diff --git a/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.test.ts b/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.test.ts
new file mode 100644
index 0000000000..f58bf19fcf
--- /dev/null
+++ b/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.test.ts
@@ -0,0 +1,15 @@
+import { difference, union } from './set-functions';
+
+test('union', () => {
+    const a = [1, 2];
+    const b = new Set([2, 3]);
+    const c = union(a, b);
+    expect([...c]).toEqual([1, 2, 3]);
+});
+
+test('difference', () => {
+    const a = [1, 2];
+    const b = new Set([2, 3]);
+    const c = difference(a, b);
+    expect([...c]).toEqual([1]);
+});
diff --git a/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.ts b/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.ts
index 6731854459..e9dd7232bf 100644
--- a/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.ts
+++ b/frontend/src/component/feature/FeatureStrategy/FeatureStrategyConstraints/useEditableConstraint/set-functions.ts
@@ -6,7 +6,7 @@
 // todo: replace the use of this methods with set.union and set.difference when
 // it's available.
 
-export const union = <T>(a: Iterable<T>, b: Set<T>) => {
+export const union = <T>(a: Iterable<T>, b: Set<T>): Set<T> => {
     const result = new Set(a);
     for (const element of b) {
         result.add(element);
@@ -14,8 +14,8 @@ export const union = <T>(a: Iterable<T>, b: Set<T>) => {
     return result;
 };
 
-export const difference = <T>(a: Iterable<T>, b: Set<T>) => {
-    const result = new Set(a);
+export const difference = <T>(a: Iterable<T>, b: Set<T>): Set<T> => {
+    const result = new Set<T>();
     for (const element of a) {
         if (!b.has(element)) {
             result.add(element);