Conas a chinntear luach `this`?

Question

Accepted Answer

`this` a chinntear de réir **conas a bhítear ag GLAOCH ar fheidhm**, ní áit a sainmhínitear í. Tá cúig riail ann:

```js
const obj = {
  x: 1,
  getX() { return this.x; },
};

obj.getX();          // 1   → method call: `this` is `obj`
const f = obj.getX;
f();                  // undefined (or window) → plain call: `this` is global/undefined
f.call(obj);          // 1   → explicit binding with call/apply/bind
new SomeClass();      //     → `this` is the brand-new instance
```

## An eisceacht feidhm saighead (an ceann tábhachtach)

Nilíonn `this` féin ag feidhmeanna saighead — fhaigheann siad ó scóp an timpeallachta ag am sainmhínithe. Sin an fáth go bhfuil saighead oiriúnach do ghlaoigh siar:

```js
const obj = {
  items: [1, 2],
  logAll() {
    // ✅ arrow inherits `this` from logAll → `this` is obj
    this.items.forEach(i => console.log(this.items, i));

// ❌ a regular function here would have its own `this` (undefined) → crash
    // this.items.forEach(function (i) { console.log(this.items); });
  },
};
```

## An gal choitianta

A bheith ag pas modh mar ghlaoch siar **cailltear** a bhinding, toisc go ndéantar glaoch simplí air ansin:

```js
button.addEventListener("click", obj.getX);        // ❌ `this` is the button
button.addEventListener("click", () => obj.getX()); // ✅ keeps obj
button.addEventListener("click", obj.getX.bind(obj)); // ✅ also fine
```

## Cén fáth a bhíonn sé tábhachtach

Is ceann de na fabhtanna JS is coitianta gan a thuiscint `this`.

Is gnách go bhíonn an leigheas ina fheidhm saighead nó `.bind()` chun an `this` atá i gceist a bhacaint.