Add fs.extension

spec/stdlib/fs-spec.coffee

@@ -18,6 +18,14 @@ describe "fs", ->
       expect(fs.join('/a/b/', 'c', 'd')).toBe '/a/b/c/d'
       expect(fs.join('a', 'b/c/', 'd/')).toBe 'a/b/c/d/'
 
+  describe ".extension(path)", ->
+    it "returns the extension of a file", ->
+      expect(fs.extension("a/b/corey.txt")).toBe '.txt'
+      expect(fs.extension("a/b/corey.txt.coffee")).toBe '.coffee'
+
+    it "returns an empty string for paths without an extension", ->
+      expect(fs.extension("a/b.not-extension/a-dir")).toBe ''
+
   describe ".async", ->
     directoryPath = null
     beforeEach ->

src/stdlib/fs.coffee

@@ -27,6 +27,17 @@ module.exports =
   exists: (path) ->
     $native.exists path
 
+  # Returns the extension of a file. The extension of a file is the
+  # last dot (excluding any number of initial dots) followed by one or
+  # more non-dot characters. Returns an empty string if no valid
+  # extension exists.
+  extension: (path) ->
+    match = @base(path).match(/\.[^\.]+$/)
+    if match
+      match[0]
+    else
+      ""
+
   join: (paths...) ->
     return paths[0] if paths.length == 1
     [first, rest...] = paths