Live Demo — Mentaro

Mentaro demo questions

Node.js · Advanced

How do process.nextTick, promise microtasks, timers, and setImmediate interact across Node.js event loop phases, and how can their ordering cause starvation?

Node.js drains the nextTick queue before the promise microtask queue after the current JavaScript operation and between event loop callbacks. Timers run in the timers phase, while setImmediate callbacks run in the check phase. Their relative order can depend on the surrounding I/O context. Recursively scheduling nextTick callbacks can prevent the event loop from reaching I/O and timer phases, which starves other work.

Node.js · Expert

How does AsyncLocalStorage propagate request context through asynchronous operations, and which integration patterns can break or leak that context?

AsyncLocalStorage associates a store with an asynchronous execution chain using async hooks. Promises and standard Node.js callback APIs usually preserve it automatically. Context can be lost when libraries create unusual async boundaries, callbacks are invoked outside the captured resource, or enterWith is used across overlapping work. run should define a bounded request scope, and AsyncResource can bridge custom callback or event integrations.

Node.js · Advanced

Explain how backpressure works in Node.js streams and what a custom producer must do when writable.write() returns false.

A writable buffers chunks up to its highWaterMark. write returning false means the producer should stop writing, not that the chunk was rejected. The producer resumes after the drain event. Readable streams similarly stop pulling when push returns false. stream.pipeline coordinates flow control and propagates completion and errors across the chain.

TypeScript · Expert

Why do conditional types distribute over unions, how can distribution be disabled, and where does this behavior become surprising in reusable APIs?

A conditional type distributes when its checked type is a naked type parameter, so T extends U ? X : Y is evaluated for each union member. Wrapping both sides in tuples, such as [T] extends [U], tests the union as a whole. Distribution is useful for union mapping but surprising when an API intends to reject or preserve an entire union, especially with never because distributing over an empty union produces never.

Node.js · Expert

When do worker threads improve throughput in a Node.js service, and how would you size and operate a worker pool without moving the bottleneck elsewhere?

Workers help CPU-bound JavaScript and native work that would otherwise block the event loop; they rarely improve ordinary asynchronous I/O. A bounded pool should account for available CPU, container quotas, task duration, serialization or transfer costs, memory per isolate, queue limits, cancellation, and overload policy. Measure event loop delay, queue latency, utilization, memory, and end-to-end throughput before increasing concurrency.

TypeScript · Advanced

How do variance and strictFunctionTypes affect assignability of callbacks in TypeScript, and why can method parameters still permit unsound assignments?

With strictFunctionTypes, standalone function parameters are checked contravariantly, preventing a callback that accepts only a narrower subtype from being used where a broader input may arrive. Method syntax remains bivariant for compatibility with common object and class patterns. This exception can admit assignments that fail at runtime, so callback-heavy public APIs can use function properties when stricter checking is desirable.