README.md

Adding Chai Assertions

• Extend chai with the chai-date-string assertion plugin from cypress/support/index.js so it is available in every spec file.
• Extend chai with the chai-colors assertion plugin in a single spec file
• Globally extend chai for all specs.
• Write and use own Chai assertion, see cypress/support/index.js

Code completion

Even if you write your specs in JavaScript, you can have Intelligent Code Completion for custom assertions.

1. Add a .d.ts file with type definition and good JSDoc comment, for example here is cypress/support/index.d.ts.

// cypress/support/index.d.ts file
// extends Cypress assertion Chainer interface with
// the new assertion methods

/// <reference types="cypress" />

declare namespace Cypress {
  interface Chainer<Subject> {
    /**
     * Custom Chai assertion that checks if given subject is string "foo"
     *
     * @example
     ```
    expect('foo').to.be.foo()
    cy.wrap('foo').should('be.foo')
    ```
    * */
    (chainer: 'be.foo'): Chainable<Subject>

    /**
     * Custom Chai assertion that checks if given subject is NOT string "foo"
     *
     * @example
     ```
    expect('bar').to.not.be.foo()
    cy.wrap('bar').should('not.be.foo')
    ```
    * */
   (chainer: 'not.be.foo'): Chainable<Subject>
  }
}

2. In the spec file add special comment reference path pointing at the .d.ts file to load it.

/// <reference types="Cypress" />
/// <reference path="../support/index.d.ts" />

In a modern code editor like VSCode you should see IntelliSense pop up when hovering over the new assertion method.

Related: IntelliSense for custom Cypress commands