> For the complete documentation index, see [llms.txt](https://www.rxjs.es/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.rxjs.es/operadores/filtering/first.md).

# first

<details>

<summary>Signatura</summary>

#### Firma

`first<T, D>(predicate?: (value: T, index: number, source: Observable<T>) => boolean, defaultValue?: D): OperatorFunction<T, T | D>`

#### Parámetros

#### Retorna

`OperatorFunction<T, T | D>`: Un Observable del primer elemento que cumpla la condición especificada.

#### Lanza

`EmptyError`: Lanza un `EmptyError` si el Observable se completa sin emitir ninguna notificación `next`.

</details>

## Descripción

Emite únicamente el primer valor. O emite el primer valor que cumpla alguna condición.

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

Si se llama sin ningún argumento, `first` emite el primer valor del Observable fuente y se completa. Si se llama con una función `predicate`, `first` emite el valor de la fuente que cumpla la condición especificada. También puede recibir un valor por defecto, que se emite en el caso de que la fuente se complete sin emitir ningún elemento válido. Lanza un error en el caso de que no se encuentre un elemento válido y no se haya proporcionado un `defaultValue` .

## Ejemplos

**Emitir la primera cadena de una secuencia**

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

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

const fruit$ = from(["Cereza", "Fresa", "Arándano"]);

fruit$.pipe(first()).subscribe(console.log);
// Salida: Cereza
```

**Emitir la primera tecla pulsada**

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

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

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

keyPressed$
  .pipe(
    first(),
    map(({ code }) => code)
  )
  .subscribe(console.log);
// Salida: KeyX
```

**Emitir el primer elemento que cumpla una condición**

[StackBlitz](https://stackblitz.com/edit/rxjs-first-3?file=index.ts)

```javascript
import { of } from "rxjs";
import { first } from "rxjs/operators";

const user$ = of(
  { name: "NyaGarcía", age: 23 },
  { name: "zaldih", age: 21 },
  { name: "caballerog", age: 35 }
);

user$.pipe(first(({ age }) => age === 21)).subscribe(console.log);
// Salida: { name: 'zaldih', age: 21 }
```

**Proporcionar un valor por defecto, que será emitido si ningún elemento cumple la condición**

[StackBlitz](https://stackblitz.com/edit/rxjs-first-4?file=index.ts)

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

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

language$
  .pipe(
    first(({ type }) => type === "Orientado a objetos", {
      name: "Java",
      type: "Orientado a objetos",
    })
  )
  .subscribe(console.log);
// Salida: { name: "Java", type: "Orientado a objetos" }
```

### Ejemplos de la documentación oficial

**Emitir solo el primer click que ocurra en el DOM**

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

const clicks = fromEvent(document, "click");
const result = clicks.pipe(first());
result.subscribe((x) => console.log(x));
```

**Emitir el primer click que ocurra en un DIV**

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

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

<details>

<summary>Sobrecargas</summary>

#### Firma

`first(predicate?: null, defaultValue?: D): OperatorFunction<T, T | D>`

#### Parameters

#### Retorna

`OperatorFunction<T, T | D>`

#### Firma

`first(predicate: (value: T, index: number, source: Observable<T>) => value is S, defaultValue?: S): OperatorFunction<T, S>`

#### Parameters

#### Retorna

`OperatorFunction<T, S>`

#### Firma

`first(predicate: (value: T, index: number, source: Observable<T>) => boolean, defaultValue?: D): OperatorFunction<T, T | D>`

#### Parameters

#### Retorna

`OperatorFunction<T, T | D>`

</details>

### Recursos adicionales

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

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


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://www.rxjs.es/operadores/filtering/first.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.