LibJS: Start building a JavaScript engine for SerenityOS :^)

I always tell people to start building things by working on the thing
that seems the most interesting right now. The most interesting thing
here was an AST + simple interpreter, so that's where we start!

There is no lexer or parser yet, we build an AST directly and then
execute it in the interpreter, producing a return value.

This seems like the start of something interesting. :^)

Libraries/LibJS/AST.cpp

@@ -0,0 +1,144 @@
+/*
+ * 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/AST.h>
+#include <LibJS/Function.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Value.h>
+#include <stdio.h>
+
+namespace JS {
+
+Value ScopeNode::execute(Interpreter& interpreter) const
+{
+    return interpreter.run(*this);
+}
+
+Value FunctionDeclaration::execute(Interpreter& interpreter) const
+{
+    auto* function = new Function(name(), body());
+    interpreter.global_object().put(m_name, Value(function));
+    return Value(function);
+}
+
+Value CallExpression::execute(Interpreter& interpreter) const
+{
+    auto callee = interpreter.global_object().get(name());
+    ASSERT(callee.is_object());
+    auto* callee_object = callee.as_object();
+    ASSERT(callee_object->is_function());
+    auto& function = static_cast<Function&>(*callee_object);
+    return interpreter.run(function.body());
+}
+
+Value ReturnStatement::execute(Interpreter& interpreter) const
+{
+    auto value = argument().execute(interpreter);
+    interpreter.do_return();
+    return value;
+}
+
+Value add(Value lhs, Value rhs)
+{
+    ASSERT(lhs.is_number());
+    ASSERT(rhs.is_number());
+    return Value(lhs.as_double() + rhs.as_double());
+}
+
+Value sub(Value lhs, Value rhs)
+{
+    ASSERT(lhs.is_number());
+    ASSERT(rhs.is_number());
+    return Value(lhs.as_double() - rhs.as_double());
+}
+
+Value BinaryExpression::execute(Interpreter& interpreter) const
+{
+    auto lhs_result = m_lhs->execute(interpreter);
+    auto rhs_result = m_rhs->execute(interpreter);
+
+    switch (m_op) {
+    case BinaryOp::Plus:
+        return add(lhs_result, rhs_result);
+    case BinaryOp::Minus:
+        return sub(lhs_result, rhs_result);
+    }
+
+    ASSERT_NOT_REACHED();
+}
+
+static void print_indent(int indent)
+{
+    for (int i = 0; i < indent * 2; ++i)
+        putchar(' ');
+}
+
+void ASTNode::dump(int indent) const
+{
+    print_indent(indent);
+    printf("%s\n", class_name());
+}
+
+void ScopeNode::dump(int indent) const
+{
+    ASTNode::dump(indent);
+    for (auto& child : children())
+        child.dump(indent + 1);
+}
+
+void BinaryExpression::dump(int indent) const
+{
+    ASTNode::dump(indent);
+    m_lhs->dump(indent + 1);
+    m_rhs->dump(indent + 1);
+}
+
+void CallExpression::dump(int indent) const
+{
+    print_indent(indent);
+    printf("%s '%s'\n", class_name(), name().characters());
+}
+
+void Literal::dump(int indent) const
+{
+    print_indent(indent);
+    printf("%d\n", (i32)m_value.as_double());
+}
+
+void FunctionDeclaration::dump(int indent) const
+{
+    print_indent(indent);
+    printf("%s '%s'\n", class_name(), name().characters());
+    body().dump(indent + 1);
+}
+
+void ReturnStatement::dump(int indent) const
+{
+    ASTNode::dump(indent);
+    argument().dump(indent + 1);
+}
+
+}