Refactor to make it more like the Android app

web/src/app/Subscriptions.js

@@ -0,0 +1,52 @@
+class Subscriptions {
+    constructor() {
+        this.subscriptions = new Map();
+    }
+
+    add(subscription) {
+        this.subscriptions.set(subscription.id, subscription);
+        return this;
+    }
+
+    get(subscriptionId) {
+        const subscription = this.subscriptions.get(subscriptionId);
+        if (subscription === undefined) return null;
+        return subscription;
+    }
+
+    update(subscription) {
+        return this.add(subscription);
+    }
+
+    remove(subscriptionId) {
+        this.subscriptions.delete(subscriptionId);
+        return this;
+    }
+
+    forEach(cb) {
+        this.subscriptions.forEach((value, key) => cb(key, value));
+    }
+
+    map(cb) {
+        return Array.from(this.subscriptions.values())
+            .map(subscription => cb(subscription.id, subscription));
+    }
+
+    ids() {
+        return Array.from(this.subscriptions.keys());
+    }
+
+    firstOrNull() {
+        const first = this.subscriptions.values().next().value;
+        if (first === undefined) return null;
+        return first;
+    }
+
+    clone() {
+        const c = new Subscriptions();
+        c.subscriptions = new Map(this.subscriptions);
+        return c;
+    }
+}
+
+export default Subscriptions;