# Referencia Rápida de JavaScript

*ES6+, DOM, eventos, Fetch API, async/await*

> Source: MDN Web Docs (developer.mozilla.org) · MIT

## Básico

### Variables

```
let name = "Alice";    // reassignable
const PI = 3.14;       // constant
var old = "avoid";     // function-scoped (legacy)
```

### Tipos de datos

| Command | Description |
|---------|-------------|
| `string` | Texto: `"hello"` o `'hello'` |
| `number` | Entero o decimal: `42`, `3.14` |
| `boolean` | `true` / `false` |
| `null` | Valor vacío intencional |
| `undefined` | Declarado pero sin asignar |
| `object` | Pares clave-valor: `{ a: 1 }` |
| `array` | Lista ordenada: `[1, 2, 3]` |
| `symbol` | Identificador único |

### Verificación y conversión de tipos

```
typeof "hello"     // "string"
typeof 42          // "number"
Number("42")       // 42
String(100)        // "100"
parseInt("3.9")    // 3
parseFloat("3.14") // 3.14
```

## Strings

### Template Literals

```
const name = "Alice";
`Hello, ${name}!`       // Hello, Alice!
`Total: ${2 + 3}`       // Total: 5
`Multi
 line string`
```

### Métodos de string

| Command | Description |
|---------|-------------|
| `s.length` | Número de caracteres |
| `s.toUpperCase()` | Copia en MAYÚSCULAS |
| `s.toLowerCase()` | Copia en minúsculas |
| `s.trim()` | Eliminar espacios iniciales y finales |
| `s.split(",")` | Dividir en array |
| `s.includes("x")` | Verificar si contiene → bool |
| `s.indexOf("x")` | Primer índice (-1 si no existe) |
| `s.slice(1, 4)` | Subcadena por índice |
| `s.replace(a, b)` | Reemplazar primera coincidencia |
| `s.replaceAll(a, b)` | Reemplazar todas las coincidencias |
| `s.startsWith(x)` | Verificar prefijo → bool |
| `s.endsWith(x)` | Verificar sufijo → bool |
| `s.padStart(n, c)` | Rellenar al inicio hasta longitud n |

## Arrays

### Crear y acceder

```
const fruits = ["apple", "banana", "cherry"];
fruits[0]          // "apple"
fruits.length      // 3
fruits.at(-1)      // "cherry"
```

### Métodos que mutan

| Command | Description |
|---------|-------------|
| `arr.push(x)` | Agregar al final |
| `arr.pop()` | Eliminar y devolver el último |
| `arr.unshift(x)` | Agregar al inicio |
| `arr.shift()` | Eliminar y devolver el primero |
| `arr.splice(i, n)` | Eliminar n elementos en el índice i |
| `arr.sort()` | Ordenar en el lugar (lexicográfico) |
| `arr.reverse()` | Invertir en el lugar |

### Métodos que no mutan

| Command | Description |
|---------|-------------|
| `arr.map(fn)` | Transformar cada elemento |
| `arr.filter(fn)` | Mantener elementos donde fn sea verdadero |
| `arr.reduce(fn, init)` | Acumular en un único valor |
| `arr.find(fn)` | Primera coincidencia o undefined |
| `arr.findIndex(fn)` | Índice de la primera coincidencia (-1) |
| `arr.includes(x)` | Verificar si contiene → bool |
| `arr.slice(a, b)` | Copia superficial de una porción |
| `arr.join(",")` | Unir en string |
| `arr.forEach(fn)` | Iterar (sin valor de retorno) |
| `[...a, ...b]` | Concatenar arrays (spread) |

## Objetos

### Crear y acceder

```
const user = { name: "Alice", age: 20 };
user.name          // "Alice"
user["age"]        // 20
user.gpa = 3.85;   // add/update
```

### Destructuring y spread

```
const { name, age } = user;
const copy = { ...user, age: 21 };
```

### Métodos de objeto

| Command | Description |
|---------|-------------|
| `Object.keys(o)` | Array de claves |
| `Object.values(o)` | Array de valores |
| `Object.entries(o)` | Array de pares [clave, valor] |
| `Object.assign(t, s)` | Copiar propiedades de s a t |
| `"k" in obj` | ¿Existe la clave? → bool |
| `delete obj.k` | Eliminar propiedad |
| `Object.freeze(o)` | Hacer inmutable (superficial) |

## Flujo de control

### if / else if / else

```
if (score >= 90) {
  grade = "A";
} else if (score >= 80) {
  grade = "B";
} else {
  grade = "C";
}
```

### Ternario y coalescencia nula

```
const status = score >= 60 ? "pass" : "fail";
const name = user.name ?? "Anonymous";
```

### switch

```
switch (color) {
  case "red":   stop();   break;
  case "green": go();     break;
  default:      wait();
}
```

## Bucles

### for / for...of / for...in

```
for (let i = 0; i < 5; i++) { }

for (const item of ["a", "b"]) { }  // arrays

for (const key in obj) { }  // object keys
```

### while / do...while

```
while (count < 10) { count++; }

do { count++; } while (count < 10);
```

### break y continue

```
for (let i = 0; i < 10; i++) {
  if (i === 5) break;       // stop loop
  if (i % 2 === 0) continue; // skip
}
```

## Funciones

### Declaración y arrow function

```
function greet(name) {
  return `Hello, ${name}!`;
}
const greet = (name) => `Hello, ${name}!`;
const square = x => x * x; // single param
```

### Parámetros por defecto y rest

```
function greet(name = "World") { }

function sum(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}
```

### Callbacks

```
[1, 2, 3].map(x => x * 2);    // [2, 4, 6]
[1, 2, 3].filter(x => x > 1); // [2, 3]
setTimeout(() => console.log("done"), 1000);
```

## Clases

```
class Dog {
  constructor(name, breed) {
    this.name = name;
    this.breed = breed;
  }
  bark() { return `${this.name} says Woof!`; }
}

class Puppy extends Dog {
  constructor(name, breed, toy) {
    super(name, breed);
    this.toy = toy;
  }
}
```

## Manejo de errores

```
try {
  JSON.parse("bad json");
} catch (err) {
  console.error(err.message);
} finally {
  console.log("always runs");
}
```

### Lanzar errores

```
throw new Error("Something went wrong");
```

## DOM

### Seleccionar elementos

```
document.querySelector(".cls")    // first match
document.querySelectorAll("li")   // all matches
document.getElementById("main")
```

### Modificar elementos

```
el.textContent = "new text";
el.innerHTML = "<b>bold</b>";
el.style.color = "red";
el.classList.add("active");
el.classList.toggle("hidden");
el.setAttribute("data-id", "42");
```

### Eventos

```
btn.addEventListener("click", (e) => {
  console.log(e.target);
});
```

### Crear elementos

```
const li = document.createElement("li");
li.textContent = "New item";
ul.appendChild(li);
el.remove(); // remove element
```

## Fetch API

### Solicitud GET

```
fetch("https://api.example.com/data")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));
```

### Solicitud POST

```
fetch(url, {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ key: "value" }),
});
```

## Async / Await

```
async function loadData() {
  try {
    const res = await fetch(url);
    const data = await res.json();
    return data;
  } catch (err) {
    console.error(err);
  }
}
```

### Solicitudes en paralelo

```
const [users, posts] = await Promise.all([
  fetch("/users").then(r => r.json()),
  fetch("/posts").then(r => r.json()),
]);
```

## Módulos

### Exportaciones nombradas

```
// math.js
export const PI = 3.14;
export function add(a, b) { return a + b; }

// main.js
import { PI, add } from "./math.js";
```

### Exportación por defecto

```
// logger.js
export default function log(msg) { }

// main.js
import log from "./logger.js";
```