Backend Developer Interview Questions and Answers

TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. It catches type errors in the editor and at build time, and it documents the shape of data for the next person who reads the code.

Both describe object shapes. interface can be merged and is preferred for public object contracts; type can express unions, intersections and primitives, and cannot be reopened later.

INNER JOIN returns only rows that have a match in both tables, while LEFT JOIN returns every row from the left table and fills the right table's columns with NULL when there is no match.

WHERE filters individual rows before grouping, while HAVING filters groups after GROUP BY, so only HAVING can use aggregate functions like COUNT or SUM.

DELETE removes selected rows and can be rolled back, TRUNCATE quickly removes all rows and keeps the table structure, and DROP removes the entire table including its structure.

A primary key uniquely identifies each row in its own table and cannot be NULL, while a foreign key is a column that references the primary key of another table to link the two.

A value type variable holds its data directly and is copied on assignment, while a reference type variable holds a reference to an object, so assignment copies the reference and both variables see the same object.

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.

The JVM runs Java bytecode, the JRE is the JVM plus the standard libraries needed to run Java programs, and the JDK is the JRE plus the development tools such as the compiler (javac).

== compares primitives by value and objects by reference (whether they are the exact same object), while equals() compares the logical content of two objects when the class overrides it.

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.

final is a keyword that prevents reassigning a variable, overriding a method or extending a class; finally is a block that always runs after try/catch for cleanup; finalize() was a method called by the garbage collector and is now deprecated.

A list is mutable and can be changed after creation, while a tuple is immutable; tuples are slightly faster, use less memory and can be used as dictionary keys when their items are hashable.

== checks whether two objects have equal values, while is checks whether they are the exact same object in memory.

Node.js is a JavaScript runtime built on Chrome's V8 engine that runs your JavaScript on a single main thread and uses non-blocking I/O, so one thread can handle many concurrent connections.

package.json lists your project's dependencies with allowed version ranges, while package-lock.json records the exact versions that were installed so every machine gets an identical dependency tree.

any turns off checking. unknown is a type-safe any: you must narrow it before use. never means a value cannot exist, used for exhaustive checks and functions that always throw.

Generics are type parameters: they let a function, class or type work with many types while keeping the relationship between inputs and outputs, instead of falling back to any.

A union (A | B) is a value that is one of the members. An intersection (A & B) is a value that must satisfy both types at once.

Utility types are built-in helpers that transform an existing type: Partial makes every field optional, Required the opposite, Pick keeps a set of keys, Omit removes them, and Record builds a key/value map.

Use a subquery that takes the MAX salary below the overall MAX, or use DENSE_RANK() and pick rank 2, which also handles ties and generalises to the Nth highest.

Find duplicates with GROUP BY and HAVING COUNT(*) > 1, then delete them by numbering rows within each duplicate group with ROW_NUMBER() and removing every row after the first.

UNION combines two result sets and removes duplicate rows, while UNION ALL combines them and keeps every row, which makes it faster.

Normalization is organising tables to reduce redundancy and update anomalies: 1NF means atomic values, 2NF removes partial dependencies on part of a composite key, and 3NF removes dependencies between non-key columns.

A const is a compile-time constant that must be assigned where it is declared, while a readonly field can be assigned at declaration or in the constructor, so its value can be decided at runtime.

Both pass an argument by reference, but ref requires the variable to be initialised before the call, while out requires the method to assign a value before it returns.

An abstract class can hold state, constructors and partial implementation but a class can inherit only one, while an interface defines a contract that a class can implement many of.

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.

An abstract class can have instance fields, constructors and a mix of implemented and abstract methods but supports only single inheritance, while an interface defines a contract that a class can implement multiple times and, since Java 8, can include default methods.

ArrayList is backed by a resizable array with O(1) index access and O(n) insertion in the middle, while LinkedList is a doubly linked list with O(n) index access and O(1) insertion once you hold the node; in practice ArrayList is faster for most workloads.

Checked exceptions (subclasses of Exception other than RuntimeException) must be caught or declared with throws, while unchecked exceptions (RuntimeException and Error) do not have to be handled and usually signal programming bugs.

Java 8 introduced lambda expressions, functional interfaces, the Stream API for declarative collection processing, Optional to represent missing values, default methods in interfaces, method references and the java.time date and time API.

*args collects extra positional arguments into a tuple and **kwargs collects extra keyword arguments into a dictionary, which lets a function accept a variable number of arguments.

Default argument values are evaluated once when the function is defined, so a default list or dictionary is shared between every call that does not pass its own, causing data to leak between calls.

A shallow copy creates a new container but reuses the same nested objects, while a deep copy recursively copies everything so the new object is fully independent.

A classmethod receives the class as its first argument (cls) and can create or configure instances, while a staticmethod receives no automatic argument and is just a function placed inside the class namespace.

require is the CommonJS module system, which loads modules synchronously at runtime, while import is the ES module standard, which is statically analysed, asynchronous and supports top-level await and tree shaking.

Middleware are functions that receive the request, response and a next callback; they run in the order they are registered and can modify the request, end the response or pass control on with next().

Vertical scaling means giving one machine more CPU, memory or disk, while horizontal scaling means adding more machines and spreading the load across them.

A load balancer distributes incoming requests across several servers to improve capacity and availability, using algorithms such as round robin, least connections or consistent hashing.

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.

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.

HashMap is not thread-safe, Hashtable is thread-safe by locking the whole map on every method, and ConcurrentHashMap is thread-safe with much finer-grained locking and lock-free reads, so it scales far better.

The JVM divides memory into a per-thread stack, a shared heap (split into young and old generations) and Metaspace; the garbage collector frees heap objects that are no longer reachable, collecting short-lived young objects frequently and old objects less often.

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.

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.

Validate all input, use parameterised queries, add security headers with helmet, rate-limit requests, restrict CORS, hash passwords, keep secrets in environment variables, and keep dependencies updated and audited.

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.

An operation is idempotent if repeating it any number of times has the same effect as doing it once; it matters because network retries can otherwise create duplicate orders or payments.

Choose an algorithm such as token bucket or sliding window, store a counter per client (by user ID, API key or IP) in a fast shared store like Redis, and return HTTP 429 with a Retry-After header when the limit is exceeded.

Mapped types build a new object type by looping over keys (`{ [K in keyof T]: ... }`). Conditional types pick a result with `T extends U ? X : Y`, which is how many utility types are implemented.

Isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) control how much concurrent transactions can see of each other's changes, trading consistency against concurrency to prevent dirty reads, non-repeatable reads and phantom reads.

Read the execution plan with EXPLAIN to find full scans and expensive joins, add or fix indexes for the filter and join columns, select only the columns and rows you need, and rewrite predicates so indexes can be used.

Use async all the way down instead of blocking on tasks with .Result or .Wait(), and in library code use ConfigureAwait(false) so continuations do not need to return to a captured context.

synchronized gives mutual exclusion and memory visibility for a block of code, while volatile only guarantees that reads and writes of one variable are visible to all threads and does not make compound operations such as count++ atomic.

The Global Interpreter Lock (GIL) in CPython lets only one thread execute Python bytecode at a time, so threads do not speed up CPU-bound code but still help with I/O-bound work.

Threading runs concurrent threads that share memory and suit blocking I/O, multiprocessing runs separate processes to use multiple CPU cores for CPU-bound work, and asyncio runs many non-blocking tasks cooperatively on one thread for high-volume I/O.

Keep the main thread free by moving CPU-heavy work to worker threads or a job queue, and scale across cores and machines by running multiple processes (cluster or PM2) behind a load balancer with shared, external state.

Generate a short unique code for each long URL (for example by base62-encoding a unique ID), store the mapping in a key-value or relational database, serve redirects through a cache because reads far outnumber writes, and record analytics asynchronously.

The CAP theorem says that during a network partition a distributed system must choose between consistency (every read sees the latest write) and availability (every request gets a response); you cannot have both while the partition lasts.

Replication copies the same data to several servers to improve read capacity and availability, while sharding splits the data across servers so each holds only a part, which increases write capacity and total storage.

Start with a well-structured monolith unless you have several teams and clear service boundaries, because microservices add distributed-system complexity such as network failures, data consistency and operational overhead.