diff --git a/lib/cli.js b/lib/cli.js
index 4bd4aad83..1d71df41a 100644
--- a/lib/cli.js
+++ b/lib/cli.js
@@ -1,12 +1,14 @@
-var path = require('path')
-var optimist = require('optimist')
-var fs = require('graceful-fs')
+'use strict'
 
-var Server = require('./server')
-var helper = require('./helper')
-var constant = require('./constants')
+const path = require('path')
+const optimist = require('optimist')
+const fs = require('graceful-fs')
 
-var processArgs = function (argv, options, fs, path) {
+const Server = require('./server')
+const helper = require('./helper')
+const constant = require('./constants')
+
+function processArgs (argv, options, fs, path) {
   if (argv.help) {
     console.log(optimist.help())
     process.exit(0)
@@ -19,7 +21,7 @@ var processArgs = function (argv, options, fs, path) {
 
   // TODO(vojta): warn/throw when unknown argument (probably mispelled)
   Object.getOwnPropertyNames(argv).forEach(function (name) {
-    var argumentValue = argv[name]
+    let argumentValue = argv[name]
     if (name !== '_' && name !== '$0') {
       if (name.indexOf('_') !== -1) {
         throw new Error('Bad argument: ' + name + ' did you mean ' + name.replace('_', '-'))
@@ -45,8 +47,9 @@ var processArgs = function (argv, options, fs, path) {
   }
 
   if (helper.isString(options.formatError)) {
+    let required
     try {
-      var required = require(options.formatError)
+      required = require(options.formatError)
     } catch (err) {
       console.error('Could not require formatError: ' + options.formatError, err)
     }
@@ -59,7 +62,7 @@ var processArgs = function (argv, options, fs, path) {
   }
 
   if (helper.isString(options.logLevel)) {
-    var logConstant = constant['LOG_' + options.logLevel.toUpperCase()]
+    const logConstant = constant['LOG_' + options.logLevel.toUpperCase()]
     if (helper.isDefined(logConstant)) {
       options.logLevel = logConstant
     } else {
@@ -103,7 +106,7 @@ var processArgs = function (argv, options, fs, path) {
     options.refresh = options.refresh === 'true'
   }
 
-  var configFile = argv._.shift()
+  let configFile = argv._.shift()
 
   if (!configFile) {
     // default config file (if exists)
@@ -127,11 +130,11 @@ var processArgs = function (argv, options, fs, path) {
   return options
 }
 
-var parseClientArgs = function (argv) {
+function parseClientArgs (argv) {
   // extract any args after '--' as clientArgs
-  var clientArgs = []
+  let clientArgs = []
   argv = argv.slice(2)
-  var idx = argv.indexOf('--')
+  const idx = argv.indexOf('--')
   if (idx !== -1) {
     clientArgs = argv.slice(idx + 1)
   }
@@ -139,13 +142,13 @@ var parseClientArgs = function (argv) {
 }
 
 // return only args that occur before `--`
-var argsBeforeDoubleDash = function (argv) {
-  var idx = argv.indexOf('--')
+function argsBeforeDoubleDash (argv) {
+  const idx = argv.indexOf('--')
 
   return idx === -1 ? argv : argv.slice(0, idx)
 }
 
-var describeShared = function () {
+function describeShared () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'Usage:\n' +
@@ -160,7 +163,7 @@ var describeShared = function () {
     .describe('version', 'Print current version.')
 }
 
-var describeInit = function () {
+function describeInit () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'INIT - Initialize a config file.\n\n' +
@@ -172,7 +175,7 @@ var describeInit = function () {
     .describe('help', 'Print usage and options.')
 }
 
-var describeStart = function () {
+function describeStart () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'START - Start the server / do a single run.\n\n' +
@@ -196,7 +199,7 @@ var describeStart = function () {
     .describe('help', 'Print usage and options.')
 }
 
-var describeRun = function () {
+function describeRun () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'RUN - Run the tests (requires running server).\n\n' +
@@ -212,7 +215,7 @@ var describeRun = function () {
     .describe('no-colors', 'Do not use colors when reporting or printing logs.')
 }
 
-var describeStop = function () {
+function describeStop () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'STOP - Stop the server (requires running server).\n\n' +
@@ -223,7 +226,7 @@ var describeStop = function () {
     .describe('help', 'Print usage.')
 }
 
-var describeCompletion = function () {
+function describeCompletion () {
   optimist
     .usage('Karma - Spectacular Test Runner for JavaScript.\n\n' +
       'COMPLETION - Bash/ZSH completion for karma.\n\n' +
@@ -233,8 +236,8 @@ var describeCompletion = function () {
 }
 
 exports.process = function () {
-  var argv = optimist.parse(argsBeforeDoubleDash(process.argv.slice(2)))
-  var options = {
+  const argv = optimist.parse(argsBeforeDoubleDash(process.argv.slice(2)))
+  const options = {
     cmd: argv._.shift()
   }
 
@@ -276,7 +279,7 @@ exports.process = function () {
 }
 
 exports.run = function () {
-  var config = exports.process()
+  const config = exports.process()
 
   switch (config.cmd) {
     case 'start':
diff --git a/test/unit/cli.spec.js b/test/unit/cli.spec.js
index 04744d96d..66cd55736 100644
--- a/test/unit/cli.spec.js
+++ b/test/unit/cli.spec.js
@@ -1,38 +1,40 @@
-var optimist = require('optimist')
-var path = require('path')
-var mocks = require('mocks')
+'use strict'
 
-var cli = require('../../lib/cli')
-var constant = require('../../lib/constants')
+const optimist = require('optimist')
+const path = require('path')
+const mocks = require('mocks')
 
-var loadFile = mocks.loadFile
+const cli = require('../../lib/cli')
+const constant = require('../../lib/constants')
+
+const loadFile = mocks.loadFile
 
 describe('cli', () => {
-  var m
-  var e
-  var mockery
+  let m
+  let e
+  let mockery
 
-  var fsMock = mocks.fs.create({
+  const fsMock = mocks.fs.create({
     cwd: {'karma.conf.js': true},
     cwd2: {'karma.conf.coffee': true},
     cwd3: {'karma.conf.ts': true}
   })
 
-  var currentCwd = null
+  let currentCwd = null
 
-  var pathMock = {
+  const pathMock = {
     resolve (p) {
       return path.resolve(currentCwd, p)
     }
   }
 
-  var setCWD = (cwd) => {
+  const setCWD = (cwd) => {
     currentCwd = cwd
     fsMock._setCWD(cwd)
   }
 
-  var processArgs = (args, opts) => {
-    var argv = optimist.parse(args)
+  const processArgs = (args, opts) => {
+    const argv = optimist.parse(args)
     return e.processArgs(argv, opts || {}, fsMock, pathMock)
   }
 
@@ -61,14 +63,14 @@ describe('cli', () => {
   describe('processArgs', () => {
     it('should override if multiple options given', () => {
       // optimist parses --port 123 --port 456 as port = [123, 456] which makes no sense
-      var options = processArgs(['some.conf', '--port', '12', '--log-level', 'info', '--port', '34', '--log-level', 'debug'])
+      const options = processArgs(['some.conf', '--port', '12', '--log-level', 'info', '--port', '34', '--log-level', 'debug'])
 
       expect(options.port).to.equal(34)
       expect(options.logLevel).to.equal('DEBUG')
     })
 
     it('should return camelCased options', () => {
-      var options = processArgs(['some.conf', '--port', '12', '--single-run'])
+      const options = processArgs(['some.conf', '--port', '12', '--single-run'])
 
       expect(options.configFile).to.exist
       expect(options.port).to.equal(12)
@@ -77,7 +79,7 @@ describe('cli', () => {
 
     it('should parse options without configFile and set default', () => {
       setCWD('/cwd')
-      var options = processArgs(['--auto-watch', '--auto-watch-interval', '10'])
+      const options = processArgs(['--auto-watch', '--auto-watch-interval', '10'])
       expect(path.resolve(options.configFile)).to.equal(path.resolve('/cwd/karma.conf.js'))
       expect(options.autoWatch).to.equal(true)
       expect(options.autoWatchInterval).to.equal(10)
@@ -85,27 +87,27 @@ describe('cli', () => {
 
     it('should set default karma.conf.coffee config file if exists', () => {
       setCWD('/cwd2')
-      var options = processArgs(['--port', '10'])
+      const options = processArgs(['--port', '10'])
 
       expect(path.resolve(options.configFile)).to.equal(path.resolve('/cwd2/karma.conf.coffee'))
     })
 
     it('should set default karma.conf.ts config file if exists', () => {
       setCWD('/cwd3')
-      var options = processArgs(['--port', '10'])
+      const options = processArgs(['--port', '10'])
 
       expect(path.resolve(options.configFile)).to.equal(path.resolve('/cwd3/karma.conf.ts'))
     })
 
     it('should not set default config if neither exists', () => {
       setCWD('/')
-      var options = processArgs([])
+      const options = processArgs([])
 
       expect(options.configFile).to.equal(null)
     })
 
     it('should parse auto-watch, colors, singleRun to boolean', () => {
-      var options = processArgs(['--auto-watch', 'false', '--colors', 'false', '--single-run', 'false'])
+      let options = processArgs(['--auto-watch', 'false', '--colors', 'false', '--single-run', 'false'])
 
       expect(options.autoWatch).to.equal(false)
       expect(options.colors).to.equal(false)
@@ -119,7 +121,7 @@ describe('cli', () => {
     })
 
     it('should replace log-level constants', () => {
-      var options = processArgs(['--log-level', 'debug'])
+      let options = processArgs(['--log-level', 'debug'])
       expect(options.logLevel).to.equal(constant.LOG_DEBUG)
 
       options = processArgs(['--log-level', 'error'])
@@ -137,29 +139,29 @@ describe('cli', () => {
 
     it('should parse format-error into a function', () => {
       // root export
-      var options = processArgs(['--format-error', '../../test/unit/fixtures/format-error-root'])
-      var formatErrorRoot = require('../../test/unit/fixtures/format-error-root')
+      let options = processArgs(['--format-error', '../../test/unit/fixtures/format-error-root'])
+      const formatErrorRoot = require('../../test/unit/fixtures/format-error-root')
       expect(options.formatError).to.equal(formatErrorRoot)
 
       // property export
       options = processArgs(['--format-error', '../../test/unit/fixtures/format-error-property'])
-      var formatErrorProperty = require('../../test/unit/fixtures/format-error-property').formatError
+      const formatErrorProperty = require('../../test/unit/fixtures/format-error-property').formatError
       expect(options.formatError).to.equal(formatErrorProperty)
     })
 
     it('should parse browsers into an array', () => {
-      var options = processArgs(['--browsers', 'Chrome,ChromeCanary,Firefox'])
+      const options = processArgs(['--browsers', 'Chrome,ChromeCanary,Firefox'])
       expect(options.browsers).to.deep.equal(['Chrome', 'ChromeCanary', 'Firefox'])
     })
 
     it('should resolve configFile to absolute path', () => {
       setCWD('/cwd')
-      var options = processArgs(['some/config.js'])
+      const options = processArgs(['some/config.js'])
       expect(path.resolve(options.configFile)).to.equal(path.resolve('/cwd/some/config.js'))
     })
 
     it('should parse report-slower-than to a number', () => {
-      var options = processArgs(['--report-slower-than', '2000'])
+      let options = processArgs(['--report-slower-than', '2000'])
       expect(options.reportSlowerThan).to.equal(2000)
 
       options = processArgs(['--no-report-slower-than'])
@@ -167,7 +169,7 @@ describe('cli', () => {
     })
 
     it('should cast reporters to array', () => {
-      var options = processArgs(['--reporters', 'dots,junit'])
+      let options = processArgs(['--reporters', 'dots,junit'])
       expect(options.reporters).to.deep.equal(['dots', 'junit'])
 
       options = processArgs(['--reporters', 'dots'])
@@ -175,7 +177,7 @@ describe('cli', () => {
     })
 
     it('should parse removed/added/changed files to array', () => {
-      var options = processArgs([
+      const options = processArgs([
         '--removed-files', 'r1.js,r2.js',
         '--changed-files', 'ch1.js,ch2.js',
         '--added-files', 'a1.js,a2.js'
@@ -187,9 +189,9 @@ describe('cli', () => {
     })
 
     it('should error on args with underscores', () => {
-      var expectedException
+      let expectedException
       try {
-        var options = processArgs(['--no_browsers'])
+        const options = processArgs(['--no_browsers'])
         expectedException = 'Should have thrown but got ' + options
       } catch (e) {
         expectedException = e
@@ -201,19 +203,19 @@ describe('cli', () => {
 
   describe('parseClientArgs', () => {
     it('should return arguments after --', () => {
-      var args = cli.parseClientArgs(['node', 'karma.js', 'runArg', '--flag', '--', '--foo', '--bar', 'baz'])
+      const args = cli.parseClientArgs(['node', 'karma.js', 'runArg', '--flag', '--', '--foo', '--bar', 'baz'])
       expect(args).to.deep.equal(['--foo', '--bar', 'baz'])
     })
 
     it('should return empty args if -- is not present', () => {
-      var args = cli.parseClientArgs(['node', 'karma.js', 'runArg', '--flag', '--foo', '--bar', 'baz'])
+      const args = cli.parseClientArgs(['node', 'karma.js', 'runArg', '--flag', '--foo', '--bar', 'baz'])
       expect(args).to.deep.equal([])
     })
   })
 
   describe('argsBeforeDoubleDash', () => {
     it('should return array of args that occur before --', () => {
-      var args = cli.argsBeforeDoubleDash(['aa', '--bb', 'value', '--', 'some', '--no-more'])
+      const args = cli.argsBeforeDoubleDash(['aa', '--bb', 'value', '--', 'some', '--no-more'])
       expect(args).to.deep.equal(['aa', '--bb', 'value'])
     })
   })