V8引擎中的Typeof

Question

I am experimentation with V8 engine, i am not a C programmer and it is very difficult for me .But i need this thing for myself

I change a function RUNTIME_FUNCTION(Runtime_Typeof) ; (runtime-object.cc) And now if to create proxy in JS:

prx = Proxy.create({
 get:function(k,v){
    if( v == '%type_of%'){
       return 'string'
    }
    .......
 }
})

typeof prx  -> "string"

Ok, i want this effect

BUT if try to make compare:

  typeof prx == 'string' -> false =(
  typeof prx == 'object' -> true
  (x = typeof prx) == 'string' -> true

After many hours i found this: In V8 ( full-codegen-x64.cc ) exist this func void FullCodeGenerator :: EmitLiteralCompareTypeof

It is look like an optimizator for expression

   typeof prx == "string" || "number" || "object" and etc..
   if (String::Equals(check, factory->number_string())) {
   ...
   } else if (String::Equals(check, factory->string_string())) {
   and etc...

it do not call Runtime_Typeof and

    typeof prx -> "string"
    typeof prx == 'string' -> false =(

How can i check if typeof prx it is proxy, then v8 must to call Runtime_Typeof ..

if typeof prx -> string, then typeof == 'string' must be true! Help me please

Answer 1

You would need to emit code that distinguishes the cases at runtime . That is not too hard, but not a completely trivial change either. Moreover, you'd also need to adapt the optimising compiler.

But that's the easy part. Changing the meaning of typeof will break other parts of the system, which rely on the correct semantics. For example, all uses of the IS_STRING primitive in V8's built-in libraries will become incorrect -- you are likely to introduce crashes that way.

Even if you managed to fix all that, the idea would be very questionable from a language semantics perspective. There is a reason why proxies cannot fake strings, first and foremost, strings aren't objects. You would also break JavaScript code that relies on the spec'ed behaviour.

In short: don't . It's a really bad idea.