diff --git a/js/js.libraries/src/core/collections/InternalStringMap.kt b/js/js.libraries/src/core/collections/InternalStringMap.kt
new file mode 100644
index 00000000000..ce3b9cddbb3
--- /dev/null
+++ b/js/js.libraries/src/core/collections/InternalStringMap.kt
@@ -0,0 +1,124 @@
+/*
+ * Copyright 2010-2016 JetBrains s.r.o.
+ *
+ * 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.
+ */
+/*
+ * Based on GWT InternalStringMap
+ * Copyright 2008 Google Inc.
+ */
+package kotlin.collections
+
+/**
+ * A simple wrapper around JavaScript Map for key type is string.
+ */
+internal class InternalStringMap<K, V>(private val host: AbstractHashMap<K, V>) : Iterable<Entry<K, V>> {
+
+    private val backingMap = InternalJsMapFactory.newJsMap()
+    private var size: Int = 0
+
+    /**
+     * A mod count to track 'value' replacements in map to ensure that the 'value' that we have in the
+     * iterator entry is guaranteed to be still correct.
+     * This is to optimize for the common scenario where the values are not modified during
+     * iterations where the entries are never stale.
+     */
+    private var valueMod: Int = 0
+
+    operator fun contains(key: String): Boolean {
+        return !JsUtils.isUndefined(backingMap.get(key))
+    }
+
+    operator fun get(key: String): V {
+        return backingMap.get(key)
+    }
+
+    fun put(key: String, value: V): V {
+        val oldValue = backingMap.get(key)
+        backingMap.set(key, toNullIfUndefined(value))
+
+        if (JsUtils.isUndefined(oldValue)) {
+            size++
+            structureChanged(host)
+        }
+        else {
+            valueMod++
+        }
+        return oldValue
+    }
+
+    fun remove(key: String): V {
+        val value = backingMap.get(key)
+        if (!JsUtils.isUndefined(value)) {
+            backingMap.delete(key)
+            size--
+            structureChanged(host)
+        }
+        else {
+            valueMod++
+        }
+
+        return value
+    }
+
+    fun size(): Int {
+        return size
+    }
+
+    override fun iterator(): Iterator<Entry<K, V>> {
+        return object : Iterator<Entry<K, V>> {
+            internal var entries = backingMap.entries()
+            internal var current = entries.next()
+            internal var last: InternalJsMap.IteratorEntry<V>
+
+            override fun hasNext(): Boolean {
+                return !current.done
+            }
+
+            override fun next(): Entry<K, V> {
+                last = current
+                current = entries.next()
+                return newMapEntry(last, valueMod)
+            }
+
+            override fun remove() {
+                this@InternalStringMap.remove(last.getKey())
+            }
+        }
+    }
+
+    private fun newMapEntry(entry: InternalJsMap.IteratorEntry<V>,
+                            lastValueMod: Int): Entry<K, V> {
+        return object : AbstractMapEntry<K, V>() {
+            val key: K
+                @SuppressWarnings("unchecked")
+                get() = entry.getKey()
+            // Let's get a fresh copy as the value may have changed.
+            val value: V
+                get() {
+                    if (valueMod != lastValueMod) {
+                        return get(entry.getKey())
+                    }
+                    return entry.getValue()
+                }
+
+            fun setValue(`object`: V): V {
+                return put(entry.getKey(), `object`)
+            }
+        }
+    }
+
+    private fun <T> toNullIfUndefined(value: T): T? {
+        return if (JsUtils.isUndefined(value)) null else value
+    }
+}