LibJS: Implement basic boolean coercion

We can't expect the conditionals in "if" and "while" statements to
always return a bool, so we need to know how to boolify a JS::Value.

Libraries/LibJS/AST.cpp

@@ -70,7 +70,7 @@ Value IfStatement::execute(Interpreter& interpreter) const
 {
     auto predicate_result = m_predicate->execute(interpreter);
 
-    if (predicate_result.as_bool())
+    if (predicate_result.to_boolean())
         return interpreter.run(*m_consequent);
     else
         return interpreter.run(*m_alternate);
@@ -79,7 +79,7 @@ Value IfStatement::execute(Interpreter& interpreter) const
 Value WhileStatement::execute(Interpreter& interpreter) const
 {
     Value last_value = js_undefined();
-    while (m_predicate->execute(interpreter).as_bool()) {
+    while (m_predicate->execute(interpreter).to_boolean()) {
         last_value = interpreter.run(*m_body);
     }
 
@@ -213,9 +213,8 @@ Value BinaryExpression::execute(Interpreter& interpreter) const
 
 Value LogicalExpression::execute(Interpreter& interpreter) const
 {
-    auto lhs_result = m_lhs->execute(interpreter).as_bool();
+    auto lhs_result = m_lhs->execute(interpreter).to_boolean();
-
+    auto rhs_result = m_rhs->execute(interpreter).to_boolean();
-    auto rhs_result = m_rhs->execute(interpreter).as_bool();
     switch (m_op) {
     case LogicalOp::And:
         return Value(lhs_result && rhs_result);

Libraries/LibJS/Value.cpp

@@ -53,6 +53,25 @@ String Value::to_string() const
     ASSERT_NOT_REACHED();
 }
 
+bool Value::to_boolean() const
+{
+    switch (m_type) {
+    case Type::Boolean:
+        return m_value.as_bool;
+    case Type::Number:
+        return m_value.as_double == 0 || m_value.as_double == -0;
+    case Type::Null:
+    case Type::Undefined:
+        return false;
+    case Type::String:
+        return String(as_string()).is_empty();
+    case Type::Object:
+        return true;
+    default:
+        ASSERT_NOT_REACHED();
+    }
+}
+
 const LogStream& operator<<(const LogStream& stream, const Value& value)
 {
     return stream << value.to_string();

Libraries/LibJS/Value.h

@@ -113,6 +113,7 @@ public:
     }
 
     String to_string() const;
+    bool to_boolean() const;
 
 private:
     Type m_type { Type::Undefined };