Cypress cy.session command

session.spec.js

const testUrl = 'https://todomvc.com/examples/typescript-react/';

const todos = {
    A: ['foo', 'bar', 'baz'],
    B: ['1', '2', '3']
};

const getTodos = () => cy.get('ul.todo-list li');
const addTodo = (text) => cy.get('input.new-todo').type(`${text}{enter}`);

const validateTodos = (todos) => getTodos()
    .should('have.length', todos.length)
    .each($todo => {
    const todoText = $todo.text();
    expect(todos).to.contain(todoText);
});

const stageTodos = (todos, url = testUrl) => {
    cy.session(todos, () => {
        cy.visit(url);
        todos.forEach(addTodo);
    }, {
        validate() {
            cy.visit(url);
            validateTodos(todos);
        }
    });
    cy.visit(url);
}

const removeTodo = (todo) => getTodos().each($todo => {
    const todoText = $todo.text();
    if (todoText === todo) {
        cy.wrap($todo).find('button.destroy').click({ force: true });
    }
});

const addTodosTest = ([name, todos]) => it(`can add a todo to dataset ${name}`, () => {
    stageTodos(todos);
    const newTodo = 'quux';
    addTodo(newTodo);
    validateTodos([...todos, newTodo]);
});

const deleteTodosTest = ([name, todos]) => it(`can delete a todo from dataset ${name}`, () => {
    stageTodos(todos);
    removeTodo(todos[0]);
    const [_, ...remainingTodos] = todos;
    validateTodos(remainingTodos);
});

const validateTodosTest = ([name, todos]) => it(`can validate todos from dataset ${name}`, () => {
    stageTodos(todos);
    validateTodos(todos);
})

const multiTest = (datasets) => (test) => Object.entries(datasets).forEach(test);

const tests = [
    addTodosTest,
    deleteTodosTest
];

describe('Cypress session', () => {
    tests.forEach(multiTest(todos));
});

This will alternate back and forth between datasets A and B (or any that are added) and run tests against each one in order...
Ex: if we have tests [1,2,3] and datasets [a,b] then we will run: 1a, 1b, 2a, 2b, 3a, 3b. I wanted to alternate so it would verify that cy.session is behaving as expected.