diff --git a/index.js b/index.js
index 0462088..e1c7c26 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const fileType = require('file-type')
 const Transform = require('readable-stream/transform')
 
diff --git a/test.js b/test.js
index 36db032..ded464c 100644
--- a/test.js
+++ b/test.js
@@ -1,5 +1,7 @@
 /* eslint-env mocha */
 
+'use strict'
+
 const fs = require('fs')
 const assert = require('assert')
 const hasha = require('hasha')