Node.js internals quick intro

class: center, middle

# Node.js feature livecoding

Anna Henningsen <br />
she/her <br />
@addaleax

Slides @ https://addaleax.net/livecoding-nodeconfco19/

<br />

Node.js TSC <br />
Active Node.js contributor since December 2015

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# Node.js feature: setTimeout()

#### Where does `setTimeout()` come from???

(Hint: It’s *not* the language!)

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# Node.js feature: setTimeout()

- `setTimeout()`: Managed by event loop → libuv

```js
const socket = net.connect(80, 'google.com', () => {
  console.log('connected');
});

setTimeout(() => {
  socket.end();
}, 1000);
```

What does the event loop do here?

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# Node.js feature: setTimeout()

- `setTimeout()`: Managed by event loop → libuv
- libuv central concepts: handles vs requests
  - `uv_handle_t`: Long-lasting, multi-event (e.g. net Socket)
  - `uv_req_t`: Oneshot (e.g. net connect, fs ops)
- libuv provides timers as …
--
 **handles**

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# Data stored for a timer

- Actual timeout function
- libuv data (`uv_timer_t`)
  - includes event loop, timeout duration, native callback, etc.
- C++ object
- JS object
- Async tracking

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# Node.js helper classes

(All entirely internal, may change anytime)

- `MemoryRetainer` – C++ objects that show up in heap dump
    - `BaseObject` – JS ↔ C++ binding connection
        - `AsyncWrap` – C++ objects that enter JS
            - `ReqWrap` – Associated with libuv requests
            - `HandleWrap` – Associated with libuv handles
            - …
        - …
    - …

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# `HandleWrap` comes builtin with …

- Heap dump detection ability (`MemoryRetainer`)
- A JS object (`BaseObject`)
- Async tracking (`AsyncWrap`)
- Methods for libuv handles (`.ref()`, `.unref()`, `.close()`)

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# What we want to write in C++ (in JS)

```js
class TimerWrap extends HandleWrap {
  constructor(env, object /* actual JS object */) {
    super(env, object, this.timer, PROVIDER_TIMER);
    this.timer.init(env.event_loop());
  }

start(timeout_ms) {
    this.timer.start(TimerWrap.Callback, timeout_ms, 0);
  }

static Callback(timer) {
    timer.object.ontimeout();
  }
}
```

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# Node.js layers

- Public modules: e.g. `timers` as `lib/timers.js`
- Internal modules: `lib/internal/...`
- C++ modules: `src/...`, accessed through `internalBinding()`

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# Thank you! Now for the exciting part :)

- https://twitter.com/addaleax/

Slides @ https://addaleax.net/livecoding-nodeconfco19/