javascript - How to get proxy's handler from proxy object?

Question

For example, if I have this handler/proxy (from the MDN example)...

var handler = {
    get: function(target, name){
        return name in target?
            target[name] :
            37;
    }
};

var p = new Proxy({}, handler);
p.a = 1;
p.b = undefined;

console.log(p.a, p.b); // 1, undefined
console.log('c' in p, p.c); // false, 37

is it possible to probe the proxy, p, in some way that allows me to get the handler object back.

Something along the lines of:

p.__handler__   // returns handler object -> Object {get: handler.get(), set: handler.set(), ...}
p.__handler__.get  // returns get prop/fn of handler -> function(target, name){ ...}

Obviously, the various traps set up in the handler are still "known" to the proxy, but is there a clear-cut way to return them/ the handler from the proxy itself? If so, how?

I have no specific use-case for this at the moment, but I could see this being useful if you wanted to dynamically change a handler/traps after you already have a proxy.

Answer 1

ECMAScript provides no way to access the internal [[ProxyHandler]] nor [[ProxyTarget]] slots.

Some implementations may provide some non-standard ways, but don't take it for granted.

For example, on Firefox privileged code, you can know if an object is a proxy using

Components.utils.isProxy(object);

I proposed implementing similar methods to expose the [[ProxyHandler]] and [[ProxyTarget]]. They told me to implement them in Debugger.Object instead of Components.utils.

When the patch lands, it will be possible to use something like

Components.utils.import('resource://gre/modules/jsdebugger.jsm');
var Cc = Components.classes;

// Add a debugger to a new global
var global = new Components.utils.Sandbox(
  Cc["@mozilla.org/systemprincipal;1"].createInstance(Ci.nsIPrincipal),
  { freshZone: true }
);
addDebuggerToGlobal(global);
var dbg = new global.Debugger().addDebuggee(this);

// Create a debugger object of your object, and run proxy getters
var dbgObj = dbg.makeDebuggeeValue(object);
if(dbgObj.isProxy) { // a boolean
  dbgObj.proxyHandler.unsafeDereference(); // the [[ProxyHandler]]
  dbgObj.proxyTarget.unsafeDereference(); // the [[ProxyTarget]]
}