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Narrowing is when TypeScript shrinks a wide type (such as a union) to a more specific type after a runtime check like typeof, instanceof, in, or a custom type guard.
A string union such as "admin" | "user" exists only in the type system and erases to plain strings. A TypeScript enum compiles to a JavaScript object (unless it is a const enum), which is extra runtime code and a weaker default.
TypeScript fills in types you omit: literals get their literal or widened type, function return types are inferred from return statements, and generic arguments are inferred from the values you pass.
process.nextTick runs its callback right after the current operation, before the event loop continues; setTimeout runs in the timers phase after a minimum delay; and setImmediate runs in the check phase right after the poll (I/O) phase.
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.
Session authentication stores the login state on the server and gives the browser a session ID cookie, while JWT authentication gives the client a signed token that contains the user's claims so the server can verify it without storing state.
ACID stands for Atomicity, Consistency, Isolation and Durability: a transaction either fully succeeds or fully fails, keeps the data valid, does not interfere with other transactions, and survives crashes once committed.
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.
Transient services are created every time they are requested, scoped services are created once per HTTP request, and singleton services are created once and shared for the life of the application.
A Thread is an operating-system execution unit, while a Task represents an asynchronous operation scheduled on the thread pool; await releases the current thread while the operation is pending and resumes the method when it completes.
The .NET garbage collector automatically frees managed objects that are no longer reachable, organising them into generations 0, 1 and 2 so short-lived objects are collected cheaply, and IDisposable is used to release unmanaged resources such as files and connections promptly.
A HashMap stores entries in an array of buckets; it computes the key's hash code to choose a bucket, resolves collisions with a linked list that becomes a red-black tree when a bucket grows large, and resizes when the load factor of 0.75 is exceeded.
A decorator is a function that takes another function and returns a modified version of it, letting you add behaviour such as logging, caching or access control without changing the original code.
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.
CPython frees objects mainly through reference counting, deleting an object as soon as nothing refers to it, and uses a cyclic garbage collector to clean up groups of objects that reference each other.
Choose SQL when your data is relational and you need transactions, joins and strong consistency; choose NoSQL when you need flexible schemas, very high write throughput or horizontal scaling for a specific access pattern.
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).
REST exposes resources over HTTP URLs and is simple and cache-friendly, GraphQL lets clients ask for exactly the fields they need from a single endpoint, and gRPC uses binary Protocol Buffers over HTTP/2 for fast service-to-service calls.