Symbol vs. String when retrieving property from target object, using ES6 Proxy object

Question

Using the following code:

 const assrt = function () {
    try {
      return chaiAssert.apply(null, arguments);
    }
    catch (e) {
      return handleError(e);
    }
  };

 v.assert = new Proxy(assrt, {
    get: function (target, prop) {

      if(typeof prop === 'symbol'){
        // I don't know what to do with symbols, so return
         return Reflect.get(...arguments);
      }

      // but here! we still get properties that don't exist
      if(!chaiAssert[prop]){
        return handleError(
          new Error(`The assertion library used does not have '${prop}' property or method.`)
        );
      }

      return function () {
        try {
          return chaiAssert[prop].apply(null, arguments);
        }
        catch (e) {
          return handleError(e);
        }
      }
    }
  });

the error I get with this code is:

TypeError: Cannot convert a Symbol value to a string

and this occurs on the line:

new Error(`The assertion library used does not have '${prop}' property or method.`));

I have used Proxies before, and I have never seen Symbols being passed to the get method of the Proxy. Does anyone know how to circumvent this problem?

Why are Symbols being passed to the Proxy get function and how do I properly handle that?

the prop that's being sent to the get function is Symbol(util.inspect.custom), and think there are others that get sent as well. — Alexander Mills
– Alexander Mills, Commented Sep 28, 2017 at 22:06
I think, you have to debug the code. It doesn't seem like symbols have to be filtered out by if(typeof prop === 'symbol'){. — Qwertiy
– Qwertiy, Commented Oct 11, 2017 at 13:15

Bergi · Accepted Answer · 2017-09-28 22:31:28Z

2

Why are Symbols being passed to the Proxy get function?

We don't know, you didn't show any code that actually uses the proxy. But many symbols are accessed by builtin methods, e.g. when you iterate the proxy it uses the Symbol.iterator method.

and how do I properly handle that?

You cannot concatenate a symbol with a string, you need to be explicit about doing this. You can use either prop.toString() or just switch based on typeof prop.

answered Sep 28, 2017 at 22:31

Bergi

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7 Comments

Alexander Mills Over a year ago

can you demonstrate what you mean by switching on typeof prop

Bergi Over a year ago

typeof prop == "symbol" ? "Symbol ["+prop.toString()+"]" : prop or whatever you want to do with symbols. Maybe just ignore them completely in your proxy handler and just respond with the default Reflect.get for them.

Alexander Mills Over a year ago

Yeah I don't know what to do with the symbols but I need to pass back what they are looking for when the symbols get passed...can you show how to use Reflect as you suggest?

Alexander Mills Over a year ago

looks like this => return Reflect.get(...arguments)

Alexander Mills Over a year ago

I am so confused why a non-symbol property would get passed to the handler's get method, and the target property would not exist on the target. I will update the OP.

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