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Clear filtersThe virtual DOM is a lightweight JavaScript copy of the UI that React compares with the previous version after each update, so it only changes the parts of the real DOM that actually differ.
A string is immutable, so every change creates a new string object, while StringBuilder is a mutable buffer that modifies its contents in place, making it much faster for repeated concatenation.
String is immutable, StringBuilder is a mutable and fast but not thread-safe buffer, and StringBuffer is a mutable buffer with synchronized methods that is thread-safe but slower.
Keys give each list item a stable identity so React can match items between renders and correctly add, remove or reorder them without recreating or mixing up their state.
Event delegation means attaching one listener to a parent element and using event bubbling to handle events from its children, instead of adding a listener to each child.
Boxing converts a value type to object by allocating it on the heap, and unboxing converts it back; both add allocations and casts, so avoid them in hot code paths.
LINQ (Language Integrated Query) lets you query collections, databases and XML with a consistent syntax, and deferred execution means a query does not run when you define it but when you enumerate the results.
Use a small, pinned base image, order instructions so dependencies are cached before source code, use multi-stage builds to leave build tools out of the final image, run as a non-root user and keep secrets out of the image.
Debounce waits until the user stops triggering an event for a set time before running the function; throttle runs the function at most once per time interval no matter how often the event fires.
useMemo caches the result of a calculation, while useCallback caches a function definition; both only recompute when their dependencies change.
CSR renders in the browser after JavaScript loads, SSR renders the HTML on the server for each request, and SSG renders the HTML once at build time; SSR and SSG give crawlers and users content faster, which helps SEO.
Both download the script without blocking HTML parsing. defer waits until parsing finishes and runs scripts in document order; async runs a script as soon as it is downloaded, which can be before or after parsing and in any order.
The browser parses HTML into a DOM and CSS into a CSSOM, combines them into a render tree, computes layout (reflow), paints pixels and composites layers onto the screen.
Streams process data piece by piece as it arrives instead of loading it all into memory, which lets you handle large files and network data efficiently; they come in Readable, Writable, Duplex and Transform types.
An index is a separate sorted data structure (usually a B-tree) that lets the database find rows without scanning the whole table, at the cost of extra storage and slower writes.
A clustered index defines the physical order in which the table's rows are stored, so there can be only one per table, while a non-clustered index is a separate structure that points back to the rows.
IEnumerable runs the query in memory using delegates, while IQueryable builds an expression tree that a provider such as Entity Framework translates into SQL, so filtering happens in the database.
A generator is a function that uses yield to produce values one at a time and remembers its state between calls, so it can process large or infinite sequences without holding everything in memory.
Caching stores frequently read data in fast storage such as memory to reduce latency and database load; the main strategies are cache-aside, write-through, write-back and time-based expiry (TTL).
Take heap snapshots in Chrome DevTools before and after the suspect action, compare what grew, and fix the retaining reference, which is usually a forgotten listener, timer, global variable or unbounded cache.