# findIndex

<details>

<summary>Signatura</summary>

#### Firma

`findIndex<T>(predicate: (value: T, index: number, source: Observable<T>) => boolean, thisArg?: any): OperatorFunction<T, number>`

#### Parámetros

#### Retorna

`OperatorFunction<T, number>`: Un Observable del índice del primer elemento que cumpla la condición.

</details>

## Descripción

Es como [find](https://github.com/puntotech/rxjs-docu/blob/master/operators/conditional/find/README.md), pero emite el índice del valor encontrado, en lugar del propio valor.

![Diagrama de canicas del operador findIndex](https://github.com/puntotech/rxjs-docu/blob/master/doc/operators/conditional/assets/images/marble-diagrams/conditional-boolean/findIndex.png)

findIndex busca el primer elemento del Observable fuente que cumpla la condición especificada en el predicado y retorna su índice (de base cero.) Al contrario que [first](https://github.com/puntotech/rxjs-docu/blob/master/operators/filtering/first/README.md), el predicado es obligatorio en findIndex, y tampoco emite un error si no encuentra un valor válido.

## Ejemplos

**Emitir el índice del primer lenguaje de tipo Multiparadigma**

[StackBlitz](https://stackblitz.com/edit/rxjs-findindex-1?file=index.ts)

```javascript
import { findIndex } from "rxjs/operators";
import { from } from "rxjs";

const language$ = from([
  { name: "Java", type: "Orientado a objetos" },
  { name: "Ruby", type: "Multiparadigma" },
  { name: "Haskell", type: "Funcional" },
  { name: "Rust", type: "Multiparadigma" },
]);

language$
  .pipe(findIndex(({ type }) => type === "Multiparadigma"))
  .subscribe(console.log);
// Salida: 1
```

**Emite el índice de la primera vez que se pulse la tecla x**

[StackBlitz](https://stackblitz.com/edit/rxjs-findindex-2?file=index.ts)

```typescript
import { findIndex, map } from "rxjs/operators";
import { fromEvent } from "rxjs";

const key$ = fromEvent<KeyboardEvent>(document, "keydown");

key$
  .pipe(
    map(({ code }) => code),
    findIndex((code) => code === "KeyX")
  )
  .subscribe(console.log);
// Salida: (Pulsar tecla n) (Pulsar tecla f) (Pulsar tecla x) 2
```

### Ejemplo de la documentación oficial

**Emite el índice del primer click que ocurre en un elemento DIV**

```javascript
import { fromEvent } from "rxjs";
import { findIndex } from "rxjs/operators";

const clicks = fromEvent(document, "click");
const result = clicks.pipe(findIndex((ev) => ev.target.tagName === "DIV"));
result.subscribe((x) => console.log(x));
```

### Recursos adicionales

[![Source code](https://github.com/puntotech/rxjs-docu/blob/master/doc/operators/conditional/assets/icons/source-code.png)](https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/findIndex.ts)

[Documentación oficial en inglés](https://rxjs.dev/api/operators/findIndex)


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