base can be a function now, also add better error messaging. closes #78

README.md

@@ -92,6 +92,7 @@ This is just [glob-watcher].
     - Default is `process.cwd()`.
   - base - Specify the folder relative to the cwd. This is used to determine the file names when saving in `.dest()`.
     - Default is the `cwd` resolves to the folder path.
+    - Can also be a function that takes in a file and returns a folder path.
   - mode - Specify the mode the files should be created with.
     - Default is the mode of the input file (file.stat.mode) if any.
     - Default is the process mode if the input file has no mode property.

lib/prepareWrite.js

@@ -5,6 +5,14 @@ var path = require('path');
 var mkdirp = require('mkdirp');
 var fs = require('graceful-fs');
 
+function stringOrFunc(v, file) {
+  if (typeof v !== 'string' && typeof v !== 'function') {
+    return null;
+  }
+
+  return typeof v === 'string' ? v : v(file);
+}
+
 function prepareWrite(outFolder, file, opt, cb) {
   var options = assign({
     cwd: process.cwd(),
@@ -15,15 +23,16 @@ function prepareWrite(outFolder, file, opt, cb) {
   options.flag = (options.overwrite ? 'w' : 'wx');
 
   var cwd = path.resolve(options.cwd);
-
-  if (typeof outFolder !== 'string' && typeof outFolder !== 'function') {
+  var outFolderPath = stringOrFunc(outFolder, file);
+  if (!outFolderPath) {
     throw new Error('Invalid output folder');
   }
+  var basePath = options.base ?
+    stringOrFunc(options.base, file) : path.resolve(cwd, outFolderPath);
+  if (!basePath) {
+    throw new Error('Invalid base option');
+  }
 
-  var outFolderPath = (
-    typeof outFolder === 'string' ? outFolder : outFolder(file)
-  );
-  var basePath = options.base || path.resolve(cwd, outFolderPath);
   var writePath = path.resolve(basePath, file.relative);
   var writeFolder = path.dirname(writePath);
 

test/dest.js

@@ -572,12 +572,11 @@ describe('dest stream', function() {
     stream.end();
   });
 
-  it('should change to the specified base', function(done) {
+  it('should change to the specified base as string', function(done) {
     var inputBase = path.join(__dirname, './fixtures');
     var inputPath = path.join(__dirname, './fixtures/wow/suchempty');
 
     var firstFile = new File({
-      base: inputBase,
       cwd: __dirname,
       path: inputPath,
       stat: fs.statSync(inputPath)
@@ -601,6 +600,38 @@ describe('dest stream', function() {
     stream.end();
   });
 
+  it('should change to the specified base as function', function(done) {
+    var inputBase = path.join(__dirname, './fixtures');
+    var inputPath = path.join(__dirname, './fixtures/wow/suchempty');
+
+    var firstFile = new File({
+      cwd: __dirname,
+      path: inputPath,
+      stat: fs.statSync(inputPath)
+    });
+
+    var onEnd = function(){
+      buffered[0].base.should.equal(inputBase);
+      done();
+    };
+
+    var stream = vfs.dest('./out-fixtures/', {
+      cwd: __dirname,
+      base: function(file){
+        should.exist(file);
+        file.path.should.equal(inputPath);
+        return inputBase;
+      }
+    });
+
+    var buffered = [];
+    bufferStream = through.obj(dataWrap(buffered.push.bind(buffered)), onEnd);
+
+    stream.pipe(bufferStream);
+    stream.write(firstFile);
+    stream.end();
+  });
+
   it('should report IO errors', function(done) {
     var inputPath = path.join(__dirname, './fixtures/test.coffee');
     var inputBase = path.join(__dirname, './fixtures/');