diff --git a/cometd-nodejs-client.js b/cometd-nodejs-client.js
index 997f741..d6816c8 100644
--- a/cometd-nodejs-client.js
+++ b/cometd-nodejs-client.js
@@ -1,13 +1,30 @@
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
+
 module.exports = {
-    adapt: function(options) {
-        var url = require('url');
-        var httpc = require('http');
-        var https = require('https');
-        var HttpcProxyAgent = require('http-proxy-agent');
-        var HttpsProxyAgent = require('https-proxy-agent');
-        var ws = require('ws');
-
-        var window = global['window'];
+    adapt: options => {
+        const url = require('url');
+        const httpc = require('http');
+        const https = require('https');
+        const HttpcProxyAgent = require('http-proxy-agent');
+        const HttpsProxyAgent = require('https-proxy-agent');
+        const ws = require('ws');
+
+        let window = global['window'];
         if (!window) {
             window = global['window'] = {};
         }
@@ -19,15 +36,15 @@ module.exports = {
         window.console.debug = window.console.log;
 
         // Fields shared by all XMLHttpRequest instances.
-        var _agentOptions = {
+        const _agentOptions = {
             keepAlive: true
         };
-        var _agentc = new httpc.Agent(_agentOptions);
-        var _agents = new https.Agent(_agentOptions);
+        const _agentc = new httpc.Agent(_agentOptions);
+        const _agents = new https.Agent(_agentOptions);
 
-        var _globalCookies = {};
+        const _globalCookies = {};
 
-        var _logLevel = options && options.logLevel || 'info';
+        const _logLevel = options && options.logLevel || 'info';
 
         function _debug() {
             if (_logLevel === 'debug') {
@@ -41,20 +58,20 @@ module.exports = {
 
         // Bare minimum XMLHttpRequest implementation that works with CometD.
         window.XMLHttpRequest = function() {
-            var _localCookies = {};
-            var _config;
-            var _request;
+            const _localCookies = {};
+            let _config;
+            let _request;
 
             function _storeCookie(cookieStore, value) {
-                var host = _config.hostname;
-                var jar = cookieStore[host];
+                const host = _config.hostname;
+                let jar = cookieStore[host];
                 if (jar === undefined) {
                     cookieStore[host] = jar = {};
                 }
-                var cookies = value.split(';');
-                for (var i = 0; i < cookies.length; ++i) {
-                    var cookie = cookies[i].trim();
-                    var equal = cookie.indexOf('=');
+                const cookies = value.split(';');
+                for (let i = 0; i < cookies.length; ++i) {
+                    const cookie = cookies[i].trim();
+                    const equal = cookie.indexOf('=');
                     if (equal > 0) {
                         jar[cookie.substring(0, equal)] = cookie;
                     }
@@ -62,10 +79,10 @@ module.exports = {
             }
 
             function _concatCookies(cookieStore) {
-                var cookies = '';
-                var jar = cookieStore[_config.hostname];
+                let cookies = '';
+                const jar = cookieStore[_config.hostname];
                 if (jar) {
-                    for (var name in jar) {
+                    for (let name in jar) {
                         if (jar.hasOwnProperty(name)) {
                             if (cookies) {
                                 cookies += '; ';
@@ -78,14 +95,14 @@ module.exports = {
             }
 
             function _chooseAgent(serverURI) {
-                var serverHostPort = serverURI.host;
-                var proxy = options && options.httpProxy && options.httpProxy.uri;
+                const serverHostPort = serverURI.host;
+                const proxy = options && options.httpProxy && options.httpProxy.uri;
                 if (proxy) {
-                    var isIncluded = true;
-                    var includes = options.httpProxy.includes;
+                    let isIncluded = true;
+                    const includes = options.httpProxy.includes;
                     if (includes && Array.isArray(includes)) {
                         isIncluded = false;
-                        for (var i = 0; i < includes.length; ++i) {
+                        for (let i = 0; i < includes.length; ++i) {
                             if (new RegExp(includes[i]).test(serverHostPort)) {
                                 isIncluded = true;
                                 break;
@@ -93,9 +110,9 @@ module.exports = {
                         }
                     }
                     if (isIncluded) {
-                        var excludes = options.httpProxy.excludes;
+                        const excludes = options.httpProxy.excludes;
                         if (excludes && Array.isArray(excludes)) {
-                            for (var e = 0; e < excludes.length; ++e) {
+                            for (let e = 0; e < excludes.length; ++e) {
                                 if (new RegExp(excludes[e]).test(serverHostPort)) {
                                     isIncluded = false;
                                     break;
@@ -105,7 +122,7 @@ module.exports = {
                     }
                     if (isIncluded) {
                         _debug('proxying', serverURI.href, 'via', proxy);
-                        var agentOpts = Object.assign(url.parse(proxy), _agentOptions);
+                        const agentOpts = Object.assign(url.parse(proxy), _agentOptions);
                         return _secure(serverURI) ? new HttpsProxyAgent(agentOpts) : new HttpcProxyAgent(agentOpts);
                     }
                 }
@@ -117,7 +134,7 @@ module.exports = {
             this.readyState = window.XMLHttpRequest.UNSENT;
             this.responseText = '';
 
-            this.open = function(method, uri) {
+            this.open = (method, uri) => {
                 _config = url.parse(uri);
                 _config.agent = _chooseAgent(_config);
                 _config.method = method;
@@ -125,7 +142,7 @@ module.exports = {
                 this.readyState = window.XMLHttpRequest.OPENED;
             };
 
-            this.setRequestHeader = function(name, value) {
+            this.setRequestHeader = (name, value) => {
                 if (/^cookie$/i.test(name)) {
                     _storeCookie(_localCookies, value)
                 } else {
@@ -133,71 +150,70 @@ module.exports = {
                 }
             };
 
-            this.send = function(data) {
-                var globalCookies = this.context && this.context.cookieStore;
+            this.send = data => {
+                let globalCookies = this.context && this.context.cookieStore;
                 if (!globalCookies) {
                     globalCookies = _globalCookies;
                 }
-                var cookies1 = _concatCookies(globalCookies);
-                var cookies2 = _concatCookies(_localCookies);
-                var delim = (cookies1 && cookies2) ? '; ' : '';
-                var cookies = cookies1 + delim + cookies2;
+                const cookies1 = _concatCookies(globalCookies);
+                const cookies2 = _concatCookies(_localCookies);
+                const delim = (cookies1 && cookies2) ? '; ' : '';
+                const cookies = cookies1 + delim + cookies2;
                 if (cookies) {
                     _config.headers['Cookie'] = cookies;
                 }
 
-                var self = this;
-                var http = _secure(_config) ? https : httpc;
-                _request = http.request(_config, function(response) {
-                    var success = false;
-                    self.status = response.statusCode;
-                    self.statusText = response.statusMessage;
-                    self.readyState = window.XMLHttpRequest.HEADERS_RECEIVED;
-                    var headers = response.headers;
-                    for (var name in headers) {
+                const http = _secure(_config) ? https : httpc;
+                _request = http.request(_config, response => {
+                    let success = false;
+                    this.status = response.statusCode;
+                    this.statusText = response.statusMessage;
+                    this.readyState = window.XMLHttpRequest.HEADERS_RECEIVED;
+                    const headers = response.headers;
+                    for (let name in headers) {
                         if (headers.hasOwnProperty(name)) {
                             if (/^set-cookie$/i.test(name)) {
-                                var header = headers[name];
-                                for (var i = 0; i < header.length; ++i) {
-                                    var whole = header[i];
-                                    var parts = whole.split(';');
-                                    var cookie = parts[0];
+                                const header = headers[name];
+                                for (let i = 0; i < header.length; ++i) {
+                                    const whole = header[i];
+                                    const parts = whole.split(';');
+                                    const cookie = parts[0];
                                     _storeCookie(globalCookies, cookie);
                                 }
                             }
                         }
                     }
-                    response.on('data', function(chunk) {
-                        self.readyState = window.XMLHttpRequest.LOADING;
-                        self.responseText += chunk;
+                    response.on('data', chunk => {
+                        this.readyState = window.XMLHttpRequest.LOADING;
+                        this.responseText += chunk;
                     });
-                    response.on('end', function() {
+                    response.on('end', () => {
                         success = true;
-                        self.readyState = window.XMLHttpRequest.DONE;
-                        if (self.onload) {
-                            self.onload();
+                        this.readyState = window.XMLHttpRequest.DONE;
+                        if (this.onload) {
+                            this.onload();
                         }
                     });
-                    response.on('close', function() {
+                    response.on('close', () => {
                         if (!success) {
-                            self.readyState = window.XMLHttpRequest.DONE;
-                            if (self.onerror) {
-                                self.onerror();
+                            this.readyState = window.XMLHttpRequest.DONE;
+                            if (this.onerror) {
+                                this.onerror();
                             }
                         }
                     });
                 });
-                ['abort', 'aborted', 'error'].forEach(function(event) {
-                    _request.on(event, function(x) {
-                        self.readyState = window.XMLHttpRequest.DONE;
+                ['abort', 'aborted', 'error'].forEach(event => {
+                    _request.on(event, x => {
+                        this.readyState = window.XMLHttpRequest.DONE;
                         if (x) {
-                            var error = x.message;
+                            const error = x.message;
                             if (error) {
-                                self.statusText = error;
+                                this.statusText = error;
                             }
                         }
-                        if (self.onerror) {
-                            self.onerror(x);
+                        if (this.onerror) {
+                            this.onerror(x);
                         }
                     });
                 });
@@ -207,15 +223,13 @@ module.exports = {
                 _request.end();
             };
 
-            this.abort = function() {
+            this.abort = () => {
                 if (_request) {
                     _request.abort();
                 }
             };
 
-            this._config = function() {
-                return _config;
-            };
+            this._config = () => _config;
         };
         window.XMLHttpRequest.UNSENT = 0;
         window.XMLHttpRequest.OPENED = 1;
diff --git a/test/client.js b/test/client.js
index 9cb2d07..55e7d9c 100644
--- a/test/client.js
+++ b/test/client.js
@@ -1,31 +1,48 @@
-var cometd = require('..');
-var http = require('http');
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('client', function() {
-    var _lib;
-    var _server;
+const cometd = require('..');
+const http = require('http');
 
-    beforeEach(function() {
+describe('client', () => {
+    let _lib;
+    let _server;
+
+    beforeEach(() => {
         cometd.adapt();
         _lib = require('cometd');
     });
 
-    afterEach(function() {
+    afterEach(() => {
         if (_server) {
             _server.close();
         }
     });
 
-    it('performs handshake', function(done) {
-        _server = http.createServer(function(request, response) {
-            var content = '';
-            request.addListener('data', function(chunk) {
+    it('performs handshake', done => {
+        _server = http.createServer((request, response) => {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -37,17 +54,17 @@ describe('client', function() {
                 response.end(content, 'utf8');
             });
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
 
-            var cometd = new _lib.CometD();
+            const cometd = new _lib.CometD();
             cometd.websocketEnabled = false;
             cometd.configure({
                 url: 'http://localhost:' + port + '/cometd',
                 logLevel: 'info'
             });
-            cometd.handshake(function(r) {
+            cometd.handshake(r => {
                 if (r.successful) {
                     done();
                 }
diff --git a/test/cookies.js b/test/cookies.js
index b8f9176..39cfeb5 100644
--- a/test/cookies.js
+++ b/test/cookies.js
@@ -1,25 +1,42 @@
-var assert = require('assert');
-var cometd = require('..');
-var http = require('http');
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('cookies', function() {
-    var _runtime;
-    var _server;
+const assert = require('assert');
+const cometd = require('..');
+const http = require('http');
 
-    beforeEach(function() {
+describe('cookies', () => {
+    let _runtime;
+    let _server;
+
+    beforeEach(() => {
         cometd.adapt();
         _runtime = global.window;
     });
 
-    afterEach(function() {
+    afterEach(() => {
         if (_server) {
             _server.close();
         }
     });
 
-    it('receives, stores and sends cookie', function(done) {
-        var cookie = 'a=b';
-        _server = http.createServer(function(request, response) {
+    it('receives, stores and sends cookie', done => {
+        const cookie = 'a=b';
+        _server = http.createServer((request, response) => {
             if (/\/1$/.test(request.url)) {
                 response.setHeader('Set-Cookie', cookie);
                 response.end();
@@ -28,18 +45,18 @@ describe('cookies', function() {
                 response.end();
             }
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
-            var uri = 'http://localhost:' + port;
+            const uri = 'http://localhost:' + port;
 
-            var xhr1 = new _runtime.XMLHttpRequest();
+            const xhr1 = new _runtime.XMLHttpRequest();
             xhr1.open('GET', uri + '/1');
-            xhr1.onload = function() {
+            xhr1.onload = () => {
                 assert.strictEqual(xhr1.status, 200);
-                var xhr2 = new _runtime.XMLHttpRequest();
+                const xhr2 = new _runtime.XMLHttpRequest();
                 xhr2.open('GET', uri + '/2');
-                xhr2.onload = function() {
+                xhr2.onload = () => {
                     assert.strictEqual(xhr2.status, 200);
                     done();
                 };
@@ -49,11 +66,11 @@ describe('cookies', function() {
         });
     });
 
-    it('sends multiple cookies', function(done) {
-        var cookie1 = 'a=b';
-        var cookie2 = 'c=d';
-        var cookies = cookie1 + '; ' + cookie2;
-        _server = http.createServer(function(request, response) {
+    it('sends multiple cookies', done => {
+        const cookie1 = 'a=b';
+        const cookie2 = 'c=d';
+        const cookies = cookie1 + '; ' + cookie2;
+        _server = http.createServer((request, response) => {
             if (/\/1$/.test(request.url)) {
                 response.setHeader('Set-Cookie', cookie1);
                 response.end();
@@ -65,22 +82,22 @@ describe('cookies', function() {
                 response.end();
             }
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
-            var uri = 'http://localhost:' + port;
+            const uri = 'http://localhost:' + port;
 
-            var xhr1 = new _runtime.XMLHttpRequest();
+            const xhr1 = new _runtime.XMLHttpRequest();
             xhr1.open('GET', uri + '/1');
-            xhr1.onload = function() {
+            xhr1.onload = () => {
                 assert.strictEqual(xhr1.status, 200);
-                var xhr2 = new _runtime.XMLHttpRequest();
+                const xhr2 = new _runtime.XMLHttpRequest();
                 xhr2.open('GET', uri + '/2');
-                xhr2.onload = function() {
+                xhr2.onload = () => {
                     assert.strictEqual(xhr2.status, 200);
-                    var xhr3 = new _runtime.XMLHttpRequest();
+                    const xhr3 = new _runtime.XMLHttpRequest();
                     xhr3.open('GET', uri + '/3');
-                    xhr3.onload = function() {
+                    xhr3.onload = () => {
                         assert.strictEqual(xhr3.status, 200);
                         done();
                     };
@@ -92,10 +109,10 @@ describe('cookies', function() {
         });
     });
 
-    it('handles cookies from different hosts', function(done) {
-        var cookieA = 'a=b';
-        var cookieB = 'b=c';
-        _server = http.createServer(function(request, response) {
+    it('handles cookies from different hosts', done => {
+        const cookieA = 'a=b';
+        const cookieB = 'b=c';
+        _server = http.createServer((request, response) => {
             if (/\/hostA\//.test(request.url)) {
                 if (/\/1$/.test(request.url)) {
                     response.setHeader('Set-Cookie', cookieA);
@@ -114,26 +131,26 @@ describe('cookies', function() {
                 }
             }
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
 
-            var xhrA1 = new _runtime.XMLHttpRequest();
+            const xhrA1 = new _runtime.XMLHttpRequest();
             xhrA1.open('GET', 'http://localhost:' + port + '/hostA/1');
-            xhrA1.onload = function() {
+            xhrA1.onload = () => {
                 assert.strictEqual(xhrA1.status, 200);
-                var xhrA2 = new _runtime.XMLHttpRequest();
+                const xhrA2 = new _runtime.XMLHttpRequest();
                 xhrA2.open('GET', 'http://localhost:' + port + '/hostA/2');
-                xhrA2.onload = function() {
+                xhrA2.onload = () => {
                     assert.strictEqual(xhrA2.status, 200);
 
-                    var xhrB1 = new _runtime.XMLHttpRequest();
+                    const xhrB1 = new _runtime.XMLHttpRequest();
                     xhrB1.open('GET', 'http://127.0.0.1:' + port + '/hostB/1');
-                    xhrB1.onload = function() {
+                    xhrB1.onload = () => {
                         assert.strictEqual(xhrB1.status, 200);
-                        var xhrB2 = new _runtime.XMLHttpRequest();
+                        const xhrB2 = new _runtime.XMLHttpRequest();
                         xhrB2.open('GET', 'http://127.0.0.1:' + port + '/hostB/2');
-                        xhrB2.onload = function() {
+                        xhrB2.onload = () => {
                             assert.strictEqual(xhrB2.status, 200);
                             done();
                         };
@@ -147,11 +164,11 @@ describe('cookies', function() {
         });
     });
 
-    it('handles cookie sent multiple times', function(done) {
-        var cookieName = 'a';
-        var cookieValue = 'b';
-        var cookie = cookieName + '=' + cookieValue;
-        _server = http.createServer(function(request, response) {
+    it('handles cookie sent multiple times', done => {
+        const cookieName = 'a';
+        const cookieValue = 'b';
+        const cookie = cookieName + '=' + cookieValue;
+        _server = http.createServer((request, response) => {
             if (/\/verify$/.test(request.url)) {
                 assert.strictEqual(request.headers['cookie'], cookie);
                 response.end();
@@ -160,21 +177,21 @@ describe('cookies', function() {
                 response.end();
             }
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
 
-            var xhr1 = new _runtime.XMLHttpRequest();
+            const xhr1 = new _runtime.XMLHttpRequest();
             xhr1.open('GET', 'http://localhost:' + port + '/1');
-            xhr1.onload = function() {
+            xhr1.onload = () => {
                 assert.strictEqual(xhr1.status, 200);
-                var xhr2 = new _runtime.XMLHttpRequest();
+                const xhr2 = new _runtime.XMLHttpRequest();
                 xhr2.open('GET', 'http://localhost:' + port + '/2');
-                xhr2.onload = function() {
+                xhr2.onload = () => {
                     assert.strictEqual(xhr2.status, 200);
-                    var xhr3 = new _runtime.XMLHttpRequest();
+                    const xhr3 = new _runtime.XMLHttpRequest();
                     xhr3.open('GET', 'http://localhost:' + port + '/verify');
-                    xhr3.onload = function() {
+                    xhr3.onload = () => {
                         assert.strictEqual(xhr1.status, 200);
                         done();
                     };
@@ -186,9 +203,9 @@ describe('cookies', function() {
         });
     });
 
-    it('handles cookies as request headers', function(done) {
-        _server = http.createServer(function(request, response) {
-            var cookies = request.headers['cookie'];
+    it('handles cookies as request headers', done => {
+        _server = http.createServer((request, response) => {
+            const cookies = request.headers['cookie'];
             if (/\/1$/.test(request.url)) {
                 response.setHeader('Set-Cookie', 'a=b');
                 response.end();
@@ -204,22 +221,22 @@ describe('cookies', function() {
                 response.end();
             }
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
 
-            var xhr1 = new _runtime.XMLHttpRequest();
+            const xhr1 = new _runtime.XMLHttpRequest();
             xhr1.open('GET', 'http://localhost:' + port + '/1');
-            xhr1.onload = function() {
+            xhr1.onload = () => {
                 assert.strictEqual(xhr1.status, 200);
-                var xhr2 = new _runtime.XMLHttpRequest();
+                const xhr2 = new _runtime.XMLHttpRequest();
                 xhr2.open('GET', 'http://localhost:' + port + '/2');
                 xhr2.setRequestHeader('cookie', 'c=d; e=f');
-                xhr2.onload = function() {
+                xhr2.onload = () => {
                     assert.strictEqual(xhr2.status, 200);
-                    var xhr3 = new _runtime.XMLHttpRequest();
+                    const xhr3 = new _runtime.XMLHttpRequest();
                     xhr3.open('GET', 'http://localhost:' + port + '/3');
-                    xhr3.onload = function() {
+                    xhr3.onload = () => {
                         assert.strictEqual(xhr3.status, 200);
                         done();
                     };
diff --git a/test/https.js b/test/https.js
index 17e6b40..0fafb0a 100644
--- a/test/https.js
+++ b/test/https.js
@@ -1,40 +1,57 @@
-var assert = require('assert');
-var cometd = require('..');
-var https = require('https');
-var fs = require('fs');
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('https', function() {
-    var _runtime;
-    var _server;
+const assert = require('assert');
+const cometd = require('..');
+const https = require('https');
+const fs = require('fs');
 
-    beforeEach(function() {
+describe('https', () => {
+    let _runtime;
+    let _server;
+
+    beforeEach(() => {
         cometd.adapt();
         _runtime = global.window;
     });
 
-    afterEach(function() {
+    afterEach(() => {
         if (_server) {
             _server.close();
         }
     });
 
-    it('supports https', function(done) {
-        var options = {
+    it('supports https', done => {
+        const options = {
             key: fs.readFileSync('test/tls/private.pem'),
             cert: fs.readFileSync('test/tls/public.pem')
         };
-        _server = https.createServer(options, function(request, response) {
+        _server = https.createServer(options, (request, response) => {
             response.end();
         });
-        _server.listen(0, 'localhost', function() {
-            var port = _server.address().port;
+        _server.listen(0, 'localhost', () => {
+            const port = _server.address().port;
             console.log('listening on localhost:' + port);
-            var uri = 'https://localhost:' + port;
-            var xhr = new _runtime.XMLHttpRequest();
+            const uri = 'https://localhost:' + port;
+            const xhr = new _runtime.XMLHttpRequest();
             xhr.open('GET', uri + '/');
             // Allow self-signed certificates.
             xhr._config().rejectUnauthorized = false;
-            xhr.onload = function() {
+            xhr.onload = () => {
                 assert.strictEqual(xhr.status, 200);
                 done();
             };
diff --git a/test/library.js b/test/library.js
index 10624d0..54155e5 100644
--- a/test/library.js
+++ b/test/library.js
@@ -1,8 +1,25 @@
-var assert = require('assert');
-var cometd = require('..');
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('library', function() {
-    it('adapter method exported', function() {
+const assert = require('assert');
+const cometd = require('..');
+
+describe('library', () => {
+    it('adapter method exported', () => {
         assert.ok(cometd.adapt);
     });
 });
diff --git a/test/proxy.js b/test/proxy.js
index 019bcc9..2624bae 100644
--- a/test/proxy.js
+++ b/test/proxy.js
@@ -1,32 +1,49 @@
-var assert = require('assert');
-var nodeCometD = require('..');
-var http = require('http');
-var url = require('url');
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('proxy', function() {
-    var _lib = require('cometd');
-    var _proxy;
+const assert = require('assert');
+const nodeCometD = require('..');
+const http = require('http');
+const url = require('url');
 
-    afterEach(function() {
+describe('proxy', () => {
+    const _lib = require('cometd');
+    let _proxy;
+
+    afterEach(() => {
         if (_proxy) {
             _proxy.close();
         }
     });
 
-    it('proxies cometd calls', function(done) {
-        _proxy = http.createServer(function(request, response) {
-            var serverPort = parseInt(url.parse(request.url).port);
+    it('proxies cometd calls', done => {
+        _proxy = http.createServer((request, response) => {
+            const serverPort = parseInt(url.parse(request.url).port);
             assert.ok(Number.isInteger(serverPort));
             assert.notStrictEqual(serverPort, _proxy.address().port);
 
-            var content = '';
-            request.addListener('data', function(chunk) {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -38,8 +55,8 @@ describe('proxy', function() {
                 response.end(content, 'utf8');
             });
         });
-        _proxy.listen(0, 'localhost', function() {
-            var proxyPort = _proxy.address().port;
+        _proxy.listen(0, 'localhost', () => {
+            const proxyPort = _proxy.address().port;
             console.log('proxy listening on localhost:' + proxyPort);
 
             nodeCometD.adapt({
@@ -50,14 +67,14 @@ describe('proxy', function() {
             });
 
             // Any port will do for the server.
-            var serverPort = proxyPort + 1;
-            var cometd = new _lib.CometD();
+            const serverPort = proxyPort + 1;
+            const cometd = new _lib.CometD();
             cometd.websocketEnabled = false;
             cometd.configure({
                 url: 'http://localhost:' + serverPort + '/cometd',
                 logLevel: 'info'
             });
-            cometd.handshake(function(r) {
+            cometd.handshake(r => {
                 if (r.successful) {
                     done();
                 } else {
@@ -69,20 +86,20 @@ describe('proxy', function() {
         });
     });
 
-    it('proxies with includes list', function(done) {
-        _proxy = http.createServer(function(request, response) {
-            var serverPort = parseInt(url.parse(request.url).port);
+    it('proxies with includes list', done => {
+        _proxy = http.createServer((request, response) => {
+            const serverPort = parseInt(url.parse(request.url).port);
             assert.ok(Number.isInteger(serverPort));
             assert.notStrictEqual(serverPort, _proxy.address().port);
 
-            var content = '';
-            request.addListener('data', function(chunk) {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -94,12 +111,12 @@ describe('proxy', function() {
                 response.end(content, 'utf8');
             });
         });
-        _proxy.listen(0, 'localhost', function() {
-            var proxyPort = _proxy.address().port;
+        _proxy.listen(0, 'localhost', () => {
+            const proxyPort = _proxy.address().port;
             console.log('proxy listening on localhost:' + proxyPort);
             // Any port will do for the server.
-            var serverPort1 = proxyPort + 1;
-            var serverPort2 = proxyPort + 2;
+            const serverPort1 = proxyPort + 1;
+            const serverPort2 = proxyPort + 2;
 
             nodeCometD.adapt({
                 httpProxy: {
@@ -108,21 +125,21 @@ describe('proxy', function() {
                 }
             });
 
-            var cometd1 = new _lib.CometD();
+            const cometd1 = new _lib.CometD();
             cometd1.websocketEnabled = false;
             cometd1.configure({
                 url: 'http://localhost:' + serverPort1 + '/cometd',
                 logLevel: 'info'
             });
-            cometd1.handshake(function(r) {
+            cometd1.handshake(r => {
                 if (r.successful) {
-                    var cometd2 = new _lib.CometD();
+                    const cometd2 = new _lib.CometD();
                     cometd2.websocketEnabled = false;
                     cometd2.configure({
                         url: 'http://localhost:' + serverPort2 + '/cometd',
                         logLevel: 'info'
                     });
-                    cometd2.handshake(function(r) {
+                    cometd2.handshake(r => {
                         if (r.successful) {
                             done(new Error('must not handshake'));
                         } else {
@@ -139,20 +156,20 @@ describe('proxy', function() {
         });
     });
 
-    it('proxies with excludes list', function(done) {
-        _proxy = http.createServer(function(request, response) {
-            var serverPort = parseInt(url.parse(request.url).port);
+    it('proxies with excludes list', done => {
+        _proxy = http.createServer((request, response) => {
+            const serverPort = parseInt(url.parse(request.url).port);
             assert.ok(Number.isInteger(serverPort));
             assert.notStrictEqual(serverPort, _proxy.address().port);
 
-            var content = '';
-            request.addListener('data', function(chunk) {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -164,12 +181,12 @@ describe('proxy', function() {
                 response.end(content, 'utf8');
             });
         });
-        _proxy.listen(0, 'localhost', function() {
-            var proxyPort = _proxy.address().port;
+        _proxy.listen(0, 'localhost', () => {
+            const proxyPort = _proxy.address().port;
             console.log('proxy listening on localhost:' + proxyPort);
             // Any port will do for the server.
-            var serverPort1 = proxyPort + 1;
-            var serverPort2 = proxyPort + 2;
+            const serverPort1 = proxyPort + 1;
+            const serverPort2 = proxyPort + 2;
 
             nodeCometD.adapt({
                 httpProxy: {
@@ -178,25 +195,25 @@ describe('proxy', function() {
                 }
             });
 
-            var cometd1 = new _lib.CometD();
+            const cometd1 = new _lib.CometD();
             cometd1.websocketEnabled = false;
             cometd1.configure({
                 url: 'http://localhost:' + serverPort1 + '/cometd',
                 logLevel: 'info'
             });
-            cometd1.handshake(function(r) {
+            cometd1.handshake(r => {
                 if (r.successful) {
                     done(new Error('could not handshake'));
                 } else {
                     // Stop /meta/handshake retries.
                     cometd1.disconnect();
-                    var cometd2 = new _lib.CometD();
+                    const cometd2 = new _lib.CometD();
                     cometd2.websocketEnabled = false;
                     cometd2.configure({
                         url: 'http://localhost:' + serverPort2 + '/cometd',
                         logLevel: 'info'
                     });
-                    cometd2.handshake(function(r) {
+                    cometd2.handshake(r => {
                         if (r.successful) {
                             done();
                         } else {
@@ -210,20 +227,20 @@ describe('proxy', function() {
         });
     });
 
-    it('proxies with includes and excludes list', function(done) {
-        _proxy = http.createServer(function(request, response) {
-            var serverPort = parseInt(url.parse(request.url).port);
+    it('proxies with includes and excludes list', done => {
+        _proxy = http.createServer((request, response) => {
+            const serverPort = parseInt(url.parse(request.url).port);
             assert.ok(Number.isInteger(serverPort));
             assert.notStrictEqual(serverPort, _proxy.address().port);
 
-            var content = '';
-            request.addListener('data', function(chunk) {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -235,12 +252,12 @@ describe('proxy', function() {
                 response.end(content, 'utf8');
             });
         });
-        _proxy.listen(0, 'localhost', function() {
-            var proxyPort = _proxy.address().port;
+        _proxy.listen(0, 'localhost', () => {
+            const proxyPort = _proxy.address().port;
             console.log('proxy listening on localhost:' + proxyPort);
             // Any port will do for the server.
-            var serverPort1 = proxyPort + 1;
-            var serverPort2 = proxyPort + 2;
+            const serverPort1 = proxyPort + 1;
+            const serverPort2 = proxyPort + 2;
 
             nodeCometD.adapt({
                 httpProxy: {
@@ -250,25 +267,25 @@ describe('proxy', function() {
                 }
             });
 
-            var cometd1 = new _lib.CometD();
+            const cometd1 = new _lib.CometD();
             cometd1.websocketEnabled = false;
             cometd1.configure({
                 url: 'http://localhost:' + serverPort1 + '/cometd',
                 logLevel: 'info'
             });
-            cometd1.handshake(function(r) {
+            cometd1.handshake(r => {
                 if (r.successful) {
                     done(new Error('could not handshake'));
                 } else {
                     // Stop /meta/handshake retries.
                     cometd1.disconnect();
-                    var cometd2 = new _lib.CometD();
+                    const cometd2 = new _lib.CometD();
                     cometd2.websocketEnabled = false;
                     cometd2.configure({
                         url: 'http://localhost:' + serverPort2 + '/cometd',
                         logLevel: 'info'
                     });
-                    cometd2.handshake(function(r) {
+                    cometd2.handshake(r => {
                         if (r.successful) {
                             done();
                         } else {
@@ -282,20 +299,20 @@ describe('proxy', function() {
         });
     });
 
-    it('proxies with authentication', function(done) {
-        _proxy = http.createServer(function(request, response) {
-            var proxyAuth = request.headers['proxy-authorization'];
+    it('proxies with authentication', done => {
+        _proxy = http.createServer((request, response) => {
+            const proxyAuth = request.headers['proxy-authorization'];
             assert.ok(proxyAuth);
             assert.ok(proxyAuth.startsWith('Basic '));
 
-            var content = '';
-            request.addListener('data', function(chunk) {
+            let content = '';
+            request.addListener('data', chunk => {
                 content += chunk;
             });
-            request.addListener('end', function() {
+            request.addListener('end', () => {
                 response.statusCode = 200;
                 response.setHeader('Content-Type', 'application/json');
-                var content = '[{' +
+                const content = '[{' +
                     '"id":"1",' +
                     '"version":"1.0",' +
                     '"channel":"/meta/handshake",' +
@@ -307,8 +324,8 @@ describe('proxy', function() {
                 response.end(content, 'utf8');
             });
         });
-        _proxy.listen(0, 'localhost', function() {
-            var proxyPort = _proxy.address().port;
+        _proxy.listen(0, 'localhost', () => {
+            const proxyPort = _proxy.address().port;
             console.log('proxy listening on localhost:' + proxyPort);
 
             nodeCometD.adapt({
@@ -318,14 +335,14 @@ describe('proxy', function() {
             });
 
             // Any port will do for the server.
-            var serverPort = proxyPort + 1;
-            var cometd = new _lib.CometD();
+            const serverPort = proxyPort + 1;
+            const cometd = new _lib.CometD();
             cometd.websocketEnabled = false;
             cometd.configure({
                 url: 'http://localhost:' + serverPort + '/cometd',
                 logLevel: 'info'
             });
-            cometd.handshake(function(r) {
+            cometd.handshake(r => {
                 if (r.successful) {
                     done();
                 } else {
diff --git a/test/websocket.js b/test/websocket.js
index ca8e207..15e73ec 100644
--- a/test/websocket.js
+++ b/test/websocket.js
@@ -1,25 +1,42 @@
-var assert = require('assert');
-var nodeCometD = require('..');
-var ws = require("ws");
+/*
+ * Copyright (c) 2017-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
 
-describe('websocket', function() {
-    var _lib = require('cometd');
-    var _server;
+const assert = require('assert');
+const nodeCometD = require('..');
+const ws = require("ws");
 
-    afterEach(function() {
+describe('websocket', () => {
+    const _lib = require('cometd');
+    let _server;
+
+    afterEach(() => {
         if (_server) {
             _server.close();
         }
     });
 
-    it('handshakes with websocket', function(done) {
-        _server = new ws.Server({port: 0}, function() {
-            _server.on('connection', function(s) {
-                s.on('message', function(m) {
-                    var handshake = JSON.parse(m)[0];
+    it('handshakes with websocket', done => {
+        _server = new ws.Server({port: 0}, () => {
+            _server.on('connection', s => {
+                s.on('message', m => {
+                    const handshake = JSON.parse(m)[0];
                     assert.strictEqual('/meta/handshake', handshake.channel);
                     assert.ok(handshake.supportedConnectionTypes.indexOf('websocket') >= 0);
-                    var reply = [{
+                    const reply = [{
                         id: handshake.id,
                         channel: handshake.channel,
                         successful: true,
@@ -35,12 +52,12 @@ describe('websocket', function() {
             });
 
             nodeCometD.adapt();
-            var cometd = new _lib.CometD();
+            const cometd = new _lib.CometD();
             cometd.configure({
                 url: 'http://localhost:' + _server.address().port,
                 logLevel: 'info'
             });
-            cometd.handshake(function(r) {
+            cometd.handshake(r => {
                 if (r.successful) {
                     done();
                 } else {