Testing and TDD

Why do testing?

Automated tests check that your code does what it should. They give you confidence to change the code without fear of breaking something (the tests warn you).

A basic assertion

An assertion checks that a condition holds; if not, it fails.

function assert(condition, message) {
  if (!condition) throw new Error(message || "Assertion failed");
}

function double(x) {
  return x * 2;
}

assert(double(4) === 8, "double(4) should be 8");
assert(double(0) === 0, "double(0) should be 0");
console.log("✅ All tests passed");

Testing frameworks

In real projects, tools like Jest or Vitest are used, with a declarative syntax:

test("double multiplies by two", () => {
  expect(double(4)).toBe(8);
});

TDD: Test-Driven Development

Test-driven development reverses the usual order. The cycle is red → green → refactor:

1. Red: write a test that fails (the function does not exist yet).
2. Green: write the minimum code to make it pass.
3. Refactor: improve the code while keeping the tests green.

TDD forces you to think first about what the code should do before how.

Examples

Homemade mini test suite

function assert(cond, msg) {
  if (!cond) throw new Error(msg);
}
function isEven(n) {
  return n % 2 === 0;
}
assert(isEven(4) === true, "4 is even");
assert(isEven(7) === false, "7 is odd");
console.log("All tests passed");