Added a helper to check for JS exceptions

Essential for add-on testing

src/main/java/in/virit/mopo/Mopo.java

@@ -1,8 +1,12 @@
 package in.virit.mopo;
 
+import com.microsoft.playwright.ElementHandle;
+import com.microsoft.playwright.Locator;
 import com.microsoft.playwright.Page;
+import com.microsoft.playwright.assertions.PlaywrightAssertions;
 
 import static com.microsoft.playwright.assertions.PlaywrightAssertions.assertThat;
+import static com.microsoft.playwright.assertions.PlaywrightAssertions.setDefaultAssertionTimeout;
 
 /**
  * General utilities for Playwright & Vaadin.
@@ -38,4 +42,31 @@ public static void waitForConnectionToSettle(Page page, int minWait) {
         // System.out.println("Waited for" + (System.currentTimeMillis() - start) + "ms");
 
     }
+
+    /**
+     * Asserts that there are no JS errors in the dev console.
+     */
+    public void assertNoJsErrors() {
+        assertNoJsErrors(page);
+    }
+
+    /**
+     * Asserts that there are no JS errors in the dev console.
+     */
+    public static void assertNoJsErrors(Page page) {
+
+        try {
+            ElementHandle elementHandle = page.waitForSelector("vaadin-dev-tools>div.error",
+                    //wait for it just a tiny moment
+                    new Page.WaitForSelectorOptions().setTimeout(100.0)
+            );
+            if (elementHandle != null) {
+                String msg = page.locator("vaadin-dev-tools div.message.error").last().textContent();
+                throw new AssertionError("JS error in dev console: " + msg);
+            }
+        } catch (com.microsoft.playwright.TimeoutError e) {
+            // expected
+            return;
+        }
+    }
 }

src/test/java/firitin/pw/AddonHelpersIT.java

@@ -0,0 +1,69 @@
+package firitin.pw;
+
+import com.microsoft.playwright.Browser;
+import com.microsoft.playwright.BrowserType;
+import com.microsoft.playwright.Page;
+import com.microsoft.playwright.Playwright;
+import in.virit.mopo.Mopo;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+
+import static com.microsoft.playwright.assertions.PlaywrightAssertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@Tag("playwright")
+public class AddonHelpersIT {
+    private final int port = 9998;
+
+    static Playwright playwright = Playwright.create();
+
+    static {
+    }
+
+    private Browser browser;
+    private Page page;
+    private Mopo mopo;
+
+    @BeforeEach
+    public void setup() {
+        browser = playwright.chromium()
+                .launch(new BrowserType.LaunchOptions()
+                        .setHeadless(false)
+                        .setDevtools(true)
+                );
+
+        page = browser.newPage();
+        page.setDefaultTimeout(5000); // die faster if needed
+        mopo = new Mopo(page);
+    }
+
+    @AfterEach
+    public  void closePlaywright() {
+        page.close();
+        browser.close();
+    }
+
+    @Test
+    public void doRandomStuffAndChangeFirstRow() throws InterruptedException {
+        page.navigate("http://localhost:" + port + "/addonhelpers");
+
+        // There should be a JS error in the dev console, assert that
+        mopo.assertNoJsErrors();
+
+        page.getByText("Throw JS exception").click();
+
+        boolean exceptionThrown = false;
+        try {
+            mopo.assertNoJsErrors();
+        } catch (java.lang.AssertionError e) {
+            // Expected
+            System.out.println(e.getMessage());
+            exceptionThrown = true;
+        }
+        assertTrue(exceptionThrown);
+        assertThat(page.locator("vaadin-dev-tools>div.error")).isAttached();
+   }
+}

src/test/java/firitin/ui/AddOnHelpersView.java

@@ -0,0 +1,18 @@
+package firitin.ui;
+
+import com.vaadin.flow.component.button.Button;
+import com.vaadin.flow.component.orderedlayout.VerticalLayout;
+import com.vaadin.flow.router.Route;
+
+@Route
+public class AddOnHelpersView extends VerticalLayout {
+
+    public AddOnHelpersView() {
+        add("Test view");
+
+        add(new Button("Throw JS exception", e -> {
+            // deliberately cause a JS exception
+            getElement().executeJs("window.foo();");
+        }));
+    }
+}