diff --git a/implementations/javascript/package.json b/implementations/javascript/package.json
index ee61172..5e4b6c8 100644
--- a/implementations/javascript/package.json
+++ b/implementations/javascript/package.json
@@ -1,6 +1,6 @@
 {
   "name": "preserves",
-  "version": "0.6.2",
+  "version": "0.6.3",
   "description": "Experimental data serialization format",
   "homepage": "https://gitlab.com/preserves/preserves",
   "license": "Apache-2.0",
diff --git a/implementations/javascript/src/flex.ts b/implementations/javascript/src/flex.ts
index 687a167..c237185 100644
--- a/implementations/javascript/src/flex.ts
+++ b/implementations/javascript/src/flex.ts
@@ -76,6 +76,16 @@ export class FlexMap<K, V> implements Map<K, V> {
         return (e === void 0) ? defaultValue : e[1];
     }
 
+    getOrSet(k: K, initializer: () => V): V {
+        const ks = this._key(k);
+        let e = this.items.get(ks);
+        if (e === void 0) {
+            e = [k, initializer()];
+            this.items.set(ks, e);
+        }
+        return e[1];
+    }
+
     set(k: K, v: V): this {
         this.items.set(this._key(k), [k, v]);
         return this;
@@ -101,6 +111,14 @@ export class FlexMap<K, V> implements Map<K, V> {
         return this.items.delete(this._key(k));
     }
 
+    getAndDelete(k: K, defaultValue?: V): V | undefined {
+        const ks = this._key(k);
+        const e = this.items.get(ks);
+        if (e === void 0) return defaultValue;
+        this.items.delete(ks);
+        return e[1];
+    }
+
     clear() {
         this.items.clear();
     }