Writing a file upon test fail.

on-fail.js

describe('test, () => {
  it('should do the same thing it did last time', () => {
    const snapshot = cy.readFile('./cypress/snapshots/snapshot.html');
    let bodyHtml = '';
    cy.visit('/home');
    cy.get('body').then(b => bodyHtml = b.innerHTML);
    cy.on('fail', () => {
      // This throws a "You can't promise inside a promise"
      // https://on.cypress.io/returning-promise-and-commands-in-another-command
      cy.writeFile('./cypress/snapshots/snapshot.html', bodyHtml);
    });
    cy.on('test:after:run', () => {
      // This never runs
      cy.writeFile('./cypress/snapshots/snapshot.html', bodyHtml);
    });
    cy.wrap(null).should(() => {
      expect(bodyHtml).to.be(snapshot);
    });
  })
})

on-test-after-run.js

describe('test, () => {
  it('should do the same thing it did last time', () => {
    const snapshot = cy.readFile('./cypress/snapshots/snapshot.html');
    let bodyHtml = '';
    cy.visit('/home');
    cy.get('body').then(b => bodyHtml = b.innerHTML);
    cy.on('test:after:run', () => {
      // This never runs
      cy.writeFile('./cypress/snapshots/snapshot.html', bodyHtml);
    });
    cy.wrap(null).should(() => {
      expect(bodyHtml).to.be(snapshot);
    });
  })
})

then.js

describe('test, () => {
  it('should do the same thing it did last time', () => {
    const snapshot = cy.readFile('./cypress/snapshots/snapshot.html');
    let bodyHtml = '';
    cy.visit('/home');
    cy.get('body').then(b => bodyHtml = b.innerHTML);
    cy.wrap(null).should(() => {
      expect(bodyHtml).to.be(snapshot);
    }).then(() => {
      // This never runs
      cy.writeFile('./cypress/snapshots/snapshot.html', bodyHtml);
    });
  })
})