diff --git a/lib/preprocessor.js b/lib/preprocessor.js
index db59bd4bf..0728ecac1 100644
--- a/lib/preprocessor.js
+++ b/lib/preprocessor.js
@@ -1,16 +1,12 @@
-var fs = require('graceful-fs')
-var crypto = require('crypto')
-var mm = require('minimatch')
-var isBinaryFile = require('isbinaryfile')
-var combineLists = require('combine-lists')
-
-var log = require('./logger').create('preprocess')
-
-function sha1 (data) {
-  var hash = crypto.createHash('sha1')
-  hash.update(data)
-  return hash.digest('hex')
-}
+'use strict'
+
+const fs = require('graceful-fs')
+const mm = require('minimatch')
+const isBinaryFile = require('isbinaryfile')
+const combineLists = require('combine-lists')
+const CryptoUtils = require('./utils/crypto-utils')
+
+const log = require('./logger').create('preprocess')
 
 function createNextProcessor (preprocessors, file, done) {
   return function nextPreprocessor (error, content) {
@@ -29,7 +25,7 @@ function createNextProcessor (preprocessors, file, done) {
     if (!preprocessors.length) {
       file.contentPath = null
       file.content = content
-      file.sha = sha1(content)
+      file.sha = CryptoUtils.sha1(content)
       return done()
     }
 
@@ -38,27 +34,25 @@ function createNextProcessor (preprocessors, file, done) {
 }
 
 function createPreprocessor (config, basePath, injector) {
-  var alreadyDisplayedErrors = {}
-  var instances = {}
-  var patterns = Object.keys(config)
-
-  var emitter = injector.get('emitter')
+  const emitter = injector.get('emitter')
+  const alreadyDisplayedErrors = {}
+  const instances = {}
+  let patterns = Object.keys(config)
 
   function instantiatePreprocessor (name) {
     if (alreadyDisplayedErrors[name]) {
       return
     }
 
-    var p
+    let p
 
     try {
       p = injector.get('preprocessor:' + name)
     } catch (e) {
-      if (e.message.indexOf('No provider for "preprocessor:' + name + '"') !== -1) {
-        log.error('Can not load "%s", it is not registered!\n  ' +
-          'Perhaps you are missing some plugin?', name)
+      if (e.message.indexOf(`No provider for "preprocessor:${name}"`) !== -1) {
+        log.error(`Can not load "${name}", it is not registered!\n  Perhaps you are missing some plugin?`)
       } else {
-        log.error('Can not load "%s"!\n  ' + e.stack, name)
+        log.error(`Can not load "${name}"!\n  ` + e.stack)
       }
       alreadyDisplayedErrors[name] = true
       emitter.emit('load_error', 'preprocessor', name)
@@ -67,16 +61,16 @@ function createPreprocessor (config, basePath, injector) {
     return p
   }
 
-  var allPreprocessors = []
-  patterns.forEach(function (pattern) {
+  let allPreprocessors = []
+  patterns.forEach((pattern) => {
     allPreprocessors = combineLists(allPreprocessors, config[pattern])
   })
   allPreprocessors.forEach(instantiatePreprocessor)
 
   return function preprocess (file, done) {
     patterns = Object.keys(config)
-    var retryCount = 0
-    var maxRetries = 3
+    let retryCount = 0
+    let maxRetries = 3
     function readFileCallback (err, buffer) {
       if (err) {
         log.warn(err)
@@ -90,25 +84,22 @@ function createPreprocessor (config, basePath, injector) {
         }
       }
 
-      isBinaryFile(buffer, buffer.length, function (err, thisFileIsBinary) {
+      isBinaryFile(buffer, buffer.length, function (err, isBinary) {
         if (err) {
           throw err
         }
 
         var preprocessorNames = []
-        for (var i = 0; i < patterns.length; i++) {
-          if (mm(file.originalPath, patterns[i], {dot: true})) {
-            preprocessorNames = combineLists(preprocessorNames, config[patterns[i]])
+        patterns.forEach((pattern) => {
+          if (mm(file.originalPath, pattern, {dot: true})) {
+            preprocessorNames = combineLists(preprocessorNames, config[pattern])
           }
-        }
+        })
 
-        var preprocessors = []
-        var nextPreprocessor = createNextProcessor(preprocessors, file, done)
-        preprocessorNames.forEach(function (name) {
-          var p = instances[name]
-          if (p == null) {
-            p = instantiatePreprocessor(name)
-          }
+        let preprocessors = []
+        const nextPreprocessor = createNextProcessor(preprocessors, file, done)
+        preprocessorNames.forEach((name) => {
+          const p = instances[name] || instantiatePreprocessor(name)
 
           if (p == null) {
             if (!alreadyDisplayedErrors[name]) {
@@ -120,7 +111,7 @@ function createPreprocessor (config, basePath, injector) {
           }
 
           instances[name] = p
-          if (!thisFileIsBinary || p.handleBinaryFiles) {
+          if (!isBinary || p.handleBinaryFiles) {
             preprocessors.push(p)
           } else {
             log.warn('Ignored preprocessing %s because %s has handleBinaryFiles=false.',
@@ -128,12 +119,13 @@ function createPreprocessor (config, basePath, injector) {
           }
         })
 
-        nextPreprocessor(null, thisFileIsBinary ? buffer : buffer.toString())
+        nextPreprocessor(null, isBinary ? buffer : buffer.toString())
       })
     }
     return fs.readFile(file.originalPath, readFileCallback)
   }
 }
+
 createPreprocessor.$inject = ['config.preprocessors', 'config.basePath', 'injector']
 
 exports.createPreprocessor = createPreprocessor
diff --git a/lib/utils/crypto-utils.js b/lib/utils/crypto-utils.js
new file mode 100644
index 000000000..9dc9a073d
--- /dev/null
+++ b/lib/utils/crypto-utils.js
@@ -0,0 +1,14 @@
+'use strict'
+
+const crypto = require('crypto')
+
+const CryptoUtils = {
+  sha1 (data) {
+    return crypto
+      .createHash('sha1')
+      .update(data)
+      .digest('hex')
+  }
+}
+
+module.exports = CryptoUtils
diff --git a/test/unit/preprocessor.spec.js b/test/unit/preprocessor.spec.js
index 2bf6f9a62..1e4439a6c 100644
--- a/test/unit/preprocessor.spec.js
+++ b/test/unit/preprocessor.spec.js
@@ -1,16 +1,18 @@
-var mocks = require('mocks')
-var di = require('di')
-var path = require('path')
+'use strict'
 
-var events = require('../../lib/events')
+const mocks = require('mocks')
+const di = require('di')
+const path = require('path')
+
+const events = require('../../lib/events')
 
 describe('preprocessor', () => {
-  var pp
-  var m
-  var mockFs
-  var emitterSetting
+  let pp
+  let m
+  let mockFs
+  let emitterSetting
   // mimic first few bytes of a pdf file
-  var binarydata = new Buffer([0x25, 0x50, 0x44, 0x66, 0x46, 0x00]) // eslint-disable-line node/no-deprecated-api
+  const binarydata = new Buffer([0x25, 0x50, 0x44, 0x66, 0x46, 0x00]) // eslint-disable-line node/no-deprecated-api
 
   beforeEach(() => {
     mockFs = mocks.fs.create({
@@ -26,7 +28,7 @@ describe('preprocessor', () => {
       }
     })
 
-    var mocks_ = {
+    const mocks_ = {
       'graceful-fs': mockFs,
       minimatch: require('minimatch')
     }
@@ -35,19 +37,19 @@ describe('preprocessor', () => {
   })
 
   it('should preprocess matching file', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       file.path = file.path + '-preprocessed'
       done(null, 'new-content')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': [
         'factory', function () { return fakePreprocessor }
       ]
     }, emitterSetting])
     pp = m.createPreprocessor({'**/*.js': ['fake']}, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, () => {
       expect(fakePreprocessor).to.have.been.called
@@ -58,17 +60,17 @@ describe('preprocessor', () => {
   })
 
   it('should match directories starting with a dot', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       file.path = file.path + '-preprocessed'
       done(null, 'new-content')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
     pp = m.createPreprocessor({'**/*.js': ['fake']}, null, injector)
 
-    var file = {originalPath: '/some/.dir/a.js', path: 'path'}
+    const file = {originalPath: '/some/.dir/a.js', path: 'path'}
 
     pp(file, () => {
       expect(fakePreprocessor).to.have.been.called
@@ -79,18 +81,18 @@ describe('preprocessor', () => {
   })
 
   it('should check patterns after creation when invoked', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       file.path = file.path + '-preprocessed'
       done(null, 'new-content')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
-    var config = {'**/*.txt': ['fake']}
+    const config = {'**/*.txt': ['fake']}
     pp = m.createPreprocessor(config, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     config['**/*.js'] = ['fake']
 
@@ -103,16 +105,16 @@ describe('preprocessor', () => {
   })
 
   it('should ignore not matching file', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       done(null, '')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
     pp = m.createPreprocessor({'**/*.js': ['fake']}, null, injector)
 
-    var file = {originalPath: '/some/a.txt', path: 'path'}
+    const file = {originalPath: '/some/a.txt', path: 'path'}
 
     pp(file, () => {
       expect(fakePreprocessor).to.not.have.been.called
@@ -121,24 +123,24 @@ describe('preprocessor', () => {
   })
 
   it('should apply all preprocessors', (done) => {
-    var fakePreprocessor1 = sinon.spy((content, file, done) => {
+    const fakePreprocessor1 = sinon.spy((content, file, done) => {
       file.path = file.path + '-p1'
       done(null, content + '-c1')
     })
 
-    var fakePreprocessor2 = sinon.spy((content, file, done) => {
+    const fakePreprocessor2 = sinon.spy((content, file, done) => {
       file.path = file.path + '-p2'
       done(content + '-c2')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake1': ['factory', function () { return fakePreprocessor1 }],
       'preprocessor:fake2': ['factory', function () { return fakePreprocessor2 }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/*.js': ['fake1', 'fake2']}, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, () => {
       expect(fakePreprocessor1).to.have.been.calledOnce
@@ -151,12 +153,12 @@ describe('preprocessor', () => {
 
   it('should compute SHA', (done) => {
     pp = m.createPreprocessor({}, null, new di.Injector([emitterSetting]))
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, () => {
       expect(file.sha).to.exist
       expect(file.sha.length).to.equal(40)
-      var previousSHA = file.sha
+      const previousSHA = file.sha
 
       pp(file, () => {
         expect(file.sha).to.equal(previousSHA)
@@ -172,18 +174,18 @@ describe('preprocessor', () => {
   })
 
   it('should compute SHA from content returned by a processor', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       done(null, content + '-processed')
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/a.js': ['fake']}, null, injector)
 
-    var fileProcess = {originalPath: '/some/a.js', path: 'path'}
-    var fileSkip = {originalPath: '/some/b.js', path: 'path'}
+    const fileProcess = {originalPath: '/some/a.js', path: 'path'}
+    const fileSkip = {originalPath: '/some/b.js', path: 'path'}
 
     pp(fileProcess, () => {
       pp(fileSkip, () => {
@@ -198,17 +200,17 @@ describe('preprocessor', () => {
   })
 
   it('should return error if any preprocessor fails', (done) => {
-    var failingPreprocessor = sinon.spy((content, file, done) => {
+    const failingPreprocessor = sinon.spy((content, file, done) => {
       done(new Error('Some error'), null)
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:failing': ['factory', function () { return failingPreprocessor }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/*.js': ['failing']}, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, (err) => {
       expect(err).to.exist
@@ -217,22 +219,22 @@ describe('preprocessor', () => {
   })
 
   it('should stop preprocessing after an error', (done) => {
-    var failingPreprocessor = sinon.spy((content, file, done) => {
+    const failingPreprocessor = sinon.spy((content, file, done) => {
       done(new Error('Some error'), null)
     })
 
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       done(null, content)
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:failing': ['factory', function () { return failingPreprocessor }],
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/*.js': ['failing', 'fake']}, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, () => {
       expect(fakePreprocessor).not.to.have.been.called
@@ -241,8 +243,8 @@ describe('preprocessor', () => {
   })
 
   describe('when fs.readFile fails', () => {
-    var file = {originalPath: '/some/a.js', path: 'path'}
-    var getReadFileCallback = (nthCall) => {
+    const file = {originalPath: '/some/a.js', path: 'path'}
+    const getReadFileCallback = (nthCall) => {
       return mockFs.readFile.args[nthCall][1]
     }
 
@@ -251,15 +253,15 @@ describe('preprocessor', () => {
     })
 
     it('should retry up to 3 times', (done) => {
-      var fakePreprocessor = sinon.spy((content, file, done) => {
+      const fakePreprocessor = sinon.spy((content, file, done) => {
         done(null, content)
       })
 
-      var injector = new di.Injector([{
+      const injector = new di.Injector([{
         'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
       }, emitterSetting])
 
-      var pp = m.createPreprocessor({'**/*.js': ['fake']}, null, injector)
+      const pp = m.createPreprocessor({'**/*.js': ['fake']}, null, injector)
 
       pp(file, () => {
         expect(fakePreprocessor).to.have.been.called
@@ -267,15 +269,15 @@ describe('preprocessor', () => {
       })
       getReadFileCallback(0)('error')
       getReadFileCallback(1)('error')
-      var thirdCallback = getReadFileCallback(2)
+      const thirdCallback = getReadFileCallback(2)
       mockFs.readFile.restore()
       thirdCallback('error')
     })
 
     it('should throw after 3 retries', (done) => {
-      var injector = new di.Injector([{}, emitterSetting])
+      const injector = new di.Injector([{}, emitterSetting])
 
-      var pp = m.createPreprocessor({'**/*.js': []}, null, injector)
+      const pp = m.createPreprocessor({'**/*.js': []}, null, injector)
 
       pp(file, () => { })
 
@@ -289,17 +291,17 @@ describe('preprocessor', () => {
   })
 
   it('should not preprocess binary files by default', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       done(null, content)
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { return fakePreprocessor }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/*': ['fake']}, null, injector)
 
-    var file = {originalPath: '/some/photo.png', path: 'path'}
+    const file = {originalPath: '/some/photo.png', path: 'path'}
 
     pp(file, (err) => {
       if (err) throw err
@@ -334,17 +336,17 @@ describe('preprocessor', () => {
   })
 
   it('should not preprocess binary files with capital letters in extension', (done) => {
-    var fakePreprocessor = sinon.spy((content, file, done) => {
+    const fakePreprocessor = sinon.spy((content, file, done) => {
       done(null, content)
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fake': ['factory', function () { fakePreprocessor }]
     }, emitterSetting])
 
     pp = m.createPreprocessor({'**/*': ['fake']}, null, injector)
 
-    var file = {originalPath: '/some/CAM_PHOTO.JPG', path: 'path'}
+    const file = {originalPath: '/some/CAM_PHOTO.JPG', path: 'path'}
 
     pp(file, (err) => {
       if (err) throw err
@@ -356,25 +358,25 @@ describe('preprocessor', () => {
   })
 
   it('should merge lists of preprocessors', (done) => {
-    var callOrder = []
-    var fakePreprocessorA = sinon.spy((content, file, done) => {
+    const callOrder = []
+    const fakePreprocessorA = sinon.spy((content, file, done) => {
       callOrder.push('a')
       done(null, content)
     })
-    var fakePreprocessorB = sinon.spy((content, file, done) => {
+    const fakePreprocessorB = sinon.spy((content, file, done) => {
       callOrder.push('b')
       done(null, content)
     })
-    var fakePreprocessorC = sinon.spy((content, file, done) => {
+    const fakePreprocessorC = sinon.spy((content, file, done) => {
       callOrder.push('c')
       done(null, content)
     })
-    var fakePreprocessorD = sinon.spy((content, file, done) => {
+    const fakePreprocessorD = sinon.spy((content, file, done) => {
       callOrder.push('d')
       done(null, content)
     })
 
-    var injector = new di.Injector([{
+    const injector = new di.Injector([{
       'preprocessor:fakeA': ['factory', function () { return fakePreprocessorA }],
       'preprocessor:fakeB': ['factory', function () { return fakePreprocessorB }],
       'preprocessor:fakeC': ['factory', function () { return fakePreprocessorC }],
@@ -387,7 +389,7 @@ describe('preprocessor', () => {
       '/some/a.js': ['fakeD']
     }, null, injector)
 
-    var file = {originalPath: '/some/a.js', path: 'path'}
+    const file = {originalPath: '/some/a.js', path: 'path'}
 
     pp(file, (err) => {
       if (err) throw err
diff --git a/test/unit/utils/crypto-utils.spec.js b/test/unit/utils/crypto-utils.spec.js
new file mode 100644
index 000000000..671712ecf
--- /dev/null
+++ b/test/unit/utils/crypto-utils.spec.js
@@ -0,0 +1,9 @@
+'use strict'
+
+const CryptoUtils = require('../../../lib/utils/crypto-utils')
+
+describe('CryptoUtils.sha1', () => {
+  it('create sha1 digest from string', () => {
+    expect(CryptoUtils.sha1('Example text')).to.equal('cf3df620b86f0c9f586359136950217cb3b8c035')
+  })
+})