Cypress 9.2.0 will throw an error when a user attempts to update a read-only config value

Cypress provides two options to override the configuration while the tests are running - Cypress.config() and suite-specific or test-specific configuration overrides.

Before

Before version 9.2.0, it was not documented which config options are readonly and which ones are writable at the runtime, which created a lot of confusion.

For example: If we were to change screenshotsFolder during test using Cypress.config():

describe('Test readonly config options', () => {
  it('updating screenshots location in version 8.7.0', () => {
    cy.visit('http://www.google.com')
    Cypress.config('screenshotsFolder', 'cypress/screenshots/desktop')
    expect(Cypress.config('screenshotsFolder')).to.eq('cypress/screenshots/desktop')
    cy.screenshot('google_ss')
  })
});

The test mentioned above passes, but Cypress does not change the location of the screenshotsFolder, nor does it throw an error telling that the screenshotsFolder was not set.

After

After 9.2.0, Cypress will throw an error when a user attempts to set these non-writable config values using Cypress.config().

For example: If we wanted to change screenshotsFolder during test now using Cypress.config():

describe('Test readonly config options', () => {
  it('updating screenshots location in version 9.2.0', () => {
    cy.visit('http://www.google.com')
    Cypress.config('screenshotsFolder', 'cypress/screenshots/desktop')
    expect(Cypress.config('screenshotsFolder')).to.eq('cypress/screenshots/desktop')
    cy.screenshot('google_ss')
  })
});

The above test run will throw a validation error mentioning it is a read-only property.

List of config options which ‘can’ be updated on runtime

animationDistanceThreshold
baseUrl
blockHosts
defaultCommandTimeout
env
execTimeout
experimentalSessionSupport
includeShadowDom
keystrokeDelay
numTestsKeptInMemory
pageLoadTimeout
redirectionLimit
requestTimeout
responseTimeout
retries
screenshotOnRunFailure
scrollBehavior
slowTestThreshold
taskTimeout
viewportHeight
viewportWidth
waitForAnimations

List of config options which ‘cannot’ be updated and are ‘read-only’ on runtime

autoOpen (internal)
browsers
chromeWebSecurity
clientCertificates
clientRoute (internal)
component
componentFolder
configFile (internal)
devServerPublicPathRoute (internal)
downloadsFolder
e2e
exit
experimentalFetchPolyfill
experimentalInteractiveRunEvents
experimentalSourceRewriting
experimentalStudio
fileServerFolder
fixturesFolder
hosts
ignoreTestFiles
integrationFolder
isInteractive
isTextTerminal
modifyObstructiveCode
morgan (internal)
namespace (internal)
nodeVersion
pluginsFile
port
projectId
reporter
reporterOptions
reporterRoute (internal)
resolvedNodePath
resolvedNodeVersion
socketId (internal)
socketIoCookie (internal)
socketIoRoute (internal)
supportFile
testFiles
trashAssetsBeforeRuns
userAgent
video
videoCompression
videoUploadOnPasses
videosFolder
watchForFileChanges
xhrRoute (internal)

To know more about this, check out the PR and Cypress-Documentation.