diff --git a/benchmark/.eslintrc.yaml b/benchmark/.eslintrc.yaml
index 7de962dc9002af..8ce0f9f6e148c1 100644
--- a/benchmark/.eslintrc.yaml
+++ b/benchmark/.eslintrc.yaml
@@ -5,6 +5,7 @@ env:
   es6: true
 
 rules:
+  no-var: error
   comma-dangle:
     - error
     - arrays: 'always-multiline'
diff --git a/benchmark/async_hooks/async-resource-vs-destroy.js b/benchmark/async_hooks/async-resource-vs-destroy.js
index 4464dd5f93e7de..a9cd60200e3149 100644
--- a/benchmark/async_hooks/async-resource-vs-destroy.js
+++ b/benchmark/async_hooks/async-resource-vs-destroy.js
@@ -55,7 +55,7 @@ function buildCurrentResource(getServe) {
   }
 
   function init(asyncId, type, triggerAsyncId, resource) {
-    var cr = executionAsyncResource();
+    const cr = executionAsyncResource();
     if (cr !== null) {
       resource[cls] = cr[cls];
     }
diff --git a/benchmark/common.js b/benchmark/common.js
index c5791c2bacfd5d..1ab7e263f7ed2b 100644
--- a/benchmark/common.js
+++ b/benchmark/common.js
@@ -227,7 +227,7 @@ function formatResult(data) {
     conf += ` ${key}=${JSON.stringify(data.conf[key])}`;
   }
 
-  var rate = data.rate.toString().split('.');
+  let rate = data.rate.toString().split('.');
   rate[0] = rate[0].replace(/(\d)(?=(?:\d\d\d)+(?!\d))/g, '$1,');
   rate = (rate[1] ? rate.join('.') : rate[0]);
   return `${data.name}${conf}: ${rate}`;
diff --git a/benchmark/dns/lookup.js b/benchmark/dns/lookup.js
index 164e490bd3bc24..691daa8a879e2b 100644
--- a/benchmark/dns/lookup.js
+++ b/benchmark/dns/lookup.js
@@ -10,7 +10,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ name, n, all }) {
-  var i = 0;
+  let i = 0;
 
   if (all === 'true') {
     const opts = { all: true };
diff --git a/benchmark/run.js b/benchmark/run.js
index 8e81a2c5e16ab7..07da6c547f551c 100644
--- a/benchmark/run.js
+++ b/benchmark/run.js
@@ -60,7 +60,7 @@ if (format === 'csv') {
       conf = conf.replace(/"/g, '""');
       console.log(`"${data.name}", "${conf}", ${data.rate}, ${data.time}`);
     } else {
-      var rate = data.rate.toString().split('.');
+      let rate = data.rate.toString().split('.');
       rate[0] = rate[0].replace(/(\d)(?=(?:\d\d\d)+(?!\d))/g, '$1,');
       rate = (rate[1] ? rate.join('.') : rate[0]);
       console.log(`${data.name} ${conf}: ${rate}`);
diff --git a/benchmark/timers/immediate.js b/benchmark/timers/immediate.js
index d12106a0e030fc..3bd4c097dce5a0 100644
--- a/benchmark/timers/immediate.js
+++ b/benchmark/timers/immediate.js
@@ -31,7 +31,7 @@ function main({ n, type }) {
 
 // setImmediate tail recursion, 0 arguments
 function depth(N) {
-  var n = 0;
+  let n = 0;
   bench.start();
   setImmediate(cb);
   function cb() {
@@ -45,7 +45,7 @@ function depth(N) {
 
 // setImmediate tail recursion, 1 argument
 function depth1(N) {
-  var n = 0;
+  let n = 0;
   bench.start();
   setImmediate(cb, 1);
   function cb(a1) {
@@ -59,7 +59,7 @@ function depth1(N) {
 
 // Concurrent setImmediate, 0 arguments
 function breadth(N) {
-  var n = 0;
+  let n = 0;
   bench.start();
   function cb() {
     n++;
@@ -73,7 +73,7 @@ function breadth(N) {
 
 // Concurrent setImmediate, 1 argument
 function breadth1(N) {
-  var n = 0;
+  let n = 0;
   bench.start();
   function cb(a1) {
     n++;
@@ -88,7 +88,7 @@ function breadth1(N) {
 // Concurrent setImmediate, 4 arguments
 function breadth4(N) {
   N /= 2;
-  var n = 0;
+  let n = 0;
   bench.start();
   function cb(a1, a2, a3, a4) {
     n++;
diff --git a/benchmark/timers/timers-breadth-args.js b/benchmark/timers/timers-breadth-args.js
index 63a301dc9bc7dc..bf1727ab865b6d 100644
--- a/benchmark/timers/timers-breadth-args.js
+++ b/benchmark/timers/timers-breadth-args.js
@@ -6,7 +6,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n }) {
-  var j = 0;
+  let j = 0;
   function cb1(arg1) {
     j++;
     if (j === n)
diff --git a/benchmark/timers/timers-breadth.js b/benchmark/timers/timers-breadth.js
index 78bd5a97ae84b8..7af5d380018898 100644
--- a/benchmark/timers/timers-breadth.js
+++ b/benchmark/timers/timers-breadth.js
@@ -6,7 +6,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n }) {
-  var j = 0;
+  let j = 0;
   bench.start();
   function cb() {
     j++;
diff --git a/benchmark/timers/timers-cancel-pooled.js b/benchmark/timers/timers-cancel-pooled.js
index 5045983210263e..30dbd7bc007d7a 100644
--- a/benchmark/timers/timers-cancel-pooled.js
+++ b/benchmark/timers/timers-cancel-pooled.js
@@ -8,11 +8,11 @@ const bench = common.createBenchmark(main, {
 
 function main({ n }) {
 
-  var timer = setTimeout(() => {}, 1);
+  let timer = setTimeout(() => {}, 1);
   for (let i = 0; i < n; i++) {
     setTimeout(cb, 1);
   }
-  var next = timer._idlePrev;
+  let next = timer._idlePrev;
   clearTimeout(timer);
 
   bench.start();
diff --git a/benchmark/timers/timers-cancel-unpooled.js b/benchmark/timers/timers-cancel-unpooled.js
index df203ee4810b1d..70f8fd96c9cbf1 100644
--- a/benchmark/timers/timers-cancel-unpooled.js
+++ b/benchmark/timers/timers-cancel-unpooled.js
@@ -14,14 +14,13 @@ function main({ n, direction }) {
     timersList.push(setTimeout(cb, i + 1));
   }
 
-  var j;
   bench.start();
   if (direction === 'start') {
-    for (j = 0; j < n; j++) {
+    for (let j = 0; j < n; j++) {
       clearTimeout(timersList[j]);
     }
   } else {
-    for (j = n - 1; j >= 0; j--) {
+    for (let j = n - 1; j >= 0; j--) {
       clearTimeout(timersList[j]);
     }
   }
diff --git a/benchmark/timers/timers-depth.js b/benchmark/timers/timers-depth.js
index bfc6dd02cf5c65..766733ba1862a9 100644
--- a/benchmark/timers/timers-depth.js
+++ b/benchmark/timers/timers-depth.js
@@ -6,7 +6,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n }) {
-  var i = 0;
+  let i = 0;
   bench.start();
   setTimeout(cb, 1);
   function cb() {
diff --git a/benchmark/timers/timers-insert-unpooled.js b/benchmark/timers/timers-insert-unpooled.js
index 11d25d5fe79670..5ee255b5e15fd2 100644
--- a/benchmark/timers/timers-insert-unpooled.js
+++ b/benchmark/timers/timers-insert-unpooled.js
@@ -10,14 +10,13 @@ const bench = common.createBenchmark(main, {
 function main({ direction, n }) {
   const timersList = [];
 
-  var i;
   bench.start();
   if (direction === 'start') {
-    for (i = 1; i <= n; i++) {
+    for (let i = 1; i <= n; i++) {
       timersList.push(setTimeout(cb, i));
     }
   } else {
-    for (i = n; i > 0; i--) {
+    for (let i = n; i > 0; i--) {
       timersList.push(setTimeout(cb, i));
     }
   }
diff --git a/benchmark/tls/convertprotocols.js b/benchmark/tls/convertprotocols.js
index 74bb942bfcec4f..87c447436a44db 100644
--- a/benchmark/tls/convertprotocols.js
+++ b/benchmark/tls/convertprotocols.js
@@ -9,7 +9,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ n }) {
   const input = ['ABC', 'XYZ123', 'FOO'];
-  var m = {};
+  let m = {};
   // First call dominates results
   if (n > 1) {
     tls.convertALPNProtocols(input, m);
diff --git a/benchmark/tls/throughput.js b/benchmark/tls/throughput.js
index a8f2d19649d04a..da26eba177b2cf 100644
--- a/benchmark/tls/throughput.js
+++ b/benchmark/tls/throughput.js
@@ -7,12 +7,12 @@ const bench = common.createBenchmark(main, {
 });
 
 const fixtures = require('../../test/common/fixtures');
-var options;
+let options;
 const tls = require('tls');
 
 function main({ dur, type, size }) {
-  var encoding;
-  var chunk;
+  let encoding;
+  let chunk;
   switch (type) {
     case 'buf':
       chunk = Buffer.alloc(size, 'b');
@@ -37,7 +37,7 @@ function main({ dur, type, size }) {
   };
 
   const server = tls.createServer(options, onConnection);
-  var conn;
+  let conn;
   server.listen(common.PORT, () => {
     const opt = { port: common.PORT, rejectUnauthorized: false };
     conn = tls.connect(opt, () => {
@@ -52,7 +52,7 @@ function main({ dur, type, size }) {
     }
   });
 
-  var received = 0;
+  let received = 0;
   function onConnection(conn) {
     conn.on('data', (chunk) => {
       received += chunk.length;
diff --git a/benchmark/tls/tls-connect.js b/benchmark/tls/tls-connect.js
index 1cb04d98a52721..3fc2ecb614978b 100644
--- a/benchmark/tls/tls-connect.js
+++ b/benchmark/tls/tls-connect.js
@@ -8,11 +8,11 @@ const bench = common.createBenchmark(main, {
   dur: [5]
 });
 
-var clientConn = 0;
-var serverConn = 0;
-var dur;
-var concurrency;
-var running = true;
+let clientConn = 0;
+let serverConn = 0;
+let dur;
+let concurrency;
+let running = true;
 
 function main(conf) {
   dur = conf.dur;
diff --git a/benchmark/url/legacy-vs-whatwg-url-parse.js b/benchmark/url/legacy-vs-whatwg-url-parse.js
index 6e5e25d231242f..94115d1305bb95 100644
--- a/benchmark/url/legacy-vs-whatwg-url-parse.js
+++ b/benchmark/url/legacy-vs-whatwg-url-parse.js
@@ -13,7 +13,7 @@ const bench = common.createBenchmark(main, {
 
 function useLegacy(data) {
   const len = data.length;
-  var result = url.parse(data[0]);  // Avoid dead code elimination
+  let result = url.parse(data[0]);  // Avoid dead code elimination
   bench.start();
   for (let i = 0; i < len; ++i) {
     result = url.parse(data[i]);
@@ -24,7 +24,7 @@ function useLegacy(data) {
 
 function useWHATWGWithBase(data) {
   const len = data.length;
-  var result = new URL(data[0][0], data[0][1]);  // Avoid dead code elimination
+  let result = new URL(data[0][0], data[0][1]);  // Avoid dead code elimination
   bench.start();
   for (let i = 0; i < len; ++i) {
     const item = data[i];
@@ -36,7 +36,7 @@ function useWHATWGWithBase(data) {
 
 function useWHATWGWithoutBase(data) {
   const len = data.length;
-  var result = new URL(data[0]);  // Avoid dead code elimination
+  let result = new URL(data[0]);  // Avoid dead code elimination
   bench.start();
   for (let i = 0; i < len; ++i) {
     result = new URL(data[i]);
@@ -47,8 +47,8 @@ function useWHATWGWithoutBase(data) {
 
 function main({ e, method, type, withBase }) {
   withBase = withBase === 'true';
-  var noDead;  // Avoid dead code elimination.
-  var data;
+  let noDead;  // Avoid dead code elimination.
+  let data;
   switch (method) {
     case 'legacy':
       data = common.bakeUrlData(type, e, false, false);
diff --git a/benchmark/url/legacy-vs-whatwg-url-serialize.js b/benchmark/url/legacy-vs-whatwg-url-serialize.js
index 5523e549ceb233..e3254ede3acee8 100644
--- a/benchmark/url/legacy-vs-whatwg-url-serialize.js
+++ b/benchmark/url/legacy-vs-whatwg-url-serialize.js
@@ -13,7 +13,7 @@ const bench = common.createBenchmark(main, {
 function useLegacy(data) {
   const obj = url.parse(data[0]);
   const len = data.length;
-  var noDead = url.format(obj);
+  let noDead = url.format(obj);
   bench.start();
   for (let i = 0; i < len; i++) {
     noDead = data[i].toString();
@@ -25,7 +25,7 @@ function useLegacy(data) {
 function useWHATWG(data) {
   const obj = new URL(data[0]);
   const len = data.length;
-  var noDead = obj.toString();
+  let noDead = obj.toString();
   bench.start();
   for (let i = 0; i < len; i++) {
     noDead = data[i].toString();
@@ -37,7 +37,7 @@ function useWHATWG(data) {
 function main({ type, e, method }) {
   const data = common.bakeUrlData(type, e, false, false);
 
-  var noDead;  // Avoid dead code elimination.
+  let noDead;  // Avoid dead code elimination.
   switch (method) {
     case 'legacy':
       noDead = useLegacy(data);
diff --git a/benchmark/url/whatwg-url-properties.js b/benchmark/url/whatwg-url-properties.js
index ac71ff4f636d66..f0ba2931e51f1e 100644
--- a/benchmark/url/whatwg-url-properties.js
+++ b/benchmark/url/whatwg-url-properties.js
@@ -12,7 +12,7 @@ const bench = common.createBenchmark(main, {
 
 function setAndGet(data, prop) {
   const len = data.length;
-  var result = data[0][prop];
+  let result = data[0][prop];
   bench.start();
   for (let i = 0; i < len; ++i) {
     result = data[i][prop];
@@ -24,7 +24,7 @@ function setAndGet(data, prop) {
 
 function get(data, prop) {
   const len = data.length;
-  var result = data[0][prop];
+  let result = data[0][prop];
   bench.start();
   for (let i = 0; i < len; ++i) {
     result = data[i][prop]; // get
diff --git a/benchmark/util/inspect-array.js b/benchmark/util/inspect-array.js
index 4fd73785f789d1..f78b1d92420df5 100644
--- a/benchmark/util/inspect-array.js
+++ b/benchmark/util/inspect-array.js
@@ -15,8 +15,8 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n, len, type }) {
-  var arr = Array(len);
-  var i, opts;
+  let arr = Array(len);
+  let opts;
 
   switch (type) {
     case 'denseArray_showHidden':
@@ -31,14 +31,14 @@ function main({ n, len, type }) {
     case 'sparseArray':
       break;
     case 'mixedArray':
-      for (i = 0; i < n; i += 2)
+      for (let i = 0; i < n; i += 2)
         arr[i] = i;
       break;
     default:
       throw new Error(`Unsupported type ${type}`);
   }
   bench.start();
-  for (i = 0; i < n; i++) {
+  for (let i = 0; i < n; i++) {
     util.inspect(arr, opts);
   }
   bench.end(n);
diff --git a/benchmark/util/inspect.js b/benchmark/util/inspect.js
index 159d831f762884..0a19e65ea234be 100644
--- a/benchmark/util/inspect.js
+++ b/benchmark/util/inspect.js
@@ -37,7 +37,7 @@ function benchmark(n, obj, options) {
 }
 
 function main({ method, n, option }) {
-  var obj;
+  let obj;
   const options = opts[option];
   switch (method) {
     case 'Object':
diff --git a/benchmark/util/normalize-encoding.js b/benchmark/util/normalize-encoding.js
index 464bda52f35c44..03769cb3b229cb 100644
--- a/benchmark/util/normalize-encoding.js
+++ b/benchmark/util/normalize-encoding.js
@@ -46,11 +46,11 @@ function getInput(input) {
 function main({ input, n }) {
   const { normalizeEncoding } = require('internal/util');
   const inputs = getInput(input);
-  var noDead = '';
+  let noDead = '';
 
   bench.start();
-  for (var i = 0; i < n; ++i) {
-    for (var j = 0; j < inputs.length; ++j) {
+  for (let i = 0; i < n; ++i) {
+    for (let j = 0; j < inputs.length; ++j) {
       noDead = normalizeEncoding(inputs[j]);
     }
   }
diff --git a/benchmark/worker/echo.js b/benchmark/worker/echo.js
index 8d45a1f76b5eac..7988ffc7e121eb 100644
--- a/benchmark/worker/echo.js
+++ b/benchmark/worker/echo.js
@@ -55,7 +55,7 @@ function main({ n, workers, sendsPerBroadcast: sends, payload: payloadType }) {
       return;
     }
     for (const worker of workerObjs) {
-      for (var i = 0; i < sends; ++i)
+      for (let i = 0; i < sends; ++i)
         worker.postMessage(payload);
     }
   }
diff --git a/benchmark/zlib/creation.js b/benchmark/zlib/creation.js
index 30e6afe6b4355f..c2d063c20d8d02 100644
--- a/benchmark/zlib/creation.js
+++ b/benchmark/zlib/creation.js
@@ -15,17 +15,16 @@ function main({ n, type, options }) {
   const fn = zlib[`create${type}`];
   if (typeof fn !== 'function')
     throw new Error('Invalid zlib type');
-  var i = 0;
 
   if (options === 'true') {
     const opts = {};
     bench.start();
-    for (; i < n; ++i)
+    for (let i = 0; i < n; ++i)
       fn(opts);
     bench.end(n);
   } else {
     bench.start();
-    for (; i < n; ++i)
+    for (let i = 0; i < n; ++i)
       fn();
     bench.end(n);
   }
diff --git a/benchmark/zlib/deflate.js b/benchmark/zlib/deflate.js
index 6d3bda90c6fdfa..5baedc924d4bc8 100644
--- a/benchmark/zlib/deflate.js
+++ b/benchmark/zlib/deflate.js
@@ -13,11 +13,11 @@ function main({ n, method, inputLen }) {
   method = method || 'deflate';
   const chunk = Buffer.alloc(inputLen, 'a');
 
-  var i = 0;
   switch (method) {
     // Performs `n` writes for a single deflate stream
-    case 'createDeflate':
-      var deflater = zlib.createDeflate();
+    case 'createDeflate': {
+      let i = 0;
+      const deflater = zlib.createDeflate();
       deflater.resume();
       deflater.on('finish', () => {
         bench.end(n);
@@ -30,9 +30,11 @@ function main({ n, method, inputLen }) {
         deflater.write(chunk, next);
       })();
       break;
+    }
     // Performs `n` single deflate operations
-    case 'deflate':
-      var deflate = zlib.deflate;
+    case 'deflate': {
+      let i = 0;
+      const deflate = zlib.deflate;
       bench.start();
       (function next(err, result) {
         if (i++ === n)
@@ -40,14 +42,16 @@ function main({ n, method, inputLen }) {
         deflate(chunk, next);
       })();
       break;
+    }
     // Performs `n` single deflateSync operations
-    case 'deflateSync':
-      var deflateSync = zlib.deflateSync;
+    case 'deflateSync': {
+      const deflateSync = zlib.deflateSync;
       bench.start();
-      for (; i < n; ++i)
+      for (let i = 0; i < n; ++i)
         deflateSync(chunk);
       bench.end(n);
       break;
+    }
     default:
       throw new Error('Unsupported deflate method');
   }