Next.js

Code splitting is an optimization technique that breaks a JavaScript bundle into smaller chunks that are loaded on demand, rather than forcing users to download the entire application upfront. In React, this is achieved through `React.lazy()` and dynamic `import()` statements, which create separate bundles loaded only when the corresponding component or route is accessed. This dramatically improves initial load times, especially for large single-page applications with many routes.

Blue-green deployment is a release strategy that maintains two identical production environments — "blue" (current) and "green" (new version). Traffic is switched from blue to green only after the new version passes all health checks, enabling zero-downtime releases. If issues are detected, traffic can be instantly routed back to the blue environment, making rollbacks trivial and fully automated.

Focus management is the practice of controlling which element receives keyboard focus in a web application, especially during dynamic content changes. When a modal opens, focus should move into it; when it closes, focus should return to the trigger element. Proper focus management is essential for keyboard and screen reader users, and involves techniques like focus trapping, roving tabindex, and programmatic focus with `element.focus()`.

A webhook is a mechanism where one system sends an automated HTTP request to another system when a specific event occurs. Unlike polling, where the client repeatedly checks for updates, webhooks push data in real time, making them far more efficient. They are widely used to connect services — for example, triggering a deployment when code is merged or sending a Slack notification when a payment is processed.

A canary release is a deployment strategy where a new version is gradually rolled out to a small subset of users before reaching the full audience. Automated monitoring compares error rates, latency, and key metrics between the canary and the stable version. If the canary performs well, traffic is incrementally shifted; if anomalies are detected, the release is automatically rolled back, minimizing the blast radius of potential issues.

A toast notification is a small, non-intrusive message that appears briefly on screen — usually at the top or bottom — to provide feedback about an action without interrupting the user's workflow. Unlike modals, toasts auto-dismiss after a few seconds and don't require user interaction. They are commonly used to confirm saves, report errors, or show status updates. Accessible implementations include ARIA live regions so screen readers announce the message.