TypeError: 'set' on proxy: trap returned falsish for property

[xeaone]

Hello awesome polly! Got an error not sure if it is me but it only happens when I use strict;. I am not adding a new path just editing existing path. If it is a bug lets see if we can fix it. If it is me slap me in the face but tell me what I am doing wrong please.

error: TypeError: 'set' on proxy: trap returned falsish for property 'port'

Example

'use strict'; // this makes the error throw

function ObserveObject(object, callback) {
    function createProxy(prefix, object) {
        return new Proxy(object, {
            set: function(target, property, value) {
                target[property] = value;
                callback(prefix + property, value);
            },
            get: function(target, property) {
                var value = target[property];

                if (isObject(value)) return createProxy(prefix + property + '.', value); 
                else return value; 
            }
        });
    }

    return createProxy('', object);
}

function isObject(value) {
    if (value === null || value === undefined) return false;
    else return value.constructor === Object;
}

var o = {
    hello: 'blue',
    app: {
        env: {
            port: 7777,
            la: 1
        }
    },
    more: 'stuff'
};

var m = ObserveObject (o, function(path, value) {
    console.log(path);
    console.log(value);
});

m.app.env.port = 8888;
console.log(m);

[xeaone]

Oops...

I think I figured it out.

Link: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/set

"Return value

The set method should return a boolean value. Return true to indicate that assignment succeeded. If the set method returns false, and the assignment happened in strict-mode code, a TypeError will be thrown."

Should return true in the set method.

This is pretty weird though can anyone explain why?

[samthor]

That's right.

This actually isn't enforced in the polyfills, since we don't know if you're in strict mode (see here for a comment on our thoughts). The error you're seeing is actually from your browser.

[ORESoftware]

I have this same error:

https://stackoverflow.com/questions/46227721/typeerror-with-proxy-class-typeerror-set-on-proxy-trap-returned-truish-for

If I return false, I get a TypeError, and when I return true I get a TypeError, I can't win! Help!

[xeaone]

Hi @ORESoftware,

I am pretty sure the issue you posted on stackoverflow has nothing to do with Proxy or this pollyfill. But I think the issue has to do with the options you passed to your Object.defineProperty method. Change your writable option from false to true and I think your issue should be resolved.

[ORESoftware]

I know I just figured I get a good answer here and I appreciate your input :) I think you're right. But why are you right? I can't figure out why...setting writable to false should allow it be written the first time.

[xeaone]

MDN has the following description for the writable property.

writable
true if and only if the value associated with the property may be changed with an assignment operator.
Defaults to false.

So technically Object.defineProperty is the first setting of the property. But that's a side note. As we can see from MDN's description writable set to false disallows us from changing the property via an assignment operator = is an example of an assignment operator.

MDN Link: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

[ORESoftware]

yeah I was looking at that thanks

[mutuadavid93]

// NOTE: When Intercepting Sets using Proxy, it throws an Error in "use strict"
// mode, due to polyfills. Babel on it's own can transpile it without an error.
//
// Solution: Inside the set function return true and you good.
//
// i.e.
// var yourSetProxy = new Proxy(targetObject, {
//     set : function (target, property, value) {
//
//          //    Your logic here.
//
//         return true;
//      }
// });

[Velkan33]

Hello, I had a different approach to this one and absolutely work:
I just change in my html <script src="./file.js" type="module"></script> to <script src="./file" type="text/javascript"></script>