src/view/download.js

/**
 * Triggers a browser file download from a Blob or a Base64-encoded string.
 * @param {Blob|string} data - Blob or Base64 encoded binary (treated as audio/midi).
 * @param {string} [filename] - Output name; defaults to UTC-timestamp .mid.
 * @return {string} Filename stem (without extension).
 * @example
 * downloadBlob(blob);                  // "2026-05-09T14-23-07" (default UTC timestamp)
 * downloadBlob(b64String, "song.mid"); // "song" (custom filename, returns stem)
 */
export function downloadBlob(
  data,
  filename = `${new Date().toISOString().replace(/[:.]/g, "-").slice(0, -5)}.mid`,
) {
  const blob =
    typeof data === "string"
      ? new Blob([Uint8Array.from(atob(data), (c) => c.charCodeAt(0))], {
          type: "audio/midi",
        })
      : data;
  const url = URL.createObjectURL(blob);
  const a = document.createElement("a");
  a.href = url;
  a.download = filename;
  document.body.appendChild(a);
  a.click();
  URL.revokeObjectURL(url);
  a.remove();
  return filename.replace(/\.[^.]+$/, "");
}