diff --git a/externs/webassembly.js b/externs/webassembly.js
new file mode 100644
index 0000000..d551ace
--- /dev/null
+++ b/externs/webassembly.js
@@ -0,0 +1,159 @@
+/*
+ * Copyright 2017 The Closure Compiler 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.
+ */
+
+/**
+ * @fileoverview Definitions for WebAssembly JS API
+ *
+ *  @see http://webassembly.org/docs/js/
+ *
+ * @externs
+ * @author loorongjie@gmail.com (Loo Rong Jie)
+ */
+
+ /**
+ * @const
+ */
+var WebAssembly = {};
+
+/**
+ * @constructor
+ * @param {!BufferSource} bytes
+ */
+WebAssembly.Module = function(bytes) {};
+
+/**
+ * @constructor
+ * @param {!WebAssembly.Module} moduleObject
+ * @param {Object=} importObject
+ */
+WebAssembly.Instance = function(moduleObject, importObject) {};
+
+/**
+ * @typedef {{initial:number, maximum:(number|undefined)}}
+ */
+var MemoryDescriptor;
+
+/**
+ * @constructor
+ * @param {MemoryDescriptor} memoryDescriptor
+ */
+WebAssembly.Memory = function(memoryDescriptor) {};
+
+/**
+ * @typedef {{element:string, initial:number, maximum:(number|undefined)}}
+ */
+var TableDescriptor;
+
+/**
+ * @constructor
+ * @param {TableDescriptor} tableDescriptor
+ */
+WebAssembly.Table = function(tableDescriptor) {};
+
+/**
+ * @constructor
+ * @extends {Error}
+ */
+WebAssembly.CompileError = function() {};
+
+/**
+ * @constructor
+ * @extends {Error}
+ */
+WebAssembly.LinkError = function() {};
+
+/**
+ * @constructor
+ * @extends {Error}
+ */
+WebAssembly.RuntimeError = function() {};
+
+// Note: Closure compiler does not support function overloading, omit this overload for now.
+// {function(!WebAssembly.Module, Object=):!Promise<!WebAssembly.Instance>}
+/**
+ * @param {!BufferSource} moduleObject
+ * @param {Object=} importObject
+ * @return {!Promise<{module:WebAssembly.Module, instance:WebAssembly.Instance}>}
+ */
+WebAssembly.instantiate = function(moduleObject, importObject) {};
+
+/**
+ * @param {!BufferSource} bytes
+ * @return {!Promise<!WebAssembly.Module>}
+ */
+WebAssembly.compile = function(bytes) {};
+
+/**
+ * @param {!BufferSource} bytes
+ * @return {boolean}
+ * @nosideeffects
+ */
+WebAssembly.validate = function(bytes) {};
+
+/**
+ * @param {!WebAssembly.Module} moduleObject
+ * @return {!Array<{name:string, kind:string}>}
+ */
+WebAssembly.Module.exports = function(moduleObject) {};
+
+/**
+ * @param {!WebAssembly.Module} moduleObject
+ * @return {!Array<{module:string, name:string, kind:string}>}
+ */
+WebAssembly.Module.imports = function(moduleObject) {};
+
+/**
+ * @param {!WebAssembly.Module} moduleObject
+ * @param {string} sectionName
+ * @return {!Array<!ArrayBuffer>}
+ */
+WebAssembly.Module.customSections = function(moduleObject, sectionName) {};
+
+WebAssembly.Instance.prototype.exports;
+
+/**
+ * @param {number} delta
+ * @return {number}
+ */
+WebAssembly.Memory.prototype.grow = function(delta) {};
+
+/**
+ * @type {!ArrayBuffer}
+ */
+WebAssembly.Memory.prototype.buffer;
+
+/**
+ * @param {number} delta
+ * @return {number}
+ */
+WebAssembly.Table.prototype.grow = function(delta) {};
+
+/**
+ * @type {number}
+ */
+WebAssembly.Table.prototype.length;
+
+/**
+ * @param {number} index
+ * @return {function(...)}
+ */
+WebAssembly.Table.prototype.get = function(index) {};
+
+/**
+ * @param {number} index
+ * @param {?function(...)} value
+ */
+WebAssembly.Table.prototype.set = function(index, value) {};
diff --git a/webpack.config.js b/webpack.config.js
index 745ecf6..d1505eb 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -10,7 +10,8 @@ function makeClosureCompiler() {
     const externDir = path.join(__dirname, "externs");
     const externs = [
         path.join(externDir, "googleAnalytics.js"),
-        path.join(externDir, "performance.js")
+        path.join(externDir, "performance.js"),
+        path.join(externDir, "webassembly.js")
     ];
 
     var pluginOptions = {