refactor: res.download with lower complexity

lib/response.js

@@ -417,31 +417,38 @@ res.sendFile = function sendFile(path, options, callback) {
  * to set the attachment and filename.
  *
  * This method uses `res.sendFile()`.
+ * 
+ * Examples:
+ *
+ *      // Basic usage with path
+ *      res.download('/path/to/file.csv');
+ *
+ *      // With filename and callback
+ *      res.download('/path/to/file.csv', 'report.csv', (err) => { ... });
  *
+ *      // With options (no filename)
+ *      res.download('/path/to/file.csv', { headers: { 'Cache-Control': 'no-cache' } });
+ *
+ * @param {String} path
+ * @param {String|Function|Object} [filename]
+ * @param {Object|Function} [options]
+ * @param {Function} [callback]
  * @public
  */
 
 res.download = function download (path, filename, options, callback) {
-  var done = callback;
-  var name = filename;
-  var opts = options || null
-
-  // support function as second or third arg
-  if (typeof filename === 'function') {
-    done = filename;
-    name = null;
-    opts = null
-  } else if (typeof options === 'function') {
-    done = options
-    opts = null
-  }
+  // identify the callback among the [filename, options, callback]
+  const done = [filename, options, callback].find(arg => typeof arg === 'function');
 
-  // support optional filename, where options may be in it's place
-  if (typeof filename === 'object' &&
-    (typeof options === 'function' || options === undefined)) {
-    name = null
-    opts = filename
-  }
+  var name = null;
+  var opts = {};
+
+  // determine if filename is a object or string
+  if (typeof filename === 'string') name = filename;
+  else if (typeof filename === 'object') opts = filename;
+
+  // if options is a function, set it as callback
+  if (typeof options === 'object') opts = Object.assign({}, opts, options);
 
   // set Content-Disposition when file is sent
   var headers = {
@@ -450,23 +457,19 @@ res.download = function download (path, filename, options, callback) {
 
   // merge user-provided headers
   if (opts && opts.headers) {
-    var keys = Object.keys(opts.headers)
-    for (var i = 0; i < keys.length; i++) {
-      var key = keys[i]
+    Object.keys(opts.headers).forEach(key => {
       if (key.toLowerCase() !== 'content-disposition') {
-        headers[key] = opts.headers[key]
+        headers[key] = opts.headers[key];
       }
-    }
+    });
   }
 
   // merge user-provided options
   opts = Object.create(opts)
   opts.headers = headers
 
   // Resolve the full path for sendFile
-  var fullPath = !opts.root
-    ? resolve(path)
-    : path
+  var fullPath = !opts.root ? resolve(path) : path
 
   // send file
   return this.sendFile(fullPath, opts, done)
@@ -865,7 +868,7 @@ res.vary = function(field){
  *
  *  - `cache`     boolean hinting to the engine it should cache
  *  - `filename`  filename of the view being rendered
- *
+ * 
  * @public
  */