Node.js runtime Basic

Many performance problems in a Node.js service are not in the database or the network, but in a piece of JavaScript holding the main thread, or in data arriving faster than its destination can write it. This section gathers three foundations of the runtime.

Scope of this section: only the Node.js runtime. There are no TypeScript articles yet (type system, tsconfig configuration, builds); they will be added here when they exist.

Reading path

Order	Article	Level	Question it answers
1	Node.js event loop	Intermediate	In what order do callbacks run, why are timers late, what blocks the main thread?
2	Streams and backpressure	Intermediate	What happens to memory when data arrives faster than the destination writes?
3	AsyncLocalStorage	Intermediate	How do you keep a `requestId` or tenant per request when many requests interleave?

Pick an article by symptom

Symptom	Start with
Every route is slow at the same time, even the one that returns `pong`	Event loop (the main thread is occupied)
`setTimeout(fn, 0)` runs late or the `setImmediate` order is hard to predict	Event loop
RSS grows with the number of uploads or slow clients	Streams and backpressure
Logs from one request carry another request’s `requestId`	AsyncLocalStorage

If you use NestJS, the request lifecycle and provider scopes assume the model above: a CPU-bound handler slows every request in the same process, no matter how the pipeline is configured.