File Utilities

// Download from a URL
const downloadFromUrl = (url, filename) => {
  const a = document.createElement("a");
  a.href = url;
  a.download = filename || url.split("/").pop();
  document.body.appendChild(a);
  a.click();
  document.body.removeChild(a);
};

downloadFromUrl("https://example.com/report.pdf", "report.pdf");

// Download a string as a text file
const downloadText = (content, filename = "file.txt", type = "text/plain") => {
  const blob = new Blob([content], { type });
  const url = URL.createObjectURL(blob);
  downloadFromUrl(url, filename);
  URL.revokeObjectURL(url);
};

downloadText("Hello, World!", "hello.txt");
downloadText(JSON.stringify(data, null, 2), "data.json", "application/json");
downloadText("<h1>Hi</h1>", "page.html", "text/html");

const downloadCSV = (data, filename = "export.csv") => {
  if (!data.length) return;
  const headers = Object.keys(data[0]);
  const rows = data.map((row) =>
    headers.map((h) => JSON.stringify(row[h] ?? "")).join(",")
  );
  const csv = [headers.join(","), ...rows].join("\n");
  downloadText(csv, filename, "text/csv;charset=utf-8;");
};

downloadCSV([
  { name: "Alice", age: 30, city: "New York" },
  { name: "Bob",   age: 25, city: "London" },
], "users.csv");

const readFileAsText = (file) =>
  new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.onload  = (e) => resolve(e.target.result);
    reader.onerror = (e) => reject(new Error("Failed to read file"));
    reader.readAsText(file);
  });

// Usage
input.addEventListener("change", async (e) => {
  const file = e.target.files[0];
  const content = await readFileAsText(file);
  console.log(content); // full text content
});

const readFileAsDataURL = (file) =>
  new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.onload  = (e) => resolve(e.target.result);
    reader.onerror = () => reject(new Error("Failed to read file"));
    reader.readAsDataURL(file);
  });

// Image preview before upload
input.addEventListener("change", async (e) => {
  const file = e.target.files[0];
  const dataUrl = await readFileAsDataURL(file);
  img.src = dataUrl; // instant preview
});

const fileToBase64 = async (file) => {
  const dataUrl = await readFileAsDataURL(file);
  return dataUrl.split(",")[1]; // strip "data:image/png;base64,"
};

const base64 = await fileToBase64(file);
// "iVBORw0KGgoAAAANSUhEUgAA..."

const base64ToBlob = (base64, mimeType = "application/octet-stream") => {
  const binary = atob(base64);
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i);
  return new Blob([bytes], { type: mimeType });
};

const blob = base64ToBlob(base64String, "image/png");
const url = URL.createObjectURL(blob);
img.src = url;

const MIME_TYPES = {
  // Images
  jpg: "image/jpeg", jpeg: "image/jpeg", png: "image/png",
  gif: "image/gif",  webp: "image/webp", svg: "image/svg+xml",
  ico: "image/x-icon",
  // Documents
  pdf: "application/pdf",
  doc: "application/msword",
  docx: "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
  xls: "application/vnd.ms-excel",
  xlsx: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
  // Text
  txt: "text/plain", html: "text/html", css: "text/css",
  js: "text/javascript", ts: "text/typescript",
  json: "application/json", xml: "application/xml",
  csv: "text/csv", md: "text/markdown",
  // Archives
  zip: "application/zip", tar: "application/x-tar",
  gz: "application/gzip",
  // Audio / Video
  mp3: "audio/mpeg", wav: "audio/wav", ogg: "audio/ogg",
  mp4: "video/mp4", webm: "video/webm", avi: "video/x-msvideo",
};

const getMimeType = (filename) => {
  const ext = filename.split(".").pop().toLowerCase();
  return MIME_TYPES[ext] || "application/octet-stream";
};

getMimeType("photo.jpg");   // "image/jpeg"
getMimeType("report.pdf");  // "application/pdf"
getMimeType("data.json");   // "application/json"
getMimeType("unknown.xyz"); // "application/octet-stream"

const formatFileSize = (bytes, decimals = 1) => {
  if (bytes === 0) return "0 B";
  const units = ["B", "KB", "MB", "GB", "TB"];
  const i = Math.floor(Math.log(bytes) / Math.log(1024));
  return `${(bytes / 1024 ** i).toFixed(decimals)} ${units[i]}`;
};

formatFileSize(0);          // "0 B"
formatFileSize(1500);       // "1.5 KB"
formatFileSize(1048576);    // "1.0 MB"
formatFileSize(1073741824); // "1.0 GB"

const IMAGE_EXTS    = new Set(["jpg","jpeg","png","gif","webp","svg","ico","bmp","tiff"]);
const VIDEO_EXTS    = new Set(["mp4","webm","avi","mov","mkv","flv"]);
const DOCUMENT_EXTS = new Set(["pdf","doc","docx","xls","xlsx","ppt","pptx","txt","md"]);

const getExt = (filename) => filename.split(".").pop().toLowerCase();

const isImageFile    = (filename) => IMAGE_EXTS.has(getExt(filename));
const isVideoFile    = (filename) => VIDEO_EXTS.has(getExt(filename));
const isDocumentFile = (filename) => DOCUMENT_EXTS.has(getExt(filename));

isImageFile("photo.jpg");    // true
isVideoFile("clip.mp4");     // true
isDocumentFile("notes.pdf"); // true
isImageFile("data.json");    // false