Converts a string to a URL-friendly slug (TypeScript)

slugify.ts

/**
 * Converts a string to a URL-friendly slug by normalizing Unicode characters,
 * filtering to allowed characters, and cleaning up the result.
 *
 * @param input - The input string to slugify (supports UTF-8)
 * @param allowedChars - Regex character class of allowed characters (default: letters, digits, underscore)
 * @returns A normalized, lowercase slug with no leading/trailing underscores
 *
 * @example
 * slugify("[email protected]") // "saint_14_vanguard_tower"
 * slugify("Café & Restaurant") // "cafe_restaurant"
 * slugify("Product #123", "a-zA-Z0-9\\-") // "product-123"
 */
function slugify(input: string, allowedChars: string = "a-zA-Z0-9_"): string {
  // Create regex to match characters NOT in the allowed set
  const regex = new RegExp(`[^${allowedChars}]`, "g");

  return (
    input
      // Normalize Unicode to decomposed form (separates base chars from diacritics)
      .normalize("NFKD")
      // Replace disallowed characters with underscores
      .replace(regex, "_")
      // Convert to lowercase
      .toLowerCase()
      // Collapse multiple consecutive underscores into single underscore
      .replace(/_{2,}/g, "_")
      // Remove leading and trailing underscores
      .replace(/^_+|_+$/g, "")
  );
}