Backend Developer 路 Intern to senior
Backend interviews test how you store, process and serve data reliably: language fundamentals, SQL and indexing, concurrency, API design and how systems behave under load.
131 questions
Walk through how you noticed the issue, limited the impact, found the root cause, fixed it, and what you changed afterwards so the same class of bug could not happen again.
Choose a real mistake, take ownership, explain the impact, what you did to recover, and what you changed so it would not happen again.
Describe feedback that was uncomfortable but useful, explain how you responded, and show the measurable or observable improvement that followed.
Choose a genuine failure, explain why it failed, what you owned, how you recovered, and what you changed in future projects.
Show how you prioritized the essential outcome, communicated trade-offs, reduced risk and delivered without pretending everything could be done.
Show that you can defend a technical decision with evidence while remaining open to a better argument.
Keep the answer professional and future-focused: explain what you are looking for next without attacking your current or former employer.
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.