Assertions & edge cases

Test your own code

Lesson 14 of 20 · 12 min · Module 4: Reliable code

Professional developers write tests: small functions that run the code with known inputs and complain when the result is wrong. Tests catch mistakes immediately and let you change code with confidence.

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Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Write assertions that fail loudly
Pick edge cases that reveal bugs
Make sure a test can actually fail

The idea

An assertion checks one expectation and throws an Error when it fails. A test function contains several of them, covering a normal case, edge cases (empty input, spaces at the edges, repeated symbols), and tricky input. Tools such as Vitest or Jest run many such tests automatically.

function assertEqual(actual, expected) {
  if (actual !== expected) throw new Error(`expected ${expected}, got ${actual}`);
}

assertEqual(2 + 2, 4);
console.log("passed");

01An assertion is a tiny if

assertEqual compares and throws a descriptive Error when the values differ. If testSlugify finishes without throwing, every expectation held.

02Edges hide the bugs

Most broken slugify versions still turn "Hello World" into "hello-world". They fail on leading spaces, trailing punctuation, or repeated symbols, so those cases matter most.

const broken = t => t.toLowerCase().replace(/[^a-z0-9]+/g, "-");
console.log(broken("  Hi!  "));

Output

-hi-

03Break it on purpose

A test that cannot fail proves nothing. Changing the code briefly, as these checks do, shows that the test really guards the behaviour you care about.

Your turn

Write function slugify(text): lowercase, every run of characters that are not a–z or 0–9 becomes one hyphen, no hyphens at the start or end. Then write function testSlugify() with at least four assertions that throw when slugify is wrong. The checks will replace slugify with broken versions and expect your test to throw, so declare both with function.

What the checks look for

slugify handles titles, punctuation and spaces
testSlugify passes with your slugify
testSlugify catches a version that keeps edge hyphens
testSlugify catches a version that only lowercases

Try it in the editor →

Take this with you

Good tests describe the behaviour precisely enough that broken code fails them.

Go a little further

Install Vitest in a small project on your computer and rewrite testSlugify with its expect(...).toBe(...) syntax.