Assertions and matchers in Deno - Performance & Optimization

Performance: Assertions and matchers

LOW IMPACT

Assertions and matchers affect test execution speed and resource use during development but do not impact production page load or rendering.

Writing tests with assertions to verify code behavior

Deno

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

Deno.test("fast test", () => {
  assertEquals(1 + 1, 2);
});

Minimal assertions with direct checks reduce CPU usage and speed up test runs.

📈 Performance GainTest runs complete instantly, improving developer productivity

Writing tests with assertions to verify code behavior

Deno

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

Deno.test("slow test", () => {
  for (let i = 0; i < 1000000; i++) {
    assertEquals(i, i);
  }
});

Running heavy or redundant assertions inside large loops slows down test execution and wastes CPU.

📉 Performance CostBlocks test runner for multiple seconds, increasing development feedback time

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy assertions in loops	0	0	0	[X] Bad
Simple direct assertions	0	0	0	[OK] Good

Rendering Pipeline

Assertions and matchers run during test execution and do not affect browser rendering pipeline.

⚠️ BottleneckNone in rendering pipeline; bottleneck is CPU during test execution.

Optimization Tips

1Avoid heavy computations inside assertions to keep tests fast.

2Assertions and matchers do not affect production page rendering performance.

3Use simple, direct assertions to reduce CPU usage during testing.

Performance Quiz - 3 Questions

Which assertion pattern leads to faster test execution in Deno?

A Using complex matchers with heavy computations

B Running assertions inside large loops repeatedly

C Using simple, direct assertions outside loops

D Adding console logs inside assertions

DevTools: Performance

How to check: Run tests with Deno's built-in test runner and profile CPU usage using the Performance panel in browser DevTools or Deno profiling tools.

What to look for: Look for long CPU tasks or high CPU usage during test execution indicating slow assertions.

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