diff --git a/lib/util/assign.js b/lib/util/assign.js
index 8e41f9a..317e5ec 100644
--- a/lib/util/assign.js
+++ b/lib/util/assign.js
@@ -1,9 +1,11 @@
+'use strict'
+
 // simple mutable assign
 function assign () {
-  var args = [].slice.call(arguments).filter(function (i) { return i })
-  var dest = args.shift()
-  args.forEach(function (src) {
-    Object.keys(src).forEach(function (key) {
+  const args = [].slice.call(arguments).filter(i => i)
+  const dest = args.shift()
+  args.forEach(src => {
+    Object.keys(src).forEach(key => {
       dest[key] = src[key]
     })
   })