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TypeError: 'get' on proxy: property 'constants' is a read-only and non-configurable data property on the proxy target but the proxy did not return its actual value #62

karthikv opened this Issue Mar 6, 2017 · 6 comments


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7 participants

karthikv commented Mar 6, 2017

Noticed this while trying to require a dependency in a vm. Some code to reproduce:

const { NodeVM } = require("vm2");

const vm = new NodeVM({
  console: "redirect",
  sandbox: {},
  require: {
    builtin: ["fs"]
  wrapper: "none"
});"const fs = require('fs'); fs.constants");

Full stack trace:

			throw this._internal.Decontextify.value(e);
TypeError: 'get' on proxy: property 'constants' is a read-only and non-configurable data property on the proxy target but the proxy did not return its actual value (expected '[object Object]' but got '[object Object]')
    at Object.<anonymous> (vm.js:1:91)
    at (node_modules/vm2/lib/main.js:325:27)
    at Object.<anonymous> (script.js:12:4)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.runMain (module.js:590:10)
    at run (bootstrap_node.js:394:7)

Let me know if there's any other information I can give you to be helpful!


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kellym commented Mar 14, 2017

I'm also having quite a few issues around proxies, specifically with inheritance. I'm not sure if these issues are due to the implementation or a limitation of Proxies, but it's worth some investigation.


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mjbvz commented Jun 11, 2017

Also ran into this error. I'm not familiar enough with proxies to understand why the current pattern is not ok but I was able to make the error go away by adding an extra check for readonly non-configurable properties in Contextify.object.get:

	object: function(object, traps, deepTraps, flags, mock) {
		const proxy = new host.Proxy(object, host.Object.assign({
			get: (target, key, receiver) => {
				if (key === 'vmProxyTarget' && DEBUG) return object;
				if (key === 'isVMProxy') return true;
				if (mock && key in mock) return mock[key];
				if (key === 'constructor') return Object;
				if (key === '__proto__') return Object.prototype;
                                // Added code
				const config = host.Object.getOwnPropertyDescriptor(object, key);
				if (config && config.configurable === false && config.writable === false) {
					return object[key];

This seems to follow the get invariant MDN describes:

The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable data property.

But it seems to me like it would break the sandboxing


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terryrgrs commented Oct 16, 2017

Just chiming in to note that we've run into this issue locally as well, with the above code change allowing us to bypass the problem. Also noting that this happened while using the Google Sheets node module in an almost exact copy of their quickstart guide (

Are there any updates on this issue?


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ghost commented Oct 26, 2017

Does this fix have an effect on the known issue "It is not possible to define class that extends proxied class?" What do you mean it would break the sandboxing?


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Madd0g commented Jun 14, 2018

Hey, any progress on this issue?
I'm tried to use libraries like graceful-fs and got the same error.


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fnjoe commented Sep 21, 2018

I met this error when i exported a ES6 class in the script used to run. it was ok after i translated it into

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