diff --git a/implementations/javascript/packages/core/src/record.ts b/implementations/javascript/packages/core/src/record.ts
index eb94f0a..7a53b2c 100644
--- a/implementations/javascript/packages/core/src/record.ts
+++ b/implementations/javascript/packages/core/src/record.ts
@@ -20,6 +20,7 @@ export interface RecordConstructor<L extends Value<T>, Fs, Names extends Tuple<k
     constructorInfo: RecordConstructorInfo<L, T>;
     isClassOf(v: any): v is Record<L, CtorTypes<Fs, Names>, T>;
     _: RecordGetters<Fs, Record<L, CtorTypes<Fs, Names>, T>>;
+    fieldNumbers: { [K in string & keyof Fs]: number };
 };
 
 export interface RecordConstructorInfo<L extends Value<T>, T = GenericEmbedded> {
@@ -74,8 +75,12 @@ export namespace Record {
             const constructorInfo = { label, arity: fieldNames.length };
             ctor.constructorInfo = constructorInfo;
             ctor.isClassOf = (v: any): v is Record<L, CtorTypes<Fs, Names>, T> => Record.isClassOf<L, CtorTypes<Fs, Names>, T>(constructorInfo, v);
+            (ctor as any).fieldNumbers = {};
             (ctor as any)._ = {};
-            fieldNames.forEach((name, i) => (ctor._ as any)[name] = (r: Record<L, CtorTypes<Fs, Names>, T>) => r[i]);
+            fieldNames.forEach((name, i) => {
+                (ctor._ as any)[name] = (r: Record<L, CtorTypes<Fs, Names>, T>) => r[i];
+                (ctor.fieldNumbers as any)[name] = i;
+            });
             return ctor;
         };
     }