javascript - 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 OBJECT]"

Question

How can we emulate this in non-ES5 compliant browsers?

Browser compat table

In this case we care about potentially solving this for

• IE < 9 (IE8 only works on DOM objects)
• Firefox 3.6
• Safari 4
• Opera 11.5 (Opera 11.6 solves this).

The ES5-shim does not have a solution for this.

The ES5 shim has a solution for most ES5 features that will break your code if it doesn't work.

Is there any black magic that can be done with propiotory IE only APIs? Maybe with VBScript?

Answer 1

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.