Controlling access to global variables via an ES6 proxy

[2016-11-27] dev, javascript, esnext
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The following function evalCode() traces the global variables that are accessed while evaluating a piece of JavaScript code.

// Simple solution
const _glob = typeof global !== 'undefined' ? global : self;

function evalCode(code) {
    const func = new Function ('proxy',
        `with (proxy) {${code}}`); // (A)
    const proxy = new Proxy(_glob, {
        get(target, propKey, receiver) {
            console.log(`GET ${String(propKey)}`); // (B)
            return Reflect.get(target, propKey, receiver);
        set(target, propKey, value, receiver) { // (C)
            console.log(`SET ${String(propKey)}=${value}`);
            return Reflect.set(target, propKey, value, receiver);
    return func(proxy);

The way this works is as follows:

  • The with statement wrapped around the code (line A) means that every variable access that “leaves” the scope of the code becomes a property access of proxy.
  • The proxy observes what properties are accessed via its handler, which traps the operations “get” (line B) and “set” (line C),

Unsing evalCode():

> evalCode('String.prototype')
GET Symbol(Symbol.unscopables)
GET String
> evalCode('String = 123')
GET Symbol(Symbol.unscopables)
SET String=123


  • We don’t return what code does, which is why the result is undefined.
  • Symbol.unscopables shows up, because with checks its operand for a property with this key to determine which properties it should not expose as variables to its body. This mechanism is explained in “Exploring ES6”.

This is very hacky! with is a deprecated sloppy mode feature that is used in conjunction with a brand new ES6 feature.

Source of this hack: Vue.js, explained by qgustavor on reddit.

Further reading