diff --git a/lib/completion.js b/lib/completion.js
index 4ffe3b7a8..25b646904 100644
--- a/lib/completion.js
+++ b/lib/completion.js
@@ -1,7 +1,9 @@
-var CUSTOM = ['']
-var BOOLEAN = false
+'use strict'
 
-var options = {
+const CUSTOM = ['']
+const BOOLEAN = false
+
+const options = {
   start: {
     '--port': CUSTOM,
     '--auto-watch': BOOLEAN,
@@ -30,8 +32,8 @@ var options = {
   }
 }
 
-var parseEnv = function (argv, env) {
-  var words = argv.slice(5)
+function parseEnv (argv, env) {
+  const words = argv.slice(5)
 
   return {
     words: words,
@@ -41,7 +43,7 @@ var parseEnv = function (argv, env) {
   }
 }
 
-var opositeWord = function (word) {
+function opositeWord (word) {
   if (word.charAt(0) !== '-') {
     return null
   }
@@ -49,11 +51,11 @@ var opositeWord = function (word) {
   return word.substr(0, 5) === '--no-' ? '--' + word.substr(5) : '--no-' + word.substr(2)
 }
 
-var sendCompletionNoOptions = function () {}
+function sendCompletionNoOptions () {}
 
-var sendCompletion = function (possibleWords, env) {
-  var regexp = new RegExp('^' + env.last)
-  var filteredWords = possibleWords.filter(function (word) {
+function sendCompletion (possibleWords, env) {
+  const regexp = new RegExp('^' + env.last)
+  const filteredWords = possibleWords.filter(function (word) {
     return regexp.test(word) && env.words.indexOf(word) === -1 &&
       env.words.indexOf(opositeWord(word)) === -1
   })
@@ -67,13 +69,13 @@ var sendCompletion = function (possibleWords, env) {
   })
 }
 
-var glob = require('glob')
-var globOpts = {
+const glob = require('glob')
+const globOpts = {
   mark: true,
   nocase: true
 }
 
-var sendCompletionFiles = function (env) {
+function sendCompletionFiles (env) {
   glob(env.last + '*', globOpts, function (err, files) {
     if (err) return console.error(err)
 
@@ -85,11 +87,11 @@ var sendCompletionFiles = function (env) {
   })
 }
 
-var sendCompletionConfirmLast = function (env) {
+function sendCompletionConfirmLast (env) {
   console.log(env.last)
 }
 
-var complete = function (env) {
+function complete (env) {
   if (env.count === 1) {
     if (env.words[0].charAt(0) === '-') {
       return sendCompletion(['--help', '--version'], env)
@@ -103,8 +105,8 @@ var complete = function (env) {
     return sendCompletionFiles(env)
   }
 
-  var cmdOptions = options[env.words[0]]
-  var previousOption = cmdOptions[env.prev]
+  const cmdOptions = options[env.words[0]]
+  const previousOption = cmdOptions[env.prev]
 
   if (!cmdOptions) {
     // no completion, wrong command
@@ -124,14 +126,14 @@ var complete = function (env) {
   return sendCompletion(Object.keys(cmdOptions), env)
 }
 
-var completion = function () {
+function completion () {
   if (process.argv[3] === '--') {
     return complete(parseEnv(process.argv, process.env))
   }
 
   // just print out the karma-completion.sh
-  var fs = require('graceful-fs')
-  var path = require('path')
+  const fs = require('graceful-fs')
+  const path = require('path')
 
   fs.readFile(path.resolve(__dirname, '../scripts/karma-completion.sh'), 'utf8', function (err, data) {
     if (err) return console.error(err)
diff --git a/test/unit/completion.spec.js b/test/unit/completion.spec.js
index 33f66ee9b..92509666b 100644
--- a/test/unit/completion.spec.js
+++ b/test/unit/completion.spec.js
@@ -1,10 +1,12 @@
-var c = require('../../lib/completion')
+'use strict'
+
+const c = require('../../lib/completion')
 
 describe('completion', () => {
-  var completion
+  let completion
 
   function mockEnv (line) {
-    var words = line.split(' ')
+    const words = line.split(' ')
 
     return {
       words: words,