Allow copy's filter to return a Promise

docs/copy.md

@@ -9,7 +9,7 @@ Copy a file or directory. The directory can have contents. Like `cp -r`.
   - `errorOnExist` `<boolean>`: when `overwrite` is `false` and the destination exists, throw an error. Default is `false`.
   - `dereference` `<boolean>`: dereference symlinks, default is `false`.
   - `preserveTimestamps` `<boolean>`: will set last modification and access times to the ones of the original source files, default is `false`.
-  - `filter` `<Function>`: Function to filter copied files. Return `true` to include, `false` to exclude.
+  - `filter` `<Function>`: Function to filter copied files. Return `true` to include, `false` to exclude. Can also return a `Promise` that resolves to `true` or `false` (or pass in an `async` function).
 - `callback` `<Function>`
 
 ## Example:

lib/copy/__tests__/copy.test.js

@@ -249,6 +249,19 @@ describe('fs-extra', () => {
         })
       })
 
+      it('allows filter fn to return a promise', done => {
+        const srcFile1 = path.join(TEST_DIR, '1.css')
+        fs.writeFileSync(srcFile1, '')
+        const destFile1 = path.join(TEST_DIR, 'dest1.css')
+        const filter = s => Promise.resolve(s.split('.').pop() !== 'css')
+
+        fse.copy(srcFile1, destFile1, filter, err => {
+          assert(!err)
+          assert(!fs.existsSync(destFile1))
+          done()
+        })
+      })
+
       it('should apply filter recursively', done => {
         const FILES = 2
         // Don't match anything that ends with a digit higher than 0:

lib/copy/copy.js

@@ -13,7 +13,7 @@ function copy (src, dest, opts, cb) {
   if (typeof opts === 'function' && !cb) {
     cb = opts
     opts = {}
-  } else if (typeof opts === 'function' || opts instanceof RegExp) {
+  } else if (typeof opts === 'function') {
     opts = {filter: opts}
   }
 
@@ -49,8 +49,13 @@ function copy (src, dest, opts, cb) {
 }
 
 function startCopy (src, dest, opts, cb) {
-  if (opts.filter && !opts.filter(src, dest)) return cb()
+  if (opts.filter) {
-  return getStats(src, dest, opts, cb)
+    Promise.resolve(opts.filter(src, dest))
+      .then(include => {
+        if (include) getStats(src, dest, opts, cb)
+        else cb()
+      }, error => cb(error))
+  } else getStats(src, dest, opts, cb)
 }
 
 function getStats (src, dest, opts, cb) {