diff --git a/.eslintrc.yml b/.eslintrc.yml
index ed73eba4c6f1a66e4d4c49b381b82ef9e52f5d9a..d747f6d5d4466060fa089a4c8df7081df294033c 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -38,6 +38,7 @@ rules:
   object-shorthand: ['error', 'properties']
   prefer-template: 'error'
   promise/prefer-await-to-then: 'error'
+  func-style: ['error', 'declaration', { 'allowArrowFunctions': true }]
 
   # Mocha-related.
   mocha/no-exclusive-tests: 'error'
diff --git a/lib/error-helper.js b/lib/error-helper.js
index 4d96d31fa1b4e3f3e56d4a03aa8358caa075b077..49ff12e12121a8734ce565cd0e8b585995c9fbd7 100644
--- a/lib/error-helper.js
+++ b/lib/error-helper.js
@@ -9,7 +9,7 @@ const defaultErrorMessages = {
   404: 'not found',
 }
 
-const checkErrorResponse = function(badgeData, err, res, errorMessages = {}) {
+function checkErrorResponse(badgeData, err, res, errorMessages = {}) {
   errorMessages = { ...defaultErrorMessages, ...errorMessages }
   if (err != null) {
     badgeData.text[1] = 'inaccessible'
diff --git a/services/docker/docker-helpers.js b/services/docker/docker-helpers.js
index 10a7f53a43808f5aa31cf0de7cd2dd15f68a8b0e..410bd363643aef49c5be1160982251418bc7ddf5 100644
--- a/services/docker/docker-helpers.js
+++ b/services/docker/docker-helpers.js
@@ -2,14 +2,14 @@
 
 const dockerBlue = '066da5' // see https://github.com/badges/shields/pull/1690
 
-const buildDockerUrl = function(badgeName) {
+function buildDockerUrl(badgeName) {
   return {
     base: `docker/${badgeName}`,
     pattern: ':user/:repo',
   }
 }
 
-const getDockerHubUser = function(user) {
+function getDockerHubUser(user) {
   return user === '_' ? 'library' : user
 }
 
diff --git a/services/packagist/packagist-license.service.js b/services/packagist/packagist-license.service.js
index 3527c60c102843df8aefc9684b8b51f1a8fb4d72..e4aec2228e80db3948b255af49cb98f736238ba2 100644
--- a/services/packagist/packagist-license.service.js
+++ b/services/packagist/packagist-license.service.js
@@ -59,15 +59,13 @@ module.exports = class PackagistLicense extends LegacyService {
             // Note: if you change the latest version detection algorithm here,
             // change it above (for the actual version badge).
             let version
-            const unstable = function(ver) {
-              return /dev/.test(ver)
-            }
+            const isUnstable = ({ version }) => version.includes('dev')
             // Grab the latest stable version, or an unstable
             for (const versionName in data.package.versions) {
               const current = data.package.versions[versionName]
 
               if (version !== undefined) {
-                if (unstable(version.version) && !unstable(current.version)) {
+                if (isUnstable(version) && !isUnstable(current)) {
                   version = current
                 } else if (
                   version.version_normalized < current.version_normalized
diff --git a/services/pypi/pypi-helpers.js b/services/pypi/pypi-helpers.js
index 98bd365cb86a78b9dd09bc466422f8ab1b17d8c3..258652cdb24a68b6900262869a137153fc9b3e2b 100644
--- a/services/pypi/pypi-helpers.js
+++ b/services/pypi/pypi-helpers.js
@@ -8,7 +8,7 @@
   our own functions to parse and sort django versions
 */
 
-const parseDjangoVersionString = function(str) {
+function parseDjangoVersionString(str) {
   if (typeof str !== 'string') {
     return false
   }
@@ -21,8 +21,8 @@ const parseDjangoVersionString = function(str) {
   }
 }
 
-// sort an array of django versions low to high
-const sortDjangoVersions = function(versions) {
+// Sort an array of django versions low to high.
+function sortDjangoVersions(versions) {
   return versions.sort((a, b) => {
     if (
       parseDjangoVersionString(a).major === parseDjangoVersionString(b).major
@@ -38,8 +38,8 @@ const sortDjangoVersions = function(versions) {
   })
 }
 
-// extract classifiers from a pypi json response based on a regex
-const parseClassifiers = function(parsedData, pattern) {
+// Extract classifiers from a pypi json response based on a regex.
+function parseClassifiers(parsedData, pattern) {
   const results = []
   for (let i = 0; i < parsedData.info.classifiers.length; i++) {
     const matched = pattern.exec(parsedData.info.classifiers[i])
diff --git a/services/response-fixtures.js b/services/response-fixtures.js
index 49ba6d8a7f0ba8758ff7808a3edadb3e082e9661..56f08546e8d6af91b324a0e44d56c95b4ecb313c 100644
--- a/services/response-fixtures.js
+++ b/services/response-fixtures.js
@@ -1,9 +1,9 @@
 'use strict'
 
-const invalidJSON = function() {
-  return [200, '{{{{{invalid json}}', { 'Content-Type': 'application/json' }]
-}
-
 module.exports = {
-  invalidJSON,
+  invalidJSON: () => [
+    200,
+    '{{{{{invalid json}}',
+    { 'Content-Type': 'application/json' },
+  ],
 }