Mi az a process objektum és mit tud csinálni?

Question

Accepted Answer

**`process`** is a global object that provides information about, and control over, the **current Node.js process**. It's your interface to the environment, command-line arguments, I/O streams, lifecycle, and signals.

## Környezeti változók olvasása

```js
process.env.NODE_ENV;        // "production" | "development"
process.env.DATABASE_URL;    // configuration & secrets injected by the environment
const port = process.env.PORT || 3000;
```

`process.env` is the standard way to read configuration and secrets (set outside the code) — central to twelve-factor config.

## Parancssori argumentumok

```js
// node script.js --name=Ann
process.argv; // ["node", "/path/script.js", "--name=Ann"]
const args = process.argv.slice(2); // skip the first two
```

## Szabványos I/O streamek

```js
process.stdout.write("output
");   // console.log writes here
process.stderr.write("error
");    // console.error writes here
process.stdin.on("data", (chunk) => { /* read input */ });
```

## Folyamat életciklusa és kilépés

```js
process.exit(0);   // exit with success code (1 = error)
process.exitCode = 1; // preferred: set code, let the process finish naturally
process.pid;       // the process id
process.cwd();     // current working directory
process.uptime();  // seconds the process has run
```

## Jelek és nem kapott hibák kezelése (a kecses leállítás / biztonsági hálók érdekében)

```js
process.on("SIGTERM", () => { /* clean up, then exit — e.g. on container stop */ });
process.on("SIGINT", () => { /* Ctrl+C */ });
process.on("uncaughtException", (err) => { log(err); process.exit(1); });
process.on("unhandledRejection", (reason) => { log(reason); });
```

Listening for `SIGTERM`/`SIGINT` is how you implement **graceful shutdown** (finish in-flight requests, close DB connections before exiting), and the uncaught-error handlers are last-resort safety nets.

## Egyéb hasznos tagok

```js
process.memoryUsage(); // heap stats (leak detection)
process.nextTick(cb);  // schedule a callback before the next event-loop phase
process.platform;      // "linux" | "darwin" | "win32"
```

## Miért fontos ez

`process` is your program's connection to its runtime environment.

You use it constantly: reading config/secrets via `process.env`, handling CLI args, and — importantly for production — catching signals for graceful shutdown and last-resort error handlers.

Understanding it is essential for building configurable, robust, production-ready Node applications.