diff --git a/cjs/index.js b/cjs/index.js
index 6983934..1b50d60 100644
--- a/cjs/index.js
+++ b/cjs/index.js
@@ -2,11 +2,16 @@
 try { new Map; }
 catch (o_O) {
   (function (i, dPs) {'use strict';
-    var proto = (Map = function Map(iterable) {
+    var proto = dPs((Map = function Map(iterable) {
       dPs(this, {_k: {value: []}, _v: {value: []}});
       if (iterable)
         iterable.forEach(add, this);
-    }).prototype;
+    }).prototype, {size: {
+      configurable: true,
+      get: function () {
+        return this._k.length;
+      }
+    }});
     proto.clear = function () {
       var length = this._k.length;
       this._k.splice(0, length);
diff --git a/esm/index.js b/esm/index.js
index c53737c..db9326e 100644
--- a/esm/index.js
+++ b/esm/index.js
@@ -2,11 +2,16 @@
 try { new Map; }
 catch (o_O) {
   (function (i, dPs) {'use strict';
-    var proto = (Map = function Map(iterable) {
+    var proto = dPs((Map = function Map(iterable) {
       dPs(this, {_k: {value: []}, _v: {value: []}});
       if (iterable)
         iterable.forEach(add, this);
-    }).prototype;
+    }).prototype, {size: {
+      configurable: true,
+      get: function () {
+        return this._k.length;
+      }
+    }});
     proto.clear = function () {
       var length = this._k.length;
       this._k.splice(0, length);
diff --git a/index.js b/index.js
index 03aef6c..eec885b 100644
--- a/index.js
+++ b/index.js
@@ -2,11 +2,16 @@
 try { new Map; }
 catch (o_O) {
   (function (i, dPs) {'use strict';
-    var proto = (Map = function Map(iterable) {
+    var proto = dPs((Map = function Map(iterable) {
       dPs(this, {_k: {value: []}, _v: {value: []}});
       if (iterable)
         iterable.forEach(add, this);
-    }).prototype;
+    }).prototype, {size: {
+      configurable: true,
+      get: function () {
+        return this._k.length;
+      }
+    }});
     proto.clear = function () {
       var length = this._k.length;
       this._k.splice(0, length);
diff --git a/min.js b/min.js
index 2b01637..22f076f 100644
--- a/min.js
+++ b/min.js
@@ -1,2 +1,2 @@
 /*! (c) Andrea Giammarchi - ISC */
-try{new Map}catch(o_O){!function(t,i){"use strict";function n(t){this.set(t[0],t[1])}function s(i,n){return-1<(t=i._k.indexOf(n))}function e(t,i){return[t,this[i]]}var h=(Map=function(t){i(this,{_k:{value:[]},_v:{value:[]}}),t&&t.forEach(n,this)}).prototype;h.clear=function(){var t=this._k.length;this._k.splice(0,t),this._v.splice(0,t)},h["delete"]=function(i){var n=s(this,i);return n&&(this._k.splice(t,1),this._v.splice(t,1)),n},h.entries=function(){return this._k.map(e,this._v)},h.forEach=function(t,i){this._k.forEach(function(n,s){t.call(i,this._v[s],n,this)},this)},h.get=function(i){return s(this,i)?this._v[t]:void 0},h.has=function(t){return s(this,t)},h.keys=function(){return this._k.slice(0)},h.set=function(i,n){return this._v[s(this,i)?t:this._k.push(i)-1]=n,this},h.values=function(){return this._v.slice(0)},Map}(0,Object.defineProperties)}
\ No newline at end of file
+try{new Map}catch(o_O){!function(t,i){"use strict";function n(t){this.set(t[0],t[1])}function e(i,n){return-1<(t=i._k.indexOf(n))}function s(t,i){return[t,this[i]]}var h=i((Map=function(t){i(this,{_k:{value:[]},_v:{value:[]}}),t&&t.forEach(n,this)}).prototype,{size:{configurable:!0,get:function(){return this._k.length}}});h.clear=function(){var t=this._k.length;this._k.splice(0,t),this._v.splice(0,t)},h["delete"]=function(i){var n=e(this,i);return n&&(this._k.splice(t,1),this._v.splice(t,1)),n},h.entries=function(){return this._k.map(s,this._v)},h.forEach=function(t,i){this._k.forEach(function(n,e){t.call(i,this._v[e],n,this)},this)},h.get=function(i){return e(this,i)?this._v[t]:void 0},h.has=function(t){return e(this,t)},h.keys=function(){return this._k.slice(0)},h.set=function(i,n){return this._v[e(this,i)?t:this._k.push(i)-1]=n,this},h.values=function(){return this._v.slice(0)},Map}(0,Object.defineProperties)}
\ No newline at end of file
diff --git a/test/index.js b/test/index.js
index c299f9b..ed7a9e3 100644
--- a/test/index.js
+++ b/test/index.js
@@ -36,9 +36,10 @@ function test() {
   var ws2 = new Map([[a, b]]);
 
   console.assert(ws1.has(a) === false);
-  console.assert(ws1.set(a, 123) === ws1);
+  console.assert(ws1.set(a, 456) === ws1);
   console.assert(ws1.set(a, 123) === ws1);
   console.assert(ws1.has(a) === true);
+  console.assert(ws1.size === 1);
   ws1.clear();
   ws1.set(a, b);
   var entries = ws1.entries();
@@ -57,7 +58,6 @@ function test() {
     console.assert(values.length === 1);
     console.assert(values[0] === b);
   }
-  console.log(ws1.forEach);
   ws1.forEach(function (value, key, map) {
     console.assert(value === b);
     console.assert(key === a);