diff --git a/implementations/javascript/package.json b/implementations/javascript/package.json
index 692d070..89e1e40 100644
--- a/implementations/javascript/package.json
+++ b/implementations/javascript/package.json
@@ -1,6 +1,6 @@
 {
   "name": "preserves",
-  "version": "0.0.3",
+  "version": "0.0.4",
   "description": "Experimental data serialization format",
   "homepage": "https://gitlab.com/tonyg/preserves",
   "license": "MIT",
diff --git a/implementations/javascript/src/values.js b/implementations/javascript/src/values.js
index ff2ca05..b786cf0 100644
--- a/implementations/javascript/src/values.js
+++ b/implementations/javascript/src/values.js
@@ -73,6 +73,16 @@ Float.prototype.hashCode = function () {
   return this.value | 0; // TODO: something better?
 };
 
+Float.unwrap = function (v) {
+  if (typeof v === 'number') return v;
+  if (v instanceof Float) return v.value;
+  {
+    const e = new TypeError("Float.unwrap failed");
+    e.irritant = v;
+    throw e;
+  }
+};
+
 function Single(value) {
   if (!(this instanceof Single)) return new Single(value);
   _Float.call(this, value);