LibJS: Support array holes, encoded as empty JS::Value

This patch adds a new kind of JS::Value, the empty value.
It's what you get when you do JSValue() (or most commonly, {} in C++.)

An empty Value signifies the absence of a value, and should never be
visible to JavaScript itself. As of right now, it's used for array
holes and as a return value when an exception has been thrown and we
just want to unwind.

This patch is a bit of a mess as I had to fix a whole bunch of code
that was relying on JSValue() being undefined, etc.

1 files changed, 3 insertions, 4 deletions

diff --git a/Libraries/LibJS/Runtime/Value.h b/Libraries/LibJS/Runtime/Value.h
index b6d30c80f1..f6240c316f 100644
--- a/Libraries/LibJS/Runtime/Value.h
+++ b/Libraries/LibJS/Runtime/Value.h
@@ -36,6 +36,7 @@ namespace JS {
 class Value {
 public:
     enum class Type {
+        Empty,
         Undefined,
         Null,
         Number,
@@ -44,6 +45,7 @@ public:
         Boolean,
     };
 
+    bool is_empty() const { return m_type == Type::Empty; }
     bool is_undefined() const { return m_type == Type::Undefined; }
     bool is_null() const { return m_type == Type::Null; }
     bool is_number() const { return m_type == Type::Number; }
@@ -56,10 +58,7 @@ public:
     bool is_nan() const { return is_number() && __builtin_isnan(as_double()); }
     bool is_infinity() const { return is_number() && __builtin_isinf(as_double()); }
 
-    Value()
-        : m_type(Type::Undefined)
-    {
-    }
+    Value();
 
     explicit Value(bool value)
         : m_type(Type::Boolean)