Check performance profiles to ensure tests do not delay rendering or interaction.

Performance Quiz - 3 Questions

What is the main performance risk of running test functions inline in your Deno app runtime?

A Blocking the event loop and delaying rendering or response

B Increasing CSS selector complexity

C Adding large images to the bundle

D Causing layout shifts on the page

DevTools: Performance

How to check: Record a performance profile while loading your app with tests inline vs tests separated. Look for long scripting tasks blocking main thread.

What to look for: Long tasks in scripting phase causing delayed first paint or interaction readiness

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