diff --git a/benchmark/zlib/creation.js b/benchmark/zlib/creation.js
index 30e6afe6b43..c2d063c20d8 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 6d3bda90c6f..5baedc924d4 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');
   }