LibJS: Implement basic exception throwing

You can now throw exceptions by calling Interpreter::throw_exception().
Anyone who calls ASTNode::execute() needs to check afterwards if the
Interpreter now has an exception(), and if so, stop what they're doing
and simply return.

When catching an exception, we'll first execute the CatchClause node
if present. After that, we'll execute the finalizer block if present.

This is unlikely to be completely correct, but it's a start! :^)

11 files changed, 257 insertions, 5 deletions

diff --git a/Base/home/anon/js/try.js b/Base/home/anon/js/try.js
index 26e950aa06..7b4f892ca4 100644
--- a/Base/home/anon/js/try.js
+++ b/Base/home/anon/js/try.js
@@ -4,6 +4,7 @@ try {
     console.log("not me");
 } catch (e) {
     console.log("catch");
+    console.log(e.name);
 } finally {
     console.log("finally");
 }
diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index 936f48d18c..b6d48ad82f 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -30,6 +30,7 @@
 #include <LibJS/AST.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/Array.h>
+#include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/PrimitiveString.h>
 #include <LibJS/Runtime/ScriptFunction.h>
 #include <LibJS/Runtime/Value.h>
@@ -62,16 +63,29 @@ Value ExpressionStatement::execute(Interpreter& interpreter) const
 Value CallExpression::execute(Interpreter& interpreter) const
 {
     auto callee = m_callee->execute(interpreter);
+    if (interpreter.exception())
+        return {};
+
     ASSERT(callee.is_object());
     ASSERT(callee.as_object()->is_function());
     auto* function = static_cast<Function*>(callee.as_object());
 
     auto& call_frame = interpreter.push_call_frame();
-    for (size_t i = 0; i < m_arguments.size(); ++i)
+    for (size_t i = 0; i < m_arguments.size(); ++i) {
         call_frame.arguments.append(m_arguments[i].execute(interpreter));
+        if (interpreter.exception())
+            return {};
+    }
 
-    if (m_callee->is_member_expression())
-        call_frame.this_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter).to_object(interpreter.heap());
+    if (m_callee->is_member_expression()) {
+        auto object_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter);
+        if (interpreter.exception())
+            return {};
+        auto this_value = object_value.to_object(interpreter.heap());
+        if (interpreter.exception())
+            return {};
+        call_frame.this_value = this_value;
+    }
 
     auto result = function->call(interpreter, call_frame.arguments);
     interpreter.pop_call_frame();
@@ -464,7 +478,10 @@ void ForStatement::dump(int indent) const
 
 Value Identifier::execute(Interpreter& interpreter) const
 {
-    return interpreter.get_variable(string());
+    auto value = interpreter.get_variable(string());
+    if (value.is_undefined())
+        return interpreter.throw_exception(interpreter.heap().allocate<Error>("ReferenceError", String::format("'%s' not known", string().characters())));
+    return value;
 }
 
 void Identifier::dump(int indent) const
@@ -745,13 +762,27 @@ void CatchClause::dump(int indent) const
     body().dump(indent + 1);
 }
 
-Value TryStatement::execute(Interpreter&) const
+Value TryStatement::execute(Interpreter& interpreter) const
 {
+    interpreter.run(block(), {}, ScopeType::Try);
+    if (auto* exception = interpreter.exception()) {
+        if (m_handler) {
+            interpreter.clear_exception();
+            Vector<Argument> arguments { { m_handler->parameter(), Value(exception) } };
+            interpreter.run(m_handler->body(), move(arguments));
+        }
+    }
+
+    if (m_finalizer)
+        m_finalizer->execute(interpreter);
+
     return {};
 }
 
 Value CatchClause::execute(Interpreter&) const
 {
+    // NOTE: CatchClause execution is handled by TryStatement.
+    ASSERT_NOT_REACHED();
     return {};
 }
 
diff --git a/Libraries/LibJS/Forward.h b/Libraries/LibJS/Forward.h
index 359d6f6ab4..aa4079bc61 100644
--- a/Libraries/LibJS/Forward.h
+++ b/Libraries/LibJS/Forward.h
@@ -31,6 +31,7 @@ namespace JS {
 class ASTNode;
 class Argument;
 class Cell;
+class Error;
 class Expression;
 class Function;
 class HandleImpl;
diff --git a/Libraries/LibJS/Interpreter.cpp b/Libraries/LibJS/Interpreter.cpp
index f602336bc4..6a5bee1938 100644
--- a/Libraries/LibJS/Interpreter.cpp
+++ b/Libraries/LibJS/Interpreter.cpp
@@ -28,6 +28,8 @@
 #include <LibJS/AST.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/ArrayPrototype.h>
+#include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/ErrorPrototype.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/Object.h>
@@ -44,6 +46,7 @@ Interpreter::Interpreter()
     m_object_prototype = heap().allocate<ObjectPrototype>();
     m_string_prototype = heap().allocate<StringPrototype>();
     m_array_prototype = heap().allocate<ArrayPrototype>();
+    m_error_prototype = heap().allocate<ErrorPrototype>();
 }
 
 Interpreter::~Interpreter()
@@ -154,6 +157,9 @@ void Interpreter::gather_roots(Badge<Heap>, HashTable<Cell*>& roots)
     roots.set(m_string_prototype);
     roots.set(m_object_prototype);
     roots.set(m_array_prototype);
+    roots.set(m_error_prototype);
+
+    roots.set(m_exception);
 
     for (auto& scope : m_scope_stack) {
         for (auto& it : scope.variables) {
@@ -182,4 +188,11 @@ Value Interpreter::call(Function* function, Value this_value, const Vector<Value
     return result;
 }
 
+Value Interpreter::throw_exception(Error* exception)
+{
+    m_exception = exception;
+    unwind(ScopeType::Try);
+    return {};
+}
+
 }
diff --git a/Libraries/LibJS/Interpreter.h b/Libraries/LibJS/Interpreter.h
index fab56397d1..6c2beecd9e 100644
--- a/Libraries/LibJS/Interpreter.h
+++ b/Libraries/LibJS/Interpreter.h
@@ -40,6 +40,7 @@ enum class ScopeType {
     None,
     Function,
     Block,
+    Try,
 };
 
 struct Variable {
@@ -104,6 +105,11 @@ public:
     Object* string_prototype() { return m_string_prototype; }
     Object* object_prototype() { return m_object_prototype; }
     Object* array_prototype() { return m_array_prototype; }
+    Object* error_prototype() { return m_error_prototype; }
+
+    Error* exception() { return m_exception; }
+    void clear_exception() { m_exception = nullptr; }
+    Value throw_exception(Error*);
 
 private:
     Heap m_heap;
@@ -115,6 +121,9 @@ private:
     Object* m_string_prototype { nullptr };
     Object* m_object_prototype { nullptr };
     Object* m_array_prototype { nullptr };
+    Object* m_error_prototype { nullptr };
+
+    Error* m_exception { nullptr };
 
     ScopeType m_unwind_until { ScopeType::None };
 };
diff --git a/Libraries/LibJS/Makefile b/Libraries/LibJS/Makefile
index e550349a57..fd0b255115 100644
--- a/Libraries/LibJS/Makefile
+++ b/Libraries/LibJS/Makefile
@@ -10,6 +10,8 @@ OBJS = \
     Runtime/ArrayPrototype.o \
     Runtime/Cell.o \
     Runtime/ConsoleObject.o \
+    Runtime/Error.o \
+    Runtime/ErrorPrototype.o \
     Runtime/Function.o \
     Runtime/GlobalObject.o \
     Runtime/MathObject.o \
diff --git a/Libraries/LibJS/Runtime/Error.cpp b/Libraries/LibJS/Runtime/Error.cpp
new file mode 100644
index 0000000000..029c25d197
--- /dev/null
+++ b/Libraries/LibJS/Runtime/Error.cpp
@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/Error.h>
+
+namespace JS {
+
+Error::Error(const FlyString& name, const String& message)
+    : m_name(name)
+    , m_message(message)
+{
+    set_prototype(interpreter().error_prototype());
+}
+
+Error::~Error()
+{
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/Error.h b/Libraries/LibJS/Runtime/Error.h
new file mode 100644
index 0000000000..411454217e
--- /dev/null
+++ b/Libraries/LibJS/Runtime/Error.h
@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/FlyString.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class Error : public Object {
+public:
+    Error(const FlyString& name, const String& message);
+    virtual ~Error() override;
+
+    const FlyString& name() const { return m_name; }
+    const String& message() const { return m_message; }
+
+private:
+    virtual bool is_error() const final { return true; }
+    virtual const char* class_name() const override { return "Error"; }
+
+    FlyString m_name;
+    String m_message;
+};
+
+}
diff --git a/Libraries/LibJS/Runtime/ErrorPrototype.cpp b/Libraries/LibJS/Runtime/ErrorPrototype.cpp
new file mode 100644
index 0000000000..27b396f211
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ErrorPrototype.cpp
@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Function.h>
+#include <LibJS/Heap/Heap.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/ErrorPrototype.h>
+#include <LibJS/Runtime/PrimitiveString.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+ErrorPrototype::ErrorPrototype()
+{
+    put_native_property(
+        "name", [](Object* this_object) {
+        dbg() << "Error.prototype.name on " << this_object;
+            ASSERT(this_object);
+            ASSERT(this_object->is_error());
+            return js_string(this_object->heap(), static_cast<const Error*>(this_object)->name());
+        },
+        nullptr);
+
+    put_native_property(
+        "message", [](Object* this_object) {
+            ASSERT(this_object);
+            ASSERT(this_object->is_error());
+            return js_string(this_object->heap(), static_cast<const Error*>(this_object)->message());
+        },
+        nullptr);
+}
+
+ErrorPrototype::~ErrorPrototype()
+{
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/ErrorPrototype.h b/Libraries/LibJS/Runtime/ErrorPrototype.h
new file mode 100644
index 0000000000..8775b6aa51
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ErrorPrototype.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class ErrorPrototype final : public Object {
+public:
+    ErrorPrototype();
+    virtual ~ErrorPrototype() override;
+
+private:
+    virtual const char* class_name() const override { return "ErrorPrototype"; }
+};
+
+}
diff --git a/Libraries/LibJS/Runtime/Object.h b/Libraries/LibJS/Runtime/Object.h
index fd98b0e44f..af11e35af5 100644
--- a/Libraries/LibJS/Runtime/Object.h
+++ b/Libraries/LibJS/Runtime/Object.h
@@ -49,6 +49,7 @@ public:
     void put_native_function(const FlyString& property_name, AK::Function<Value(Object*, Vector<Value>)>);
     void put_native_property(const FlyString& property_name, AK::Function<Value(Object*)> getter, AK::Function<void(Object*, Value)> setter);
 
+    virtual bool is_error() const { return false; }
     virtual bool is_array() const { return false; }
     virtual bool is_function() const { return false; }
     virtual bool is_native_function() const { return false; }