Is it possible to emulate non-enumerable properties?

Question

ES5 has a enumerable flag. Example Example ``` var getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor , pd = getOwnPropertyDescriptor(Object.prototype, "toString"); assert(pd.enumerable === false, "enumerability has the wrong value"); ``` Partial implementation Partial implementation is do-able by having `Object.keys` and `Object.getOwnPropertyNames` filter out new non-enumerable properties using the shimmed `Object.defineProperty`. Introduction This allows for properties to be non enumerable. This clearly means that Example ``` for (var key in {}) { assert(key !== "toString", "I should never print"); } ``` This allows us to add properties to say `Object.prototype` (Example) ``` Object.defineProperty(Object.prototype, "toUpperCaseString", { value: function toUpperCaseString() { return this.toString().toUpperCase(); }, enumerable: false }); for (var key in {}) { assert(key !== "toUpperCaseString", "I should never print"); } console.log(({}).toUpperCaseString()); // "[OBJECT OBJ…

Accepted Answer

You can do it via code-rewriting. Rewrite every use of `for (p in o) body` to

```
for (p in o) {
  if (!(/^__notenum_/.test(p) || o['__notenum_' + p])) {
    body
  } 
}
```

and then you can mark properties not enumerable by defining a `__notenum_...` property. To be compatible you would have to tweak the above to make sure that `__notenum_propname` is defined at the same prototype level as `propname`, and if you use them, overwrite `eval` and `new Function` to rewrite.

That's basically what ES5/3 does.

Is it possible to emulate non-enumerable properties?

Solution

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