diff --git a/lib/transformer.js b/lib/transformer.js
index 375888c4..2d90c268 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -76,9 +76,9 @@ class Transformer {
       impl: content.impl
     }));
 
-    const interfaces = this._interfaces = Object.create(null);
-    const dictionaries = this._dictionaries = Object.create(null);
-    const typedefs = this._typedefs = Object.create(null);
+    const interfaces = this._interfaces = new Map();
+    const dictionaries = this._dictionaries = new Map();
+    const typedefs = this._typedefs = new Map();
     const customTypes = this._customTypes = new Set();
 
     // first we're gathering all full interfaces and ignore partial ones
@@ -96,7 +96,7 @@ class Transformer {
               implSuffix: this.options.implSuffix,
               customTypes
             });
-            interfaces[obj.name] = obj;
+            interfaces.set(obj.name, obj);
             break;
           case "implements":
             break; // handled later
@@ -106,13 +106,13 @@ class Transformer {
             }
 
             obj = new Dictionary(instruction, { customTypes });
-            dictionaries[obj.name] = obj;
-            customTypes.add(instruction.name);
+            dictionaries.set(obj.name, obj);
+            customTypes.add(obj.name);
             break;
           case "typedef":
             obj = new Typedef(instruction, { customTypes });
-            typedefs[obj.name] = obj;
-            customTypes.add(instruction.name);
+            typedefs.set(obj.name, obj);
+            customTypes.add(obj.name);
             break;
           default:
             if (!this.options.suppressErrors) {
@@ -133,31 +133,34 @@ class Transformer {
               break;
             }
 
-            if (this.options.suppressErrors && !interfaces[instruction.name]) {
+            const iface = interfaces.get(instruction.name);
+            if (this.options.suppressErrors && !iface) {
               break;
             }
-            oldMembers = interfaces[instruction.name].idl.members;
+            oldMembers = iface.idl.members;
             oldMembers.push.apply(oldMembers, instruction.members);
-            extAttrs = interfaces[instruction.name].idl.extAttrs;
+            extAttrs = iface.idl.extAttrs;
             extAttrs.push.apply(extAttrs, instruction.extAttrs);
             break;
           case "dictionary":
             if (!instruction.partial) {
               break;
             }
-            if (this.options.suppressErrors && !dictionaries[instruction.name]) {
+            const dict = dictionaries.get(instruction.name);
+            if (this.options.suppressErrors && !dict) {
               break;
             }
-            oldMembers = dictionaries[instruction.name].idl.members;
+            oldMembers = dict.idl.members;
             oldMembers.push.apply(oldMembers, instruction.members);
-            extAttrs = dictionaries[instruction.name].idl.extAttrs;
+            extAttrs = dict.idl.extAttrs;
             extAttrs.push.apply(extAttrs, instruction.extAttrs);
             break;
           case "implements":
-            if (this.options.suppressErrors && !interfaces[instruction.target]) {
+            const iface = interfaces.get(instruction.target);
+            if (this.options.suppressErrors && !iface) {
               break;
             }
-            interfaces[instruction.target].implements(instruction.implements);
+            iface.implements(instruction.implements);
             break;
         }
       }
@@ -169,20 +172,23 @@ class Transformer {
     yield fs.writeFile(this.options.utilPath, utilsText);
 
     let interfaceStub = yield fs.readFile(__dirname + "/output/interfaceStub.js");
-    let keys = Object.keys(this._interfaces).concat(Object.keys(this._dictionaries)).concat(Object.keys(this._typedefs));
-    for (let key of keys) {
-      if (this._interfaces[key]) {
-        yield fs.writeFile(path.join(outputDir, this._interfaces[key].name + ".js"), interfaceStub);
-      } else if (this._dictionaries[key]) {
-        yield fs.writeFile(path.join(outputDir, this._dictionaries[key].name + ".js"), "");
-      } else {
-        yield fs.writeFile(path.join(outputDir, this._typedefs[key].name + ".js"), "");
-      }
+    for (const key of this._interfaces.keys()) {
+      yield fs.writeFile(path.join(outputDir, key + ".js"), interfaceStub);
+    }
+    // TODO: use spread syntax to merge the two following blocks
+    for (const key of this._dictionaries.keys()) {
+      yield fs.writeFile(path.join(outputDir, key + ".js"), "");
+    }
+    for (const key of this._typedefs.keys()) {
+      yield fs.writeFile(path.join(outputDir, key + ".js"), "");
+    }
+
+    let relativeUtils = path.relative(outputDir, this.options.utilPath).replace(/\\/g, '/');
+    if (relativeUtils[0] !== ".") {
+      relativeUtils = "./" + relativeUtils;
     }
 
-    keys = Object.keys(this._interfaces);
-    for (let i = 0; i < keys.length; ++i) {
-      const obj = this._interfaces[keys[i]];
+    for (const obj of this._interfaces.values()) {
       let source = obj.toString();
 
       const implDir = path.relative(outputDir, obj.opts.implDir).replace(/\\/g, "/"); // fix windows file paths
@@ -191,11 +197,6 @@ class Transformer {
         implFile = "./" + implFile;
       }
 
-      let relativeUtils = path.relative(outputDir, this.options.utilPath).replace(/\\/g, '/');
-      if (relativeUtils[0] !== ".") {
-        relativeUtils = "./" + relativeUtils;
-      }
-
       source = `"use strict";
 
 const conversions = require("webidl-conversions");
@@ -205,16 +206,10 @@ const Impl = require("${implFile}.js");\n`;
       yield fs.writeFile(path.join(outputDir, obj.name + ".js"), source);
     }
 
-    keys = Object.keys(this._dictionaries).concat(Object.keys(this._typedefs));
-    for (let i = 0; i < keys.length; ++i) {
-      const obj = this._dictionaries[keys[i]] || this._typedefs[keys[i]];
+    // TODO: use spread syntax to simplify
+    for (const obj of Array.from(this._dictionaries.values()).concat(Array.from(this._typedefs.values()))) {
       let source = obj.toString();
 
-      let relativeUtils = path.relative(outputDir, this.options.utilPath).replace(/\\/g, '/');
-      if (relativeUtils[0] !== ".") {
-        relativeUtils = "./" + relativeUtils;
-      }
-
       source = `"use strict";
 
 const conversions = require("webidl-conversions");