Next.js & Playwright / Cypress

The problem

Random data in E2E tests is brittle: a DNI or an IBAN generated on the fly can clash with validations, or make a test fail once in a thousand runs for no apparent reason.

The fix: a fixed seed per test

Each test uses its own seed (for example, the test name). Same seed + same parameters ⇒ always the same data, so the test is reproducible and its assertions can use concrete values.

Playwright

tests/alta-cliente.spec.ts

import { test, expect } from "@playwright/test";

async function fetchDni(seed: string) {
  const res = await fetch(
    "https://datamockseed.firstflip.es/api/v1/es/document?sub=dni&count=1&seed=" + seed,
  );
  const { data } = await res.json();
  return data[0].documento as string;
}

test("the sign-up form accepts a fake DNI", async ({ page }) => {
  const dni = await fetchDni("signup-e2e"); // same seed => same DNI on every run
  await page.goto("/signup");
  await page.getByLabel("DNI").fill(dni);
  await page.getByRole("button", { name: "Continue" }).click();
  await expect(page.getByText("Valid DNI")).toBeVisible();
});

Cypress

cypress/e2e/alta-cliente.cy.js

Cypress.Commands.add("fixtureDni", (seed) => {
  return cy
    .request(
      `https://datamockseed.firstflip.es/api/v1/es/document?sub=dni&count=1&seed=${seed}`,
    )
    .its("body.data.0.documento");
});

it("accepts a fake DNI", () => {
  cy.fixtureDni("signup-e2e").then((dni) => {
    cy.visit("/signup");
    cy.findByLabelText("DNI").type(dni);
    cy.findByRole("button", { name: "Continue" }).click();
    cy.findByText("Valid DNI").should("be.visible");
  });
});

Complete fixture datasets

For a fixture with several consistent rows (name + DNI + email), use POST /v1/es/bulk with the test's seed instead of chaining individual requests.