feat(js): add solution and documentation for easy challenge 4 - Factorial calculator

Author: IbatoLionDev

1-EASY/JavaScript/4-factorial-calculator/article.md

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+# Challenge Description and Solution
+
+## English Version
+
+### Challenge Description
+Develop a recursive or iterative function to calculate the factorial of a non-negative integer. Consider handling special cases such as negative numbers.
+
+### Code Explanation
+The code presents two functions to calculate the factorial:
+
+- `factorialRecursive(n)`: Calculates the factorial recursively. If `n` is negative, it throws an error. If `n` is 0 or 1, it returns 1. Otherwise, it returns `n` multiplied by the factorial of `n-1`.
+
+- `factorialIterative(n)`: Calculates the factorial iteratively. If `n` is negative, it throws an error. It initializes `result` to 1 and successively multiplies by each number from 2 to `n`.
+
+### Relevant Code Snippet
+
+```javascript
+function factorialRecursive(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    if (n === 0 || n === 1) {
+        return 1;
+    }
+    return n * factorialRecursive(n - 1);
+}
+
+function factorialIterative(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    let result = 1;
+    for (let i = 2; i <= n; i++) {
+        result *= i;
+    }
+    return result;
+}
+```
+
+### Example Usage
+
+```javascript
+const testValues = [0, 1, 5, 7];
+testValues.forEach(val => {
+    console.log(`Factorial of ${val} (recursive): ${factorialRecursive(val)}`);
+    console.log(`Factorial of ${val} (iterative): ${factorialIterative(val)}`);
+});
+```
+
+---
+
+## Versión en Español
+
+### Descripción del Reto
+Desarrolla una función recursiva o iterativa para calcular el factorial de un número entero no negativo. Considera el manejo de casos especiales como números negativos.
+
+### Explicación del Código
+El código presenta dos funciones para calcular el factorial:
+
+- `factorialRecursive(n)`: Calcula el factorial de forma recursiva. Si `n` es negativo, lanza un error. Si `n` es 0 o 1, retorna 1. En otro caso, retorna `n` multiplicado por el factorial de `n-1`.
+
+- `factorialIterative(n)`: Calcula el factorial de forma iterativa. Si `n` es negativo, lanza un error. Inicializa `result` en 1 y multiplica sucesivamente por cada número desde 2 hasta `n`.
+
+### Fragmento de Código Relevante
+
+```javascript
+function factorialRecursive(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    if (n === 0 || n === 1) {
+        return 1;
+    }
+    return n * factorialRecursive(n - 1);
+}
+
+function factorialIterative(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    let result = 1;
+    for (let i = 2; i <= n; i++) {
+        result *= i;
+    }
+    return result;
+}
+```
+
+### Ejemplo de Uso
+
+```javascript
+const testValues = [0, 1, 5, 7];
+testValues.forEach(val => {
+    console.log(`Factorial de ${val} (recursivo): ${factorialRecursive(val)}`);
+    console.log(`Factorial de ${val} (iterativo): ${factorialIterative(val)}`);
+});

1-EASY/JavaScript/4-factorial-calculator/proposed-solution/main.js

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+// Challenge: Develop a recursive or iterative function to calculate the factorial of a non-negative integer.
+// Consider handling special cases such as negative numbers.
+
+// Recursive factorial function
+function factorialRecursive(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    if (n === 0 || n === 1) {
+        return 1;
+    }
+    return n * factorialRecursive(n - 1);
+}
+
+// Iterative factorial function
+function factorialIterative(n) {
+    if (n < 0) {
+        throw new Error("Factorial is not defined for negative numbers");
+    }
+    let result = 1;
+    for (let i = 2; i <= n; i++) {
+        result *= i;
+    }
+    return result;
+}
+
+// Example usage
+const testValues = [0, 1, 5, 7];
+testValues.forEach(val => {
+    console.log(`Factorial of ${val} (recursive): ${factorialRecursive(val)}`);
+    console.log(`Factorial of ${val} (iterative): ${factorialIterative(val)}`);
+});