JS: display Java exception message in uncaught exceptions in console

core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java

@@ -295,6 +295,9 @@ public class JavaScriptTarget implements TeaVMTarget, TeaVMJavaScriptHost {
             includeStackTraceMethods(dependencyAnalyzer);
         }
 
+        dependencyAnalyzer.linkMethod(new MethodReference(Throwable.class, "getMessage", String.class)).use();
+        dependencyAnalyzer.linkMethod(new MethodReference(Throwable.class, "getCause", Throwable.class)).use();
+
         dependencyAnalyzer.addDependencyListener(new AbstractDependencyListener() {
             @Override
             public void methodReached(DependencyAgent agent, MethodDependency method) {

core/src/main/java/org/teavm/backend/javascript/rendering/RuntimeRenderer.java

@@ -70,6 +70,7 @@ public class RuntimeRenderer {
             renderRuntimeUnwrapString();
             renderRuntimeObjcls();
             renderRuntimeThrowablecls();
+            renderRuntimeThrowableMethods();
             renderRuntimeNullCheck();
             renderRuntimeIntern();
             renderRuntimeThreads();
@@ -203,6 +204,18 @@ public class RuntimeRenderer {
         writer.append(";").softNewLine().outdent().append("}").newLine();
     }
 
+    private void renderRuntimeThrowableMethods() throws IOException {
+        writer.append("function $rt_throwableMessage(t)").ws().append("{").indent().softNewLine();
+        writer.append("return ");
+        writer.appendMethodBody(Throwable.class, "getMessage", String.class).append("(t);").softNewLine();
+        writer.outdent().append("}").newLine();
+
+        writer.append("function $rt_throwableCause(t)").ws().append("{").indent().softNewLine();
+        writer.append("return ");
+        writer.appendMethodBody(Throwable.class, "getCause", Throwable.class).append("(t);").softNewLine();
+        writer.outdent().append("}").newLine();
+    }
+
     private void renderRuntimeThreads() throws IOException {
         ClassReader threadCls = classSource.get(THREAD_CLASS);
         MethodReader currentThreadMethod = threadCls != null ? threadCls.getMethod(CURRENT_THREAD_METHOD) : null;

core/src/main/resources/org/teavm/backend/javascript/runtime.js

@@ -284,7 +284,10 @@ var $rt_javaExceptionProp = Symbol("javaException")
 function $rt_exception(ex) {
     var err = ex.$jsException;
     if (!err) {
-        err = new Error("Java exception thrown");
+        var javaCause = $rt_throwableCause(ex);
+        var jsCause = javaCause !== null ? javaCause.$jsException : undefined;
+        var cause = typeof jsCause === "object" ? { cause : jsCause } : undefined;
+        err = new JavaError("Java exception thrown", cause);
         if (typeof Error.captureStackTrace === "function") {
             Error.captureStackTrace(err);
         }
@@ -694,6 +697,38 @@ function $rt_intBitsToFloat(n) {
     return $rt_numberConversionView.getFloat32(0);
 }
 
+var JavaError;
+if (typeof Reflect === 'object') {
+    var defaultMessage = Symbol("defaultMessage");
+    JavaError = function JavaError(message, cause) {
+        var self = Reflect.construct(Error, [undefined, cause], JavaError);
+        Object.setPrototypeOf(self, JavaError.prototype);
+        self[defaultMessage] = message;
+        return self;
+    }
+    JavaError.prototype = Object.create(Error.prototype, {
+        constructor: {
+            configurable: true,
+            writable: true,
+            value: JavaError
+        },
+        message: {
+            get: function() {
+                var javaException = this[$rt_javaExceptionProp];
+                if (typeof javaException === 'object') {
+                    var javaMessage = $rt_throwableMessage(javaException);
+                    if (typeof javaMessage === "object") {
+                        return javaMessage.toString();
+                    }
+                }
+                return this[defaultMessage];
+            }
+        }
+    });
+} else {
+    JavaError = Error;
+}
+
 function $rt_javaException(e) {
     return e instanceof Error && typeof e[$rt_javaExceptionProp] === 'object' ? e[$rt_javaExceptionProp] : null;
 }