diff --git a/js/Makefile.am b/js/Makefile.am
index 29187da2e..323cd1b12 100644
--- a/js/Makefile.am
+++ b/js/Makefile.am
@@ -28,6 +28,7 @@ nobase_dist_js_DATA = 	\
 	misc/extensionUtils.js	\
 	misc/fileUtils.js	\
 	misc/gnomeSession.js	\
+	misc/hash.js		\
 	misc/history.js		\
 	misc/jsParse.js		\
 	misc/loginManager.js	\
diff --git a/js/misc/hash.js b/js/misc/hash.js
new file mode 100644
index 000000000..19d4df619
--- /dev/null
+++ b/js/misc/hash.js
@@ -0,0 +1,141 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
+const Lang = imports.lang;
+const System = imports.system;
+
+const Params = imports.misc.params;
+
+// This is an implementation of EcmaScript SameValue algorithm,
+// which returns true if two values are not observably distinguishable.
+// It was taken from http://wiki.ecmascript.org/doku.php?id=harmony:egal
+//
+// In the future, we may want to use the 'is' operator instead.
+function _sameValue(x, y) {
+    if (x === y) {
+        // 0 === -0, but they are not identical
+        return x !== 0 || 1 / x === 1 / y;
+    }
+
+    // NaN !== NaN, but they are identical.
+    // NaNs are the only non-reflexive value, i.e., if x !== x,
+    // then x is a NaN.
+    // isNaN is broken: it converts its argument to number, so
+    // isNaN("foo") => true
+    return x !== x && y !== y;
+}
+
+const _hashers = {
+    object: function(o) { return o ? System.addressOf(o) : 'null'; },
+    function: function(f) { return System.addressOf(f); },
+    string: function(s) { return s; },
+    number: function(n) { return String(n); },
+    undefined: function() { return 'undefined'; },
+};
+
+/* Map is meant to be similar in usage to ES6 Map, which is
+   described at http://wiki.ecmascript.org/doku.php?id=harmony:simple_maps_and_sets,
+   without requiring more than ES5 + Gjs extensions.
+
+   Known differences from other implementations:
+   Polyfills around the web usually implement HashMaps for
+   primitive values and reversed WeakMaps for object keys,
+   but we want real maps with real O(1) semantics in all cases,
+   and the easiest way is to have different hashers for different
+   types.
+
+   Known differences from the ES6 specification:
+   - Map is a Lang.Class, not a ES6 class, so inheritance,
+     prototype, sealing, etc. work differently.
+   - items(), keys() and values() don't return iterators,
+     they return actual arrays, so they incur a full copy everytime
+     they're called, and they don't see changes if you mutate
+     the table while iterating
+     (admittedly, the ES6 spec is a bit unclear on this, and
+     the reference code would just blow up)
+*/
+const Map = new Lang.Class({
+    Name: 'Map',
+
+    _init: function(iterable) {
+        this._pool = { };
+
+        if (iterable) {
+            for (let i = 0; i < iterable.length; i++) {
+                let [key, value] = iterable[i];
+                this.set(key, value);
+            }
+        }
+    },
+
+    _hashKey: function(key) {
+        let type = typeof(key);
+        return type + ':' + _hashers[type](key);
+    },
+
+    _internalGet: function(key) {
+        let hash = this._hashKey(key);
+        let node = this._pool[hash];
+
+        if (node && _sameValue(node.key, key))
+            return [true, node.value];
+        else
+            return [false, null];
+    },
+
+    get: function(key) {
+        return this._internalGet(key)[1];
+    },
+
+    has: function(key) {
+        return this._internalGet(key)[0];
+    },
+
+    set: function(key, value) {
+        let hash = this._hashKey(key);
+        let node = this._pool[hash];
+
+        if (node) {
+            node.key = key;
+            node.value = value;
+        } else {
+            this._pool[hash] = { key: key, value: value };
+        }
+    },
+
+    delete: function(key) {
+        let hash = this._hashKey(key);
+        let node = this._pool[hash];
+
+        if (node && _sameValue(node.key, key)) {
+            delete this._pool[hash];
+            return [node.key, node.value];
+        } else {
+            return [null, null];
+        }
+    },
+
+    keys: function() {
+        let pool = this._pool;
+        return Object.getOwnPropertyNames(pool).map(function(hash) {
+            return pool[hash].key;
+        });
+    },
+
+    values: function() {
+        let pool = this._pool;
+        return Object.getOwnPropertyNames(pool).map(function(hash) {
+            return pool[hash].value;
+        });
+    },
+
+    items: function() {
+        let pool = this._pool;
+        return Object.getOwnPropertyNames(pool).map(function(hash) {
+            return [pool[hash].key, pool[hash].value];
+        });
+    },
+
+    size: function() {
+        return Object.getOwnPropertyNames(this._pool).length;
+    },
+});