diff --git a/benchmark/domain/domain-fn-args.js b/benchmark/domain/domain-fn-args.js
index c889b35442d046..18906a9ce64099 100644
--- a/benchmark/domain/domain-fn-args.js
+++ b/benchmark/domain/domain-fn-args.js
@@ -15,7 +15,7 @@ function main({ n, args }) {
   bench.start();
 
   bdomain.enter();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     if (myArguments.length >= 2) {
       const args = Array.prototype.slice.call(myArguments, 1);
       fn.apply(this, args);
diff --git a/benchmark/es/defaultparams-bench.js b/benchmark/es/defaultparams-bench.js
index c568c12ae03fda..ab9cc45749c996 100644
--- a/benchmark/es/defaultparams-bench.js
+++ b/benchmark/es/defaultparams-bench.js
@@ -22,14 +22,14 @@ function defaultParams(x = 1, y = 2) {
 
 function runOldStyleDefaults(n) {
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     oldStyleDefaults();
   bench.end(n);
 }
 
 function runDefaultParams(n) {
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     defaultParams();
   bench.end(n);
 }
diff --git a/benchmark/es/destructuring-bench.js b/benchmark/es/destructuring-bench.js
index 37f3fd9ad3d601..f1b484bd47e08f 100644
--- a/benchmark/es/destructuring-bench.js
+++ b/benchmark/es/destructuring-bench.js
@@ -9,9 +9,9 @@ const bench = common.createBenchmark(main, {
 });
 
 function runSwapManual(n) {
-  var x, y, r;
+  let x, y, r;
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     x = 1, y = 2;
     r = x;
     x = y;
@@ -23,9 +23,9 @@ function runSwapManual(n) {
 }
 
 function runSwapDestructured(n) {
-  var x, y;
+  let x, y;
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     x = 1, y = 2;
     [x, y] = [y, x];
     assert.strictEqual(x, 2);
diff --git a/benchmark/es/destructuring-object-bench.js b/benchmark/es/destructuring-object-bench.js
index 0c5615fd1e667b..68dc17073badc2 100644
--- a/benchmark/es/destructuring-object-bench.js
+++ b/benchmark/es/destructuring-object-bench.js
@@ -8,10 +8,9 @@ const bench = common.createBenchmark(main, {
 });
 
 function runNormal(n) {
-  var i = 0;
   const o = { x: 0, y: 1 };
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     /* eslint-disable no-unused-vars */
     const x = o.x;
     const y = o.y;
@@ -22,10 +21,9 @@ function runNormal(n) {
 }
 
 function runDestructured(n) {
-  var i = 0;
   const o = { x: 0, y: 1 };
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     /* eslint-disable no-unused-vars */
     const { x, y, r = 2 } = o;
     /* eslint-enable no-unused-vars */
diff --git a/benchmark/es/foreach-bench.js b/benchmark/es/foreach-bench.js
index f424ca7a2552b2..88bfed00fbc27c 100644
--- a/benchmark/es/foreach-bench.js
+++ b/benchmark/es/foreach-bench.js
@@ -10,8 +10,8 @@ const bench = common.createBenchmark(main, {
 
 function useFor(n, items, count) {
   bench.start();
-  for (var i = 0; i < n; i++) {
-    for (var j = 0; j < count; j++) {
+  for (let i = 0; i < n; i++) {
+    for (let j = 0; j < count; j++) {
       // eslint-disable-next-line no-unused-vars
       const item = items[j];
     }
@@ -21,7 +21,7 @@ function useFor(n, items, count) {
 
 function useForOf(n, items) {
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     // eslint-disable-next-line no-unused-vars
     for (const item of items) {}
   }
@@ -30,8 +30,8 @@ function useForOf(n, items) {
 
 function useForIn(n, items) {
   bench.start();
-  for (var i = 0; i < n; i++) {
-    for (var j in items) {
+  for (let i = 0; i < n; i++) {
+    for (const j in items) {
       // eslint-disable-next-line no-unused-vars
       const item = items[j];
     }
@@ -41,7 +41,7 @@ function useForIn(n, items) {
 
 function useForEach(n, items) {
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     items.forEach((item) => {});
   }
   bench.end(n);
@@ -49,8 +49,8 @@ function useForEach(n, items) {
 
 function main({ n, count, method }) {
   const items = new Array(count);
-  var fn;
-  for (var i = 0; i < count; i++)
+  let fn;
+  for (let i = 0; i < count; i++)
     items[i] = i;
 
   switch (method) {
diff --git a/benchmark/es/map-bench.js b/benchmark/es/map-bench.js
index a2f9e2450a1f23..1b3ba4789db073 100644
--- a/benchmark/es/map-bench.js
+++ b/benchmark/es/map-bench.js
@@ -14,7 +14,7 @@ const bench = common.createBenchmark(main, {
 function runObject(n) {
   const m = {};
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m[`i${i}`] = i;
     m[`s${i}`] = String(i);
     assert.strictEqual(String(m[`i${i}`]), m[`s${i}`]);
@@ -27,7 +27,7 @@ function runObject(n) {
 function runNullProtoObject(n) {
   const m = Object.create(null);
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m[`i${i}`] = i;
     m[`s${i}`] = String(i);
     assert.strictEqual(String(m[`i${i}`]), m[`s${i}`]);
@@ -40,7 +40,7 @@ function runNullProtoObject(n) {
 function runNullProtoLiteralObject(n) {
   const m = { __proto__: null };
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m[`i${i}`] = i;
     m[`s${i}`] = String(i);
     assert.strictEqual(String(m[`i${i}`]), m[`s${i}`]);
@@ -56,7 +56,7 @@ StorageObject.prototype = Object.create(null);
 function runStorageObject(n) {
   const m = new StorageObject();
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m[`i${i}`] = i;
     m[`s${i}`] = String(i);
     assert.strictEqual(String(m[`i${i}`]), m[`s${i}`]);
@@ -79,7 +79,7 @@ function fakeMap() {
 function runFakeMap(n) {
   const m = fakeMap();
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m.set(`i${i}`, i);
     m.set(`s${i}`, String(i));
     assert.strictEqual(String(m.get(`i${i}`)), m.get(`s${i}`));
@@ -92,7 +92,7 @@ function runFakeMap(n) {
 function runMap(n) {
   const m = new Map();
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     m.set(`i${i}`, i);
     m.set(`s${i}`, String(i));
     assert.strictEqual(String(m.get(`i${i}`)), m.get(`s${i}`));
diff --git a/benchmark/es/restparams-bench.js b/benchmark/es/restparams-bench.js
index d9b4878cb316a0..d568e28744541f 100644
--- a/benchmark/es/restparams-bench.js
+++ b/benchmark/es/restparams-bench.js
@@ -11,7 +11,7 @@ const bench = common.createBenchmark(main, {
 function copyArguments() {
   const len = arguments.length;
   const args = new Array(len);
-  for (var i = 0; i < len; i++)
+  for (let i = 0; i < len; i++)
     args[i] = arguments[i];
   assert.strictEqual(args[0], 1);
   assert.strictEqual(args[1], 2);
@@ -34,22 +34,22 @@ function useArguments() {
 }
 
 function runCopyArguments(n) {
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     copyArguments(1, 2, 'a', 'b');
 }
 
 function runRestArguments(n) {
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     restArguments(1, 2, 'a', 'b');
 }
 
 function runUseArguments(n) {
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     useArguments(1, 2, 'a', 'b');
 }
 
 function main({ n, method }) {
-  var fn;
+  let fn;
   switch (method) {
     case '':
       // Empty string falls through to next line as default, mostly for tests.
diff --git a/benchmark/es/spread-assign.js b/benchmark/es/spread-assign.js
index bbe07e02e15110..97a5c5458e396b 100644
--- a/benchmark/es/spread-assign.js
+++ b/benchmark/es/spread-assign.js
@@ -16,26 +16,25 @@ function main({ n, context, count, rest, method }) {
     src[`p${n}`] = n;
 
   let obj;  // eslint-disable-line no-unused-vars
-  let i;
 
   switch (method) {
     case '':
       // Empty string falls through to next line as default, mostly for tests.
     case '_extend':
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         obj = util._extend({}, src);
       bench.end(n);
       break;
     case 'assign':
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         obj = Object.assign({}, src);
       bench.end(n);
       break;
     case 'spread':
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         obj = { ...src };
       bench.end(n);
       break;
diff --git a/benchmark/es/spread-bench.js b/benchmark/es/spread-bench.js
index 97c7596b6b732b..fc56c9433f2add 100644
--- a/benchmark/es/spread-bench.js
+++ b/benchmark/es/spread-bench.js
@@ -25,10 +25,10 @@ function makeTest(count, rest) {
 
 function main({ n, context, count, rest, method }) {
   const ctx = context === 'context' ? {} : null;
-  var fn = makeTest(count, rest);
+  let fn = makeTest(count, rest);
   const args = new Array(count);
-  var i;
-  for (i = 0; i < count; i++)
+
+  for (let i = 0; i < count; i++)
     args[i] = i;
 
   switch (method) {
@@ -36,7 +36,7 @@ function main({ n, context, count, rest, method }) {
       // Empty string falls through to next line as default, mostly for tests.
     case 'apply':
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         fn.apply(ctx, args);
       bench.end(n);
       break;
@@ -44,13 +44,13 @@ function main({ n, context, count, rest, method }) {
       if (ctx !== null)
         fn = fn.bind(ctx);
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         fn(...args);
       bench.end(n);
       break;
     case 'call-spread':
       bench.start();
-      for (i = 0; i < n; i++)
+      for (let i = 0; i < n; i++)
         fn.call(ctx, ...args);
       bench.end(n);
       break;
diff --git a/benchmark/events/ee-add-remove.js b/benchmark/events/ee-add-remove.js
index 2d20736b057568..715eab29a59c4c 100644
--- a/benchmark/events/ee-add-remove.js
+++ b/benchmark/events/ee-add-remove.js
@@ -8,17 +8,16 @@ function main({ n }) {
   const ee = new events.EventEmitter();
   const listeners = [];
 
-  var k;
-  for (k = 0; k < 10; k += 1)
+  for (let k = 0; k < 10; k += 1)
     listeners.push(() => {});
 
   bench.start();
-  for (var i = 0; i < n; i += 1) {
+  for (let i = 0; i < n; i += 1) {
     const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
-    for (k = listeners.length; --k >= 0; /* empty */) {
+    for (let k = listeners.length; --k >= 0; /* empty */) {
       ee.on(dummy, listeners[k]);
     }
-    for (k = listeners.length; --k >= 0; /* empty */) {
+    for (let k = listeners.length; --k >= 0; /* empty */) {
       ee.removeListener(dummy, listeners[k]);
     }
   }
diff --git a/benchmark/events/ee-emit.js b/benchmark/events/ee-emit.js
index 8f1c2761ffd4ba..39bcbb937d6c56 100644
--- a/benchmark/events/ee-emit.js
+++ b/benchmark/events/ee-emit.js
@@ -11,35 +11,34 @@ const bench = common.createBenchmark(main, {
 function main({ n, argc, listeners }) {
   const ee = new EventEmitter();
 
-  for (var k = 0; k < listeners; k += 1)
+  for (let k = 0; k < listeners; k += 1)
     ee.on('dummy', () => {});
 
-  var i;
   switch (argc) {
     case 2:
       bench.start();
-      for (i = 0; i < n; i += 1) {
+      for (let i = 0; i < n; i += 1) {
         ee.emit('dummy', true, 5);
       }
       bench.end(n);
       break;
     case 4:
       bench.start();
-      for (i = 0; i < n; i += 1) {
+      for (let i = 0; i < n; i += 1) {
         ee.emit('dummy', true, 5, 10, false);
       }
       bench.end(n);
       break;
     case 10:
       bench.start();
-      for (i = 0; i < n; i += 1) {
+      for (let i = 0; i < n; i += 1) {
         ee.emit('dummy', true, 5, 10, false, 5, 'string', true, false, 11, 20);
       }
       bench.end(n);
       break;
     default:
       bench.start();
-      for (i = 0; i < n; i += 1) {
+      for (let i = 0; i < n; i += 1) {
         ee.emit('dummy');
       }
       bench.end(n);
diff --git a/benchmark/events/ee-listener-count-on-prototype.js b/benchmark/events/ee-listener-count-on-prototype.js
index d48e96a46aafe2..04cadc8dcb64da 100644
--- a/benchmark/events/ee-listener-count-on-prototype.js
+++ b/benchmark/events/ee-listener-count-on-prototype.js
@@ -7,13 +7,13 @@ const bench = common.createBenchmark(main, { n: [5e7] });
 function main({ n }) {
   const ee = new EventEmitter();
 
-  for (var k = 0; k < 5; k += 1) {
+  for (let k = 0; k < 5; k += 1) {
     ee.on('dummy0', () => {});
     ee.on('dummy1', () => {});
   }
 
   bench.start();
-  for (var i = 0; i < n; i += 1) {
+  for (let i = 0; i < n; i += 1) {
     const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
     ee.listenerCount(dummy);
   }
diff --git a/benchmark/events/ee-listeners-many.js b/benchmark/events/ee-listeners-many.js
index 7f9099c41790ea..bd3f3538d63c92 100644
--- a/benchmark/events/ee-listeners-many.js
+++ b/benchmark/events/ee-listeners-many.js
@@ -8,13 +8,13 @@ function main({ n }) {
   const ee = new EventEmitter();
   ee.setMaxListeners(101);
 
-  for (var k = 0; k < 50; k += 1) {
+  for (let k = 0; k < 50; k += 1) {
     ee.on('dummy0', () => {});
     ee.on('dummy1', () => {});
   }
 
   bench.start();
-  for (var i = 0; i < n; i += 1) {
+  for (let i = 0; i < n; i += 1) {
     const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
     ee.listeners(dummy);
   }
diff --git a/benchmark/events/ee-listeners.js b/benchmark/events/ee-listeners.js
index 74b88b043401b5..867393efffcb04 100644
--- a/benchmark/events/ee-listeners.js
+++ b/benchmark/events/ee-listeners.js
@@ -7,13 +7,13 @@ const bench = common.createBenchmark(main, { n: [5e6] });
 function main({ n }) {
   const ee = new EventEmitter();
 
-  for (var k = 0; k < 5; k += 1) {
+  for (let k = 0; k < 5; k += 1) {
     ee.on('dummy0', () => {});
     ee.on('dummy1', () => {});
   }
 
   bench.start();
-  for (var i = 0; i < n; i += 1) {
+  for (let i = 0; i < n; i += 1) {
     const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
     ee.listeners(dummy);
   }
diff --git a/benchmark/fixtures/simple-http-server.js b/benchmark/fixtures/simple-http-server.js
index 2b4f1ea956d7cc..fb5b0d87c16e0d 100644
--- a/benchmark/fixtures/simple-http-server.js
+++ b/benchmark/fixtures/simple-http-server.js
@@ -31,14 +31,14 @@ module.exports = http.createServer((req, res) => {
   // URL format: /<type>/<length>/<chunks>/<responseBehavior>/chunkedEnc
   const params = req.url.split('/');
   const command = params[1];
-  var body = '';
+  let body = '';
   const arg = params[2];
   const n_chunks = parseInt(params[3], 10);
   const resHow = params.length >= 5 ? params[4] : 'normal';
   const chunkedEnc = params.length >= 6 && params[5] === '0' ? false : true;
-  var status = 200;
+  let status = 200;
 
-  var n, i;
+  let n, i;
   if (command === 'bytes') {
     n = ~~arg;
     if (n <= 0)
diff --git a/benchmark/fs/bench-readdirSync.js b/benchmark/fs/bench-readdirSync.js
index 5d0e97399a33ff..c0144d3460f8ff 100644
--- a/benchmark/fs/bench-readdirSync.js
+++ b/benchmark/fs/bench-readdirSync.js
@@ -15,7 +15,7 @@ function main({ n, dir, withFileTypes }) {
   withFileTypes = withFileTypes === 'true';
   const fullPath = path.resolve(__dirname, '../../', dir);
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     fs.readdirSync(fullPath, { withFileTypes });
   }
   bench.end(n);
diff --git a/benchmark/fs/bench-realpathSync.js b/benchmark/fs/bench-realpathSync.js
index 7a01bd18cb72bf..d0dbd7f891f126 100644
--- a/benchmark/fs/bench-realpathSync.js
+++ b/benchmark/fs/bench-realpathSync.js
@@ -17,7 +17,7 @@ const bench = common.createBenchmark(main, {
 function main({ n, pathType }) {
   const path = pathType === 'relative' ? relative_path : resolved_path;
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     fs.realpathSync(path);
   }
   bench.end(n);
diff --git a/benchmark/fs/bench-stat.js b/benchmark/fs/bench-stat.js
index 0b2e1972e2cb80..e1f51c762c8e5f 100644
--- a/benchmark/fs/bench-stat.js
+++ b/benchmark/fs/bench-stat.js
@@ -10,7 +10,7 @@ const bench = common.createBenchmark(main, {
 
 
 function main({ n, statType }) {
-  var arg;
+  let arg;
   if (statType === 'fstat')
     arg = fs.openSync(__filename, 'r');
   else
diff --git a/benchmark/fs/bench-statSync.js b/benchmark/fs/bench-statSync.js
index bd8754a6c3d0e3..0c9366a402cb71 100644
--- a/benchmark/fs/bench-statSync.js
+++ b/benchmark/fs/bench-statSync.js
@@ -16,7 +16,7 @@ function main({ n, statSyncType }) {
   const fn = fs[statSyncType];
 
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     fn(arg);
   }
   bench.end(n);
diff --git a/benchmark/fs/read-stream-throughput.js b/benchmark/fs/read-stream-throughput.js
index 4c8de582418960..34c25760ea9c38 100644
--- a/benchmark/fs/read-stream-throughput.js
+++ b/benchmark/fs/read-stream-throughput.js
@@ -52,7 +52,7 @@ function runTest() {
     bench.start();
   });
 
-  var bytes = 0;
+  let bytes = 0;
   rs.on('data', (chunk) => {
     bytes += chunk.length;
   });
@@ -68,7 +68,7 @@ function makeFile() {
   const buf = Buffer.allocUnsafe(filesize / 1024);
   if (encoding === 'utf8') {
     // ü
-    for (var i = 0; i < buf.length; i++) {
+    for (let i = 0; i < buf.length; i++) {
       buf[i] = i % 2 === 0 ? 0xC3 : 0xBC;
     }
   } else if (encoding === 'ascii') {
@@ -78,7 +78,7 @@ function makeFile() {
   }
 
   try { fs.unlinkSync(filename); } catch {}
-  var w = 1024;
+  let w = 1024;
   const ws = fs.createWriteStream(filename);
   ws.on('close', runTest);
   ws.on('drain', write);
diff --git a/benchmark/fs/readFileSync.js b/benchmark/fs/readFileSync.js
index c28adeb229b358..47d210b7aa0463 100644
--- a/benchmark/fs/readFileSync.js
+++ b/benchmark/fs/readFileSync.js
@@ -9,7 +9,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ n }) {
   bench.start();
-  for (var i = 0; i < n; ++i)
+  for (let i = 0; i < n; ++i)
     fs.readFileSync(__filename);
   bench.end(n);
 }
diff --git a/benchmark/fs/readfile-partitioned.js b/benchmark/fs/readfile-partitioned.js
index 90af3754ce3de2..5f452319b4d28f 100644
--- a/benchmark/fs/readfile-partitioned.js
+++ b/benchmark/fs/readfile-partitioned.js
@@ -25,15 +25,15 @@ const bench = common.createBenchmark(main, {
 function main(conf) {
   const len = +conf.len;
   try { fs.unlinkSync(filename); } catch {}
-  var data = Buffer.alloc(len, 'x');
+  let data = Buffer.alloc(len, 'x');
   fs.writeFileSync(filename, data);
   data = null;
 
   const zipData = Buffer.alloc(1024, 'a');
 
-  var reads = 0;
-  var zips = 0;
-  var benchEnded = false;
+  let reads = 0;
+  let zips = 0;
+  let benchEnded = false;
   bench.start();
   setTimeout(() => {
     const totalOps = reads + zips;
@@ -78,7 +78,7 @@ function main(conf) {
   }
 
   // Start reads
-  var cur = +conf.concurrent;
+  let cur = +conf.concurrent;
   while (cur--) read();
 
   // Start a competing zip
diff --git a/benchmark/fs/readfile.js b/benchmark/fs/readfile.js
index 7460c8e233e59f..361ffbff597d6e 100644
--- a/benchmark/fs/readfile.js
+++ b/benchmark/fs/readfile.js
@@ -21,12 +21,12 @@ const bench = common.createBenchmark(main, {
 
 function main({ len, dur, concurrent }) {
   try { fs.unlinkSync(filename); } catch {}
-  var data = Buffer.alloc(len, 'x');
+  let data = Buffer.alloc(len, 'x');
   fs.writeFileSync(filename, data);
   data = null;
 
-  var reads = 0;
-  var benchEnded = false;
+  let reads = 0;
+  let benchEnded = false;
   bench.start();
   setTimeout(() => {
     benchEnded = true;
diff --git a/benchmark/fs/write-stream-throughput.js b/benchmark/fs/write-stream-throughput.js
index 44de0900d8ed15..44a3bb23b041d1 100644
--- a/benchmark/fs/write-stream-throughput.js
+++ b/benchmark/fs/write-stream-throughput.js
@@ -17,9 +17,9 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ dur, encodingType, size }) {
-  var encoding;
+  let encoding;
 
-  var chunk;
+  let chunk;
   switch (encodingType) {
     case 'buf':
       chunk = Buffer.alloc(size, 'b');
@@ -38,8 +38,8 @@ function main({ dur, encodingType, size }) {
 
   try { fs.unlinkSync(filename); } catch {}
 
-  var started = false;
-  var ended = false;
+  let started = false;
+  let ended = false;
 
   const f = fs.createWriteStream(filename);
   f.on('drain', write);
diff --git a/benchmark/http/_chunky_http_client.js b/benchmark/http/_chunky_http_client.js
index 1bf7fa284a835b..1ab95a2b9706c1 100644
--- a/benchmark/http/_chunky_http_client.js
+++ b/benchmark/http/_chunky_http_client.js
@@ -12,7 +12,7 @@ const bench = common.createBenchmark(main, {
 
 
 function main({ len, n }) {
-  var todo = [];
+  let todo = [];
   const headers = [];
   // Chose 7 because 9 showed "Connection error" / "Connection closed"
   // An odd number could result in a better length dispersion.
@@ -48,19 +48,19 @@ function main({ len, n }) {
   }
 
   const min = 10;
-  var size = 0;
+  let size = 0;
   const mod = 317;
   const mult = 17;
   const add = 11;
-  var count = 0;
+  let count = 0;
   const PIPE = process.env.PIPE_NAME;
   const socket = net.connect(PIPE, () => {
     bench.start();
     WriteHTTPHeaders(socket, 1, len);
     socket.setEncoding('utf8');
     socket.on('data', (d) => {
-      var did = false;
-      var pattern = 'HTTP/1.1 200 OK\r\n';
+      let did = false;
+      let pattern = 'HTTP/1.1 200 OK\r\n';
       if ((d.length === pattern.length && d === pattern) ||
           (d.length > pattern.length &&
            d.slice(0, pattern.length) === pattern)) {
diff --git a/benchmark/http/client-request-body.js b/benchmark/http/client-request-body.js
index b5ac2828c6ff1f..6e2323b3f07e8a 100644
--- a/benchmark/http/client-request-body.js
+++ b/benchmark/http/client-request-body.js
@@ -12,8 +12,8 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ dur, len, type, method }) {
-  var encoding;
-  var chunk;
+  let encoding;
+  let chunk;
   switch (type) {
     case 'buf':
       chunk = Buffer.alloc(len, 'x');
@@ -27,7 +27,7 @@ function main({ dur, len, type, method }) {
       break;
   }
 
-  var nreqs = 0;
+  let nreqs = 0;
   const options = {
     headers: { 'Connection': 'keep-alive', 'Transfer-Encoding': 'chunked' },
     agent: new http.Agent({ maxSockets: 1 }),
diff --git a/benchmark/http/cluster.js b/benchmark/http/cluster.js
index c2d6d8e1a70c90..3bcd061a0894c5 100644
--- a/benchmark/http/cluster.js
+++ b/benchmark/http/cluster.js
@@ -18,7 +18,7 @@ if (cluster.isMaster) {
 
 function main({ type, len, c }) {
   process.env.PORT = PORT;
-  var workers = 0;
+  let workers = 0;
   const w1 = cluster.fork();
   const w2 = cluster.fork();
 
diff --git a/benchmark/http/end-vs-write-end.js b/benchmark/http/end-vs-write-end.js
index b4ca560b9a897c..38e9b89a97a6b4 100644
--- a/benchmark/http/end-vs-write-end.js
+++ b/benchmark/http/end-vs-write-end.js
@@ -19,7 +19,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ len, type, method, c }) {
   const http = require('http');
-  var chunk;
+  let chunk;
   switch (type) {
     case 'buf':
       chunk = Buffer.alloc(len, 'x');
diff --git a/benchmark/http2/headers.js b/benchmark/http2/headers.js
index f18a73b95e2421..56799da1987e53 100644
--- a/benchmark/http2/headers.js
+++ b/benchmark/http2/headers.js
@@ -24,7 +24,7 @@ function main({ n, nheaders }) {
     'user-agent': 'SuperBenchmarker 3000'
   };
 
-  for (var i = 0; i < nheaders; i++) {
+  for (let i = 0; i < nheaders; i++) {
     headersObject[`foo${i}`] = `some header value ${i}`;
   }
 
diff --git a/benchmark/misc/arguments.js b/benchmark/misc/arguments.js
index 5b0cb0e2ca1488..8fefe617a51d56 100644
--- a/benchmark/misc/arguments.js
+++ b/benchmark/misc/arguments.js
@@ -32,7 +32,7 @@ function usingPredefined() {
 }
 
 function main({ n, method, args }) {
-  var fn;
+  let fn;
   switch (method) {
     // '' is a default case for tests
     case '':
@@ -53,7 +53,7 @@ function main({ n, method, args }) {
   }
 
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     fn('part 1', 'part', 2, 'part 3', 'part', 4);
   bench.end(n);
 }
diff --git a/benchmark/misc/freelist.js b/benchmark/misc/freelist.js
index e6868fae1ce8c5..d03c3674fdbfac 100644
--- a/benchmark/misc/freelist.js
+++ b/benchmark/misc/freelist.js
@@ -14,7 +14,7 @@ function main({ n }) {
     FreeList = FreeList.FreeList;
   const poolSize = 1000;
   const list = new FreeList('test', poolSize, Object);
-  var j;
+  let j;
   const used = [];
 
   // First, alloc `poolSize` items
@@ -24,7 +24,7 @@ function main({ n }) {
 
   bench.start();
 
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     // Return all the items to the pool
     for (j = 0; j < poolSize; j++) {
       list.free(used[j]);
diff --git a/benchmark/misc/object-property-bench.js b/benchmark/misc/object-property-bench.js
index fae11119b6fe23..0a4d004999ed23 100644
--- a/benchmark/misc/object-property-bench.js
+++ b/benchmark/misc/object-property-bench.js
@@ -11,9 +11,9 @@ const bench = common.createBenchmark(main, {
 
 function runProperty(n) {
   const object = {};
-  var i = 0;
+
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     object.p1 = 21;
     object.p2 = 21;
     object.p1 += object.p2;
@@ -23,9 +23,9 @@ function runProperty(n) {
 
 function runString(n) {
   const object = {};
-  var i = 0;
+
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     object['p1'] = 21;
     object['p2'] = 21;
     object['p1'] += object['p2'];
@@ -37,9 +37,9 @@ function runVariable(n) {
   const object = {};
   const var1 = 'p1';
   const var2 = 'p2';
-  var i = 0;
+
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     object[var1] = 21;
     object[var2] = 21;
     object[var1] += object[var2];
@@ -51,9 +51,9 @@ function runSymbol(n) {
   const object = {};
   const symbol1 = Symbol('p1');
   const symbol2 = Symbol('p2');
-  var i = 0;
+
   bench.start();
-  for (; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     object[symbol1] = 21;
     object[symbol2] = 21;
     object[symbol1] += object[symbol2];
diff --git a/benchmark/misc/punycode.js b/benchmark/misc/punycode.js
index 18023419709ee5..5f85df758ef7e3 100644
--- a/benchmark/misc/punycode.js
+++ b/benchmark/misc/punycode.js
@@ -45,18 +45,17 @@ function usingICU(val) {
 }
 
 function runPunycode(n, val) {
-  var i = 0;
-  for (; i < n; i++)
+  for (let i = 0; i < n; i++)
     usingPunycode(val);
   bench.start();
-  for (i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     usingPunycode(val);
   bench.end(n);
 }
 
 function runICU(n, val) {
   bench.start();
-  for (var i = 0; i < n; i++)
+  for (let i = 0; i < n; i++)
     usingICU(val);
   bench.end(n);
 }
diff --git a/benchmark/misc/trace.js b/benchmark/misc/trace.js
index d76f53cf04e82d..bdbf547007e72e 100644
--- a/benchmark/misc/trace.js
+++ b/benchmark/misc/trace.js
@@ -15,7 +15,7 @@ const {
 
 function doTrace(n, trace) {
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     trace(kBeforeEvent, 'foo', 'test', 0, 'test');
   }
   bench.end(n);
@@ -23,7 +23,7 @@ function doTrace(n, trace) {
 
 function doIsTraceCategoryEnabled(n, isTraceCategoryEnabled) {
   bench.start();
-  for (var i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     isTraceCategoryEnabled('foo');
     isTraceCategoryEnabled('bar');
   }
diff --git a/benchmark/misc/util-extend-vs-object-assign.js b/benchmark/misc/util-extend-vs-object-assign.js
index 3880711b660fe1..b3d95f0e2d97a4 100644
--- a/benchmark/misc/util-extend-vs-object-assign.js
+++ b/benchmark/misc/util-extend-vs-object-assign.js
@@ -22,13 +22,13 @@ function main({ n, type }) {
 
   // Force-optimize the method to test so that the benchmark doesn't
   // get disrupted by the optimizer kicking in halfway through.
-  for (var i = 0; i < type.length * 10; i += 1)
+  for (let i = 0; i < type.length * 10; i += 1)
     fn({}, process.env);
 
   const obj = new Proxy({}, { set: function(a, b, c) { return true; } });
 
   bench.start();
-  for (var j = 0; j < n; j += 1)
+  for (let j = 0; j < n; j += 1)
     fn(obj, process.env);
   bench.end(n);
 }