meet it

A catches every failure and reports the score.

With one assertion after another, the first failure stops everything, and you never hear about the rest. A test runner wraps each test in its own try...catch, records a pass or a fail with the reason, and carries on.

Here test takes a name and a function. A group of tests like this is a . Run the suite after every change: if an old test suddenly fails, you have caught a before your users do.

const score = { passed: 0, failed: 0 };
function test(name, check) {
  try {
    if (check() !== true) { throw new Error("returned false"); }
    score.passed++;
  } catch (error) {
    score.failed++;
    console.log(`FAIL ${name} (${error.name})`);
  }
}
const shout = (text) => `${text.toUpperCase()}!`;
test("adds a bang", () => shout("hi") === "HI!");
test("copes with no text", () => shout() === "!");
console.log(`${score.passed} passed, ${score.failed} failed`);

Real runners such as Jest and Vitest work the same way, with far more helpers.

guess first

One test crashes in the middle. What will this show?

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

function test(name, check) {
  try {
    check();
    console.log(`PASS ${name}`);
  } catch (error) {
    console.log(`FAIL ${name}: ${error.name}`);
  }
}
const firstLetter = (word) => word[0].toUpperCase();
test("cat", () => { if (firstLetter("cat") !== "C") { throw new Error("wrong"); } });
test("empty word", () => { if (firstLetter("") !== "") { throw new Error("wrong"); } });
test("dog", () => { if (firstLetter("dog") !== "D") { throw new Error("wrong"); } });
console.log("all tests ran");

your turn

Your turn. Add a test to the suite.

Give the second test a name and pick an odd number, so the whole suite passes.

Your code

press run to see what happens

fix it

One crash hides the last test. Can you fix it?

Broken code to fix

press run to see what happens

After the crash, did the empty list test run at all?
The try wraps the whole loop, so one crash ends every test after it.
Move try and its catch inside the loop, around the if line, and log ${name} crashed in the catch.

nailed it

you built your own test runner. :D

A test runner contains each failure and keeps going.
A test suite guards your code against regressions.
Run the suite after every change you make.

new words in your collection

test runner
test suite
regression

This lesson isn’t saved. A free account remembers every lesson you finish.

A test runner catches every failure and reports the score.

One test crashes in the middle. What will this show?

Your turn. Add a test to the suite.

One crash hides the last test. Can you fix it?

you built your own test runner. :D

A catches every failure and reports the score.