What is a feature flag, and how would you implement one in a Node.js application to control a new feature's rollout?

A feature flag is a runtime conditional determining whether code executes, letting a feature deploy dormant and be enabled gradually without a new deployment -- decoupling deploying code from releasing features, enabling instant rollback by flipping the flag off.

if (await isFeatureEnabled('new-checkout-flow', req.user)) {
  return renderNewCheckout(req, res);
}
renderLegacyCheckout(req, res);