Cypress + Launchdarkly

cypress_launcdarkly.ts

// all code in cypress/support/commands.ts

  // Convert this file to a module for global namespace declaration
  export {};
  declare global {
    namespace Cypress {
      interface Chainable {
        /**
         * Custom command to set launchdarkly feature flag values.
         * @example cy.updateFeatureFlags({ 'flag-name': false})
         */
        updateFeatureFlags(value: string): Chainable<Element>;
      }
    }
  }

  // Cypress command to turn on a feature flag for launch darkly 
  Cypress.Commands.add('updateFeatureFlags', (featureFlags: { [key: string]: boolean }) => {
    // turn off push (EventSource) updates from LaunchDarkly
    cy.intercept({ hostname: /.*clientstream.launchdarkly.com/ }, (req) => {
      req.reply('data: no streaming feature flag data here\n\n', {
        'content-type': 'text/event-stream; charset=utf-8',
      });
    });

    // ignore api calls to events endpoint
    cy.intercept({ hostname: /.*events.launchdarkly.com/ }, { body: {} });

    // return feature flag values in format expected by launchdarkly client
    cy.intercept({ hostname: /.*app.launchdarkly.com/ }, (req) => {
      const body = {};
      Object.entries(featureFlags).forEach(([featureFlagName, featureFlagValue]) => {
        body[featureFlagName] = { value: featureFlagValue };
      });
      req.reply({ body });
    });
  });

// usage
  // must be in beforeEach (vs before) because intercepts are cleared before each test run
  beforeEach(() => {
    cy.updateFeatureFlags({ 'flag-name': true})
  })

Thanks for this! Got my Cypress + LD setup working like a dream.

One tweak to the type declaration towards the top — your params for updateFeatureFlags is typed as a string, but accepts an object of feature flags + values:

declare global {
  namespace Cypress {
    interface Chainable {
      /**
       * Custom command to set LaunchDarkly feature flag values
       * @example cy.updateFeatureFlags({ flagName: false })
       */
      updateFeatureFlags(featureFlags: Record<string, boolean>): Chainable<Element>;
    }
  }
}