Chai

Chai is a library that adds assertions to your tests.

Chai defines 3 different "flavors" to write assertions. These are all available in Cypress out of the box. However, most Cypress tests are written asynchronously which means assertions have to be written in callback functions which may make your code less readable. Therefore Cypress added the should method to simplify asserting asynchronously.

In the following examples, consider this setup:

let foo = 'bar';
let tea = { flavors: ['mint', 'lemon', 'jasmine'] };

Behavior driven

Expect

expect(foo).to.be.a('string');
expect(foo).to.equal('bar');
expect(foo).to.have.lengthOf(3);

expect(tea).to.have.property('flavors').with.lengthOf(3);

Should

foo.should.be.a('string');
foo.should.equal('bar');
foo.should.have.lengthOf(3);

tea.should.have.property('flavors').with.lengthOf(3);

Classic style

Assert

assert.typeOf(foo, 'string');
assert.equal(foo, 'bar');
assert.lengthOf(foo, 3)

assert.property(tea, 'flavors');
assert.lengthOf(tea.flavors, 3);

Via Cypress should syntax

cy.wrap(foo)
    .should('be.a', 'string')
    .should('equal', 'bar')
    .should('have.length', 3);

cy.wrap(tea)
    .should('have.property', 'flavors') // Chain subject changes at this point
    .should('have.length', 3);

Notice how some of the should assertions change the subject of the Cypress chain. In the first statement, the wrapped variable foo remains the subject and all assertions are executed against foo. In the latter example, the first should method changes the subject from tea, to tea.flavors.