index.js

// ***********************************************************
// This example support/index.js is processed and
// loaded automatically before your test files.
//
// This is a great place to put global configuration and
// behavior that modifies Cypress.
//
// You can change the location of this file or turn off
// automatically serving support files with the
// 'supportFile' configuration option.
//
// You can read more here:
// https://on.cypress.io/configuration
// ***********************************************************

require('cypress-wait-until')

const { isPlainObject, last } = Cypress._

/**
 * Adds command "cy.waitForResources(name1, name2, ...)"
 * that checks performance entries for resources that end with the given names.
 * This command will be available in every spec file.
 *
 * @example cy.waitForResources('base.css', 'app.css')
 *
 * You can pass additional options, like "timeout"
 *
 * @example cy.waitForResources('base.css', 'app.css', { timeout: 3000 })
 */
Cypress.Commands.add('waitForResources', (...args) => {
  let names
  let options

  if (isPlainObject(last(args))) {
    names = args.slice(0, args.length - 1)
    options = last(args)
  } else {
    names = args
    options = {}
  }

  const log = false // let's not log inner commands
  const timeout = options.timeout || Cypress.config('defaultCommandTimeout')

  cy.log(`Waiting for resources ${names.join(', ')}`)

  cy.window({ log }).then(
    // note that ".then" method has options first, callback second
    // https://on.cypress.io/then
    { log, timeout },
    (win) => {
      return new Cypress.Promise((resolve, reject) => {
        // flag set when we find all names
        let foundResources

        // control how long we should try finding the resource
        // and if it is still not found. An explicit "reject"
        // allows us to show nice informative message
        setTimeout(() => {
          if (foundResources) {
            // nothing needs to be done, successfully found the resource
            return
          }

          clearInterval(interval)
          reject(
            new Error(`Timed out waiting for resources ${names.join(', ')}`)
          )
        }, timeout)

        const interval = setInterval(() => {
          foundResources = names.every((name) =>
            win.performance
            .getEntriesByType('resource')
            .find((item) => item.name.endsWith(name))
          )
          if (!foundResources) {
            // some resource not found, will try again
            return
          }

          cy.log('Found all resources')
          clearInterval(interval)
          resolve()
        }, 100)
      })
    }
  )
})