meet it

Unit tests check your code so you do not have to.

A runs one small piece of code with a known input and checks the answer. Professionals keep hundreds of them and run them after every change, so a bug shows up in seconds instead of weeks.

A is the code that runs every test and reports what passed. Ours is tiny. test takes a name and a callback and runs it inside try...catch. assertEqual<T> throws when two values differ, and the generic makes both sides the same type.

function assertEqual<T>(actual: T, expected: T): void {
  if (actual !== expected) {
    throw new Error(`expected ${String(expected)}, got ${String(actual)}`);
  }
}
function test(name: string, run: () => void): void {
  try {
    run();
    console.log(`PASS ${name}`);
  } catch (e) {
    console.log(`FAIL ${name}: ${String(e)}`);
  }
}
test("trims spaces", () => assertEqual(" tea ".trim(), "tea"));

String(e) turns a thrown Error into text like Error: expected 4, got 5.

guess first

Test the rounding. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

function assertEqual<T>(actual: T, expected: T): void {
  if (actual !== expected) {
    throw new Error(`expected ${String(expected)}, got ${String(actual)}`);
  }
}
function test(name: string, run: () => void): void {
  try {
    run();
    console.log(`PASS ${name}`);
  } catch (e) {
    console.log(`FAIL ${name}: ${String(e)}`);
  }
}
test("rounds halves", () => { assertEqual(Math.round(2.5), 3); assertEqual(Math.round(-2.5), -3); });

your turn

Your turn. Write a test case of your own.

Add a second test for discount: give it a name, a price, a percentage, and the answer you expect. Make it pass.

Your code

press run to see what happens

fix it

The lists match, but the test fails. Fix the test.

Broken code to fix

press run to see what happens

The message says it expected [1,2,3] and got [1,2,3]. They look the same, so why fail?
!== on two arrays asks whether they are the very same array, not whether they hold the same items.
On line 2, compare JSON.stringify(actual) with JSON.stringify(expected) instead.

nailed it

your code now checks itself. :D

A unit test runs code with a known input and checks the answer.
A test runner catches each failure, so one FAIL never stops the rest.
=== compares arrays by identity, so compare their contents instead.

new words in your collection

unit test
test runner

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