feat: React 19 behind a feature flag (#20816)

Author: Artur-

flow-server/src/main/java/com/vaadin/experimental/FeatureFlags.java

@@ -95,6 +95,10 @@ public class FeatureFlags implements Serializable {
             "https://github.com/vaadin/web-components/issues/5340", true,
             "com.vaadin.flow.component.card.Card");
 
+    public static final Feature REACT19 = new Feature(
+            "React 19 (default in Vaadin 25)", "react19",
+            "https://react.dev/blog/2024/12/05/react-19", true, null);
+
     private List<Feature> features = new ArrayList<>();
 
     File propertiesFolder = null;
@@ -123,6 +127,7 @@ public FeatureFlags(Lookup lookup) {
         features.add(new Feature(COPILOT_EXPERIMENTAL));
         features.add(new Feature(DASHBOARD_COMPONENT));
         features.add(new Feature(CARD_COMPONENT));
+        features.add(new Feature(REACT19));
         loadProperties();
     }
 

flow-server/src/main/java/com/vaadin/flow/server/frontend/NodeUpdater.java

@@ -37,6 +37,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import com.vaadin.experimental.FeatureFlags;
 import com.vaadin.flow.server.Constants;
 import com.vaadin.flow.server.frontend.scanner.ClassFinder;
 import com.vaadin.flow.server.frontend.scanner.FrontendDependencies;
@@ -302,6 +303,10 @@ Map<String, String> getDefaultDependencies() {
         if (options.isReactEnabled()) {
             dependencies
                     .putAll(readDependencies("react-router", "dependencies"));
+            if (options.getFeatureFlags().isEnabled(FeatureFlags.REACT19)) {
+                dependencies
+                        .putAll(readDependencies("react19", "dependencies"));
+            }
         } else {
             dependencies
                     .putAll(readDependencies("vaadin-router", "dependencies"));
@@ -368,6 +373,9 @@ Map<String, String> getDefaultDevDependencies() {
         if (options.isReactEnabled()) {
             defaults.putAll(
                     readDependencies("react-router", "devDependencies"));
+            if (options.getFeatureFlags().isEnabled(FeatureFlags.REACT19)) {
+                defaults.putAll(readDependencies("react19", "devDependencies"));
+            }
         }
 
         return defaults;

flow-server/src/main/resources/com/vaadin/flow/server/frontend/dependencies/react19/package.json

@@ -0,0 +1,12 @@
+{
+  "private": true,
+  "description": "A list of Flow dependencies when using React 19",
+  "dependencies": {
+    "react": "19.0.0",
+    "react-dom": "19.0.0"
+  },
+  "devDependencies": {
+    "@types/react": "19.0.2",
+    "@types/react-dom": "19.0.2"
+  }
+}

flow-server/src/test/java/com/vaadin/flow/server/frontend/NodeUpdaterTest.java

@@ -127,6 +127,31 @@ public void getDefaultDependencies_includesAllDependencies() {
         Assert.assertEquals(expectedDependencies, actualDependendencies);
     }
 
+    @Test
+    public void react19UsedWhenFeatureFlagIsOn() {
+        Map<String, String> react18Deps = nodeUpdater.getDefaultDependencies();
+        Map<String, String> react18DevDeps = nodeUpdater
+                .getDefaultDevDependencies();
+        Mockito.when(options.getFeatureFlags().isEnabled(FeatureFlags.REACT19))
+                .thenReturn(true);
+
+        Map<String, String> react19Deps = nodeUpdater.getDefaultDependencies();
+        Map<String, String> react19DevDeps = nodeUpdater
+                .getDefaultDevDependencies();
+
+        Assert.assertTrue(react18Deps.get("react").startsWith("18."));
+        Assert.assertTrue(react18Deps.get("react-dom").startsWith("18."));
+        Assert.assertTrue(react18DevDeps.get("@types/react").startsWith("18."));
+        Assert.assertTrue(
+                react18DevDeps.get("@types/react-dom").startsWith("18."));
+
+        Assert.assertTrue(react19Deps.get("react").startsWith("19."));
+        Assert.assertTrue(react19Deps.get("react-dom").startsWith("19."));
+        Assert.assertTrue(react19DevDeps.get("@types/react").startsWith("19."));
+        Assert.assertTrue(
+                react19DevDeps.get("@types/react-dom").startsWith("19."));
+    }
+
     @Test
     public void getDefaultDevDependencies_includesAllDependencies_whenUsingVite() {
         Map<String, String> defaultDeps = nodeUpdater