How do you parameterise a test across multiple datasets in Cypress?

Cypress doesn't ship a describe.each or it.each API like Jest/Vitest. The idiomatic pattern is to iterate over your dataset at the top of the spec and call it() inside the loop:

const cases = [
  { input: 'alice@x.com',    valid: true },
  { input: 'no-at-symbol',   valid: false },
  { input: '',               valid: false },
];

cases.forEach(({ input, valid }) => {
  it(`treats "${input}" as ${valid ? 'valid' : 'invalid'}`, () => {
    cy.visit('/login');
    cy.get('[data-test=email]').type(input);
    cy.get('[data-test=submit]').click();
    cy.get('[data-test=error]').should(valid ? 'not.exist' : 'be.visible');
  });
});

Cypress sees N it calls and reports them individually. Each gets its own clean state under testIsolation: true.

For data loaded from a fixture, do the load synchronously at module level (avoid putting it inside before or you'll generate tests after the runner has fixed the list):

// Fails — tests aren't generated in time
before(() => {
  cy.fixture('users').then((users) => {
    users.forEach(...) // too late
  });
});

// Works — load with require/import at the top
const users = require('../fixtures/users.json') as User[];
users.forEach((user) => {
  it(`logs in as ${user.email}`, () => { ... });
});

For fully data-driven fixtures (not hard-coded), prefer importing the file as a module rather than cy.fixture — the latter is asynchronous and runs inside Cypress's queue, too late for test generation.