diff --git a/implementations/javascript/packages/core/src/merge.ts b/implementations/javascript/packages/core/src/merge.ts
new file mode 100644
index 0000000..39761e6
--- /dev/null
+++ b/implementations/javascript/packages/core/src/merge.ts
@@ -0,0 +1,71 @@
+import { Record, Tuple } from "./record";
+import { Bytes } from "./bytes";
+import { fold, isPointer } from "./fold";
+import { is } from "./is";
+import { Value } from "./values";
+import { Set, Dictionary } from "./dictionary";
+import { Annotated } from "./annotated";
+import { unannotate } from "./strip";
+
+export function merge<T>(
+    mergePointers: (a: T, b: T) => T | undefined,
+    item0: Value<T>,
+    ... items: Array<Value<T>>): Value<T>
+{
+    function die(): never {
+        throw new Error("Cannot merge items");
+    }
+
+    function walk(a: Value<T>, b: Value<T>): Value<T> {
+        if (a === b) return a;
+        return fold<T, Value<T>>(a, {
+            boolean: die,
+            single(_f: number) { return is(a, b) ? a : die(); },
+            double(_f: number) { return is(a, b) ? a : die(); },
+            integer: die,
+            string: die,
+            bytes(_b: Bytes) { return is(a, b) ? a : die(); },
+            symbol: die,
+
+            record(r: Record<Value<T>, Tuple<Value<T>>, T>) {
+                if (!Record.isRecord<Value<T>, Tuple<Value<T>>, T>(b)) die();
+                return Record(walk(r.label, b.label), walkMany(r, b));
+            },
+            array(a: Array<Value<T>>) {
+                if (!Array.isArray(b) || Record.isRecord(b)) die();
+                return walkMany(a, b);
+            },
+            set(_s: Set<T>) { die(); },
+            dictionary(d: Dictionary<T>) {
+                if (!Dictionary.isDictionary<T>(b)) die();
+                const r = new Dictionary<T>();
+                d.forEach((av,ak) => {
+                    const bv = b.get(ak);
+                    r.set(ak, bv === void 0 ? av : walk(av, bv));
+                });
+                b.forEach((bv, bk) => {
+                    if (!d.has(bk)) r.set(bk, bv);
+                });
+                return r;
+            },
+
+            annotated(a: Annotated<T>) {
+                return walk(a, unannotate(b));
+            },
+
+            pointer(t: T) {
+                if (!isPointer(b)) die();
+                const r = mergePointers(t, b);
+                if (r === void 0) die();
+                return r;
+            },
+        });
+    }
+
+    function walkMany(a: Array<Value<T>>, b: Array<Value<T>>): Array<Value<T>> {
+        if (a.length !== b.length) die();
+        return a.map((aa, i) => walk(aa, b[i]));
+    }
+
+    return items.reduce(walk, item0);
+}
diff --git a/implementations/javascript/packages/core/src/runtime.ts b/implementations/javascript/packages/core/src/runtime.ts
index 7eeb0ff..9befa66 100644
--- a/implementations/javascript/packages/core/src/runtime.ts
+++ b/implementations/javascript/packages/core/src/runtime.ts
@@ -10,6 +10,7 @@ export * from './float';
 export * from './fold';
 export * from './fromjs';
 export * from './is';
+export * from './merge';
 export * from './pointer';
 export * from './reader';
 export * from './record';