CSS Grid

The Virtual DOM is a lightweight in-memory representation of the actual browser DOM that React uses to optimize UI updates. When state changes, React creates a new Virtual DOM tree, compares it with the previous one through a process called reconciliation, and calculates the minimal set of real DOM mutations needed. This batching and diffing approach avoids expensive direct DOM manipulation and was a key innovation that made React's declarative programming model performant.

Git hooks are scripts that Git executes automatically before or after events like commits, pushes, and merges. Common examples include pre-commit hooks that run linters and formatters, commit-msg hooks that enforce message conventions, and pre-push hooks that run tests. Tools like Husky and lint-staged make it easy to manage hooks across a team, catching issues before they reach the remote repository.

A skeleton screen is a placeholder UI that mimics the page layout with simple shapes and animated gradients while content is loading. Unlike spinners, skeletons give users an immediate sense of the page structure, reducing perceived loading time. They are widely used in apps like Facebook, YouTube, and LinkedIn to create a smoother transition from loading to loaded state.

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.

Auto-scaling is the ability of a system to automatically adjust the number of running instances or allocated resources based on current demand. When traffic spikes, new instances are provisioned; when demand drops, excess resources are released to save costs. Cloud platforms like AWS, GCP, and Azure offer auto-scaling groups with configurable policies based on CPU usage, request count, queue depth, or custom metrics.

State management refers to the strategies and tools used to handle, store, and synchronize application data across components in a frontend application. In React, state can live locally in components via `useState`, be shared via Context, or be managed by external libraries like Redux, Zustand, Jotai, or Recoil. Choosing the right state management approach depends on your app's complexity — many apps over-adopt heavy solutions when simpler patterns would suffice.