Question about GetBindingValue

André Bargull andre.bargull at udo.edu
Fri Aug 29 04:07:52 PDT 2014


> Hello,
>
> We found one "dead part" in ES5 and we're wondering whether we're missing something here.  The question is about the 4th step in Section 10.2.1.2.4 GetBindingValue(N, S):
>
> 10.2.1.2.4 GetBindingValue(N,S)
> The concrete Environment Record method GetBindingValue for object environment records returns the value of its associated binding object's property whose name is the String value of the argument identifier N.  The property should already exist but if it does not the result depends upon the value of the S argument:
>   1. Let envRec be the object environment record for which the method was invoked.
>   2. Let bindings be the binding object for envRec.
>   3. Let value be the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.
>   4. If value is false, then
>     a. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
>   5. Return the result of calling the [[Get]] internal method of bindings, passing N for the argument.
>
> We believe that the 4th step is unreachable.  In other words, whenever GetBindingValue(N, S) is called, the result of calling the [[HasProperty]](N) is always true and here's why:
>

Yes, that reasoning looks correct.


> 10.2.1.1.4 may have a similar problem but we haven't checked it yet.

The only immutable bindings present in ES5 are directly initialized 
before user code can be executed, so yes, step 3 in 10.2.1.1.4 is never 
reachable in ES5.


> We checked with the recent ES6 draft but it seems to have the same issue.

In ES6 it's actually possible to reach that step (8.1.1.2.6 
GetBindingValue, step 5), albeit it's a somewhat tricky and involves 
doing unusual things with proxy objects:

```javascript
with(new Proxy({}, {
   has: function(t, name) {
     print("has: " +name);
     return !this.called && (this.called = true);
}})) {
   (function(){ "use strict"; ref })();
}
```

That program will give the following output:
---
has: ref
has: ref
uncaught exception: ReferenceError: cannot resolve reference: "ref"
---

Proxy objects allow you to define your own [[HasProperty]] 
implementation (the "has" method in the example above). In this case 
[[HasProperty]] will return `true` on the first call in order to report 
a binding is present in HasBinding, but then will return `false` when 
the binding's presence is checked the second time in GetBindingValue.

>
> Best,
> --
> Sukyoung


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