Interview Q&A
A collection of questions and answers for technical interview preparation. Includes JavaScript/Node.js, System Design, databases, and cloud technologies.
CORS (Cross-Origin Resource Sharing) is a mechanism that lets a server tell the browser which websites are allowed to read its responses. Imagine we’re building an online store: The frontend runs on shop.com. The…
JuniorSecurityLast updated:Sharding is a way of splitting a large database into several smaller parts (shards) that are stored on different servers. The main idea is to distribute data and load across multiple servers instead of keeping…
Junior/Middle/SeniorDatabasesDistributed SystemsLast updated:Partitioning is a way to split one large table into several smaller pieces (partitions) so the database can work more easily with a large number of records. Imagine we are building a banking application that stores all…
DatabasesJunior/Middle/SeniorLast updated:Quite often in technical interviews, when the conversation turns to distributed systems, you will hear a question about a pattern called Idempotency. In short, an Idempotency Key is a unique identifier of an operation…
API DesignSystem designMiddleSeniorLast updated:Questions about Rate Limiting come up in regular technical interviews as well as in System Design interviews. Imagine you are building an API that is available to all registered clients. Several potential problems arise…
API DesignMiddleSeniorLast updated:Tokens are the basic units of text that LLM works with. A model does not read text as words in the human sense. Before processing, the text goes through tokenization, the process of splitting a string into smaller…
JuniorMiddleAILast updated:Embeddings are vector representations of unstructured data, such as text, in the form of fixed-length numerical vectors. Such a vector encodes the semantic meaning of an object so that semantically similar objects are…
SeniorAILast updated:Triggers in databases are special procedures that are automatically executed in response to a specific event (INSERT, UPDATE, DELETE) on a table or view. Let's walk through an example Imagine we have a users table and a…
JuniorMiddleDatabasesLast updated:This question often comes up in interviews for AI Engineer roles, especially for products that work with agentic systems. It's worth stressing right away that these conversations are usually not about implementation in…
AIMiddleSeniorLast updated:FULL JOIN is a type of JOIN that returns all rows from both tables. If the tables have a match, the rows are combined into a single result row. If there is no match, the missing values are filled with NULL. In other…
JuniorMiddleDatabasesLast updated:INNER JOIN is a type of JOIN that returns only the rows that have a match in both tables. In other words, if a value in one table has no corresponding value in the other table, that row will not appear in the result.…
JuniorMiddleDatabasesLast updated:CROSS JOIN is a type of JOIN that returns all possible combinations of rows from two tables. In other words, every row from the first table is paired with every row from the second table. As a result, the number of rows…
JuniorMiddleDatabasesLast updated:A polymorphic relationship is a type of relationship where a single record can be associated with records from different tables through a single universal structure. In simpler terms, one table can reference different…
Junior/Middle/SeniorDatabasesLast updated:Foreign key is a column in a table that references a primary key in another table. The most important thing to understand is that a foreign key is not just an ID value pointing to a specific record in another table -…
DatabasesJuniorMiddleLast updated:This question often stumps even senior developers, although it is actually very simple. When you hear it, there's no need to overcomplicate things with answers like: there are arrays, objects, functions… - because from…
JuniorJavaScriptLast updated:You are unlikely to be asked this question directly, because technical interviews usually focus on classic structures like Map/Set. But if you also mention WeakMap, WeakSet, and WeakRef, it will almost certainly earn…
Junior/Middle/SeniorJavaScriptLast updated:One of those questions that can leave a Node.js developer slightly stunned. For some reason, SQL and an understanding of relational databases have become the Achilles’ heel of backend development in the JavaScript…
DatabasesJunior/Middle/SeniorLast updated:This is an old chestnut that still comes up in interviews, usually at companies where people have clearly gotten stuck in their ways and missed a ton of JS updates. In general, there are three most common ways to clone…
JuniorMiddleJavaScriptLast updated:Both methods work with an array of promises: they run them in parallel and wait for them to finish. However, they behave differently, so it is important, both in interviews and in real-world work, to understand the…
JavaScriptJuniorMiddleLast updated:It's already 2026, but the question about promisification still pops up in interviews, mostly for Junior-інтервʼю. To understand how this workaround works, we first need to look at the problem it solves. Callback hell…
JuniorJavaScriptLast updated:The difference between null and undefined is a classic question for junior developer positions. At their core they are similar and both mean that there is no value, but if you dig a little deeper, the difference becomes…
JuniorJavaScriptLast updated:This question is more related to computer science, but it’s asked quite often in Node.js interviews, so it makes sense to cover it in this category. First, let’s define a process and a thread. Process is a separately…
Node.jsMiddleSeniorLast updated:This is one of the must-know questions in Senior interviews for projects involving distributed systems. It’s a theoretical question, but it shows how deeply you’ve gone into system design and microservices architecture.…
SeniorSystem designMicroservicesLast updated:This question comes up in almost every Frontend or Backend interview at the junior/middle level. In short, an Execution Context is the environment in which our code is currently running; put simply, it is the value of…
JuniorMiddleJavaScriptLast updated:AbortController is a controller that lets you cancel one or more asynchronous operations, as long as those operations support AbortSignal. It creates a signal object that you pass into the operation, and then at the…
Junior/Middle/SeniorReact.jsJavaScriptLast updated:One of those questions that helps determine whether you’ve worked with complex systems. The first thing to understand is that a Circuit Breaker is a pattern that stops a system from attempting an operation that is very…
SeniorMicroservicesSystem designLast updated:This question comes up quite often in Frontend interviews, and it is frequently asked on projects that were started 5-7 years ago. That was exactly the time when functional programming was at its peak in the JS world,…
Junior/Middle/SeniorJavaScriptLast updated:This is a fairly common question in Senior-level interviews, and it is a standard way to check whether a candidate keeps up with security trends. Usually, you will first be asked about the essence of the project itself,…
SeniorSecurityLast updated:Question is often asked in Junior/Middle interviews and it shows how deeply you’ve understood why React Hooks were introduced in the first place. If we go back to the era of class components, we can see an obvious…
JuniorMiddleReact.jsLast updated:This question often comes up in conversations about authentication/authorization, the difference between HTTP and HTTPS, and TLS. Below we briefly go over how symmetric and asymmetric keys differ, along with their…
Junior/Middle/SeniorSecurityLast updated:This question often comes up in backend or full-stack interviews, especially for projects involving distributed systems. Each specific technology (SQS, RabbitMQ, Kafka, etc.) has its own approach to dead-letter queues…
System designSeniorLast updated:One of the common interview questions for a Node.js developer position. Most often, it comes up in discussions about the difference between HTTP and HTTPS or in the context of authorization, especially JWT. First, you…
Junior/Middle/SeniorSecurityLast updated:Noisy tenants are a common problem in multi-tenant systems when one “noisy” tenant (customer) starts consuming a disproportionate amount of resources, such as CPU, memory, or queue capacity, or exceeding API limits. As…
SeniorSystem designLast updated:Throttling is a programming technique that limits how often something can run. Put simply, we specify that the code can’t be executed more often than once every N milliseconds. The best way to understand throttling is…
API DesignSystem designMiddleSeniorLast updated:Debounce is a very common technique that delays the execution of a function until an event has stopped firing for a certain amount of time. The simplest example of its use is a search field. Imagine input where the user…
JuniorJavaScriptReact.jsOptimizationLast updated:Consistency in ACID means that every transaction moves the database from one valid state to another valid state without breaking the rules and logic of the system. When it comes to implementing Consistency, the most…
ACIDMiddleSeniorLast updated:Destructuring is syntactic sugar that lets you extract values from arrays or objects into separate variables, which makes code shorter and more readable. This is a fairly simple question, so for the most part you just…
JavaScriptJuniorMiddleLast updated:This question comes up quite often in interviews. It may be phrased exactly like the title, or it may be asked like this: "Can we change this?" The answer is yes, we can, and for that JS has built-in methods such as…
JuniorMiddleJavaScriptLast updated:This is a fairly common interview question if React is part of the project's stack. It's worth starting with the fact that both hooks, useEffect and useLayoutEffect, are used to work with side effects, and the main…
React.jsJunior/Middle/SeniorLast updated:Atomicity means that a transaction either completes in full or does not execute at all. If an error occurs during execution after the transaction has already changed something in the database, those changes НЕ be…
ACIDMiddleSeniorLast updated:When you're asked which optimization techniques you know, not mentioning React.lazy would be злочином. React.lazy() is one of the approaches to optimizing the initial page load in React. Put simply, it's an API that…
Junior/Middle/SeniorReact.jsLast updated:Most likely, in an interview this question will sound like this: have you heard anything about Suspense. It is also often asked in the context of a conversation about optimization, and in particular React.lazy(). If you…
Junior/Middle/SeniorReact.jsLast updated:Questions about code splitting often come up in the context of optimizing frontend applications. This approach is used every day, so you need to understand it for more than just interviews. Code splitting is a page load…
MiddleSeniorOptimizationLast updated:This question usually comes up in a conversation about resource optimization in SPA and hybrid apps, which often use resources inefficiently. If you aren't asked about React.memo directly, it's good practice to bring up…
React.jsMiddleSeniorLast updated:As in other programming languages, an SQL function must return a value. Conceptually, it is used to encapsulate business logic for later reuse. A function can also be used inside a query. Let's look at an example…
DatabasesJuniorMiddleLast updated:Put simply, never means something that will never happen. You probably thought of the void type just now and wondered what the difference between them is? If you look at these types from the perspective of functions,…
Junior/Middle/SeniorTypeScriptLast updated:Generics are a way to make code reusable through type abstraction. If you have a part of your system (a class, function, interface, etc.) that works with a single type, Generics allow it to work with multiple types.…
Junior/Middle/SeniorTypeScriptLast updated:This is a very common question in TypeScript conversations. If you hear it in an interview, it can be a good sign that TS is used in the project for real, not just for show. Type Narrowing means refining a type for…
MiddleSeniorTypeScriptLast updated:This question is usually asked in the context of a conversation about functions, namely the difference between arrow functions and regular ones, or when discussing the function execution context (this). So let's break…
JavaScriptJuniorMiddleLast updated:You can hear the question about regular functions versus arrow functions in almost every interview. There are several differences, and now we will go through them in detail: Syntactic sugar The most obvious, but not the…
JuniorMiddleJavaScriptLast updated:Quite a few interview questions are about array methods. You may be asked which methods you know, for example. And once you finish listing them, one of the follow-up questions might be: "What is the difference between…
JavaScriptJuniorLast updated:The N + 1 problem in GraphQL is a classic question when discussing GraphQL API design or comparing it with REST. It occurs when a single GraphQL query results in a large number of small database queries. Let's look at a…
API DesignMiddleSeniorLast updated:Memoization is an optimization technique where we store the result of a function call for specific input values, and on subsequent calls with the same arguments we return the already computed result instead of…
JavaScriptJuniorMiddleLast updated:Immutability is a programming approach in which data is not changed after it is created. Instead of modifying existing data structures, a new copy with the updated values is created. This way, we avoid unexpected…
JuniorMiddleJavaScriptLast updated:Let's start with the fact that they share the same goal: reducing the number of unnecessary component renders, which lowers CPU usage and, in theory, saves you from potential performance problems. This matters…
JuniorMiddleJavaScriptReact.jsLast updated:A pure function is a function that always returns the same result for the same input and has no side effects. The definition can be confusing, so let's go through examples of each of these characteristics: The same…
JavaScriptJuniorLast updated:This is one of those old chestnuts that these days you'll mostly meet only in interviews at so-called "trash galley" companies. Most often, the question is split into 2 parts: First, you'll be asked what gets logged to…
JavaScriptJuniorMiddleStupid interview questionsLast updated:This question often comes up when discussing Promises or the Event Loop. In short, there is no difference from a technical standpoint. You could say that async/await is syntactic sugar on top of chaining that lets you…
JavaScriptJuniorMiddleLast updated:RIGHT JOIN is a type of JOIN that returns all rows from the right table, even if there are no matching rows in the left table. If a match exists in the left table, its data is included; if not, the left table's columns…
DatabasesJuniorMiddleLast updated:This question often comes up in the context of optimizing the frontend of a product. Async and defer are attributes used on the script tag to include external JS files. Both allow the browser to NOT block HTML parsing…
MiddleSeniorJavaScriptLast updated:First, let’s clarify what algorithmic complexity is. It’s a way to estimate how many resources an algorithm uses as the input size grows. There are two ways to measure an algorithm’s complexity. Time Complexity The time…
Algorithms and data structuresJunior/Middle/SeniorLast updated:LEFT JOIN is a type of join that returns all rows from the left table, even if there are no matching rows in the right table. If a match exists in the right table, its data is included. If there is no match, the columns…
JuniorMiddleDatabasesLast updated:This question falls into the category of so-called Tricky Questions. It is asked fairly rarely, and most often when the interviewer sees ElasticSearch in your CV, just to check whether you have really worked with it.…
Tricky QuestionsSeniorLast updated:Most likely, you will hear this question in a conversation about REST API design. It's important to understand that both methods are used to update a resource, but they differ in semantics. How the resource is updated…
API DesignMiddleLast updated:So, let’s start with the definition of idempotency. It’s a concept that guarantees that repeating an operation multiple times has the same effect as performing it once. Most often, this question comes up in the context…
System designMiddleSeniorLast updated:Besides being a common interview question, understanding this difference is also useful in day-to-day work. Any Let's start with any. Put simply, when we use it, we turn TypeScript off. let num: any = 1; // ми…
TypeScriptJunior/Middle/SeniorLast updated:Most likely, you’ll be asked this question in the context of discussing indexes, and they’ll expect you to understand that the order in which an index is created and used matters. Let’s start with creation When creating…
SeniorDatabasesMiddleLast updated:This is probably a must-have interview question, especially for Junior and Middle developers. If you get asked it, your goal is to answer in an unexpected way. First, explain in simple terms what a closure is. In simple…
JuniorMiddleJavaScriptLast updated:This question isn't asked often, but it's still relevant, so it's worth understanding the main differences: Scope var has function scope, so if we declare a var variable inside a {} block, it is still visible throughout…
JavaScriptJuniorLast updated:This is a common interview question, especially for Junior and Middle positions. In general, what they usually want to hear in an interview is interface merging. Let’s break it down with an example: interface IUser {…
TypeScriptMiddleJuniorLast updated:
