Brendan Eich wrote:
> Function.setTypeOf(V, U)
>
> 1.  Let S = ToString(U)
> 2.  Let T = V
> 3.  If T is null then
> 3a.   If S is not "null" and S is not "object", throw
> 4.  Else
> 4a.   If T is not a value object constructor, throw
> 4b.   T = T.prototype
> 4c.   If TypeofToExemplarMap.has(S), throw
> 5.  Call TypeofToExemplarMap.set(S, T)
> 6.  Call ExemplarToTypeofMap.set(T, S)

Correction, see new 4b below:

1.  Let S = ToString(U)
2.  Let T = V
3.  If T is null then
3a.   If S is not "null" and S is not "object", throw
4.  Else
4a.   If T is not a value object constructor, throw
4b.   If T and Function are not same-realm, throw
4c.   T = T.prototype
4d.   If TypeofToExemplarMap.has(S), throw
5.  Call TypeofToExemplarMap.set(S, T)
6.  Call ExemplarToTypeofMap.set(T, S)

I did not add an exception when the current evaluation context's realm 
is not the same as Function's -- that seems too restrictive, forbidding 
script in the main window from fixing up typeof for same-origin code 
that it loads into an iframe.

Again, comments welcome (cc'ing Allen since he is realm-master; ES1-5 
left realm implicit and singular, contrary to reality in browsers).

/be

Brendan Eich wrote:
> Function.setTypeOf(V, U)
>
> ```
> 1.  Let S = ToString(U)
> 2.  Let T = V
> 3.  If T is null then
> 3a.   If S is not "null" and S is not "object", throw
> 4.  Else
> 4a.   If T is not a value object constructor, throw
> 4b.   T = T.prototype
> 4c.   If TypeofToExemplarMap.has(S), throw
> 5.  Call TypeofToExemplarMap.set(S, T)
> 6.  Call ExemplarToTypeofMap.set(T, S)
> ```

Correction, see new 4b below:

```
1.  Let S = ToString(U)
2.  Let T = V
3.  If T is null then
3a.   If S is not "null" and S is not "object", throw
4.  Else
4a.   If T is not a value object constructor, throw
4b.   If T and Function are not same-realm, throw
4c.   T = T.prototype
4d.   If TypeofToExemplarMap.has(S), throw
5.  Call TypeofToExemplarMap.set(S, T)
6.  Call ExemplarToTypeofMap.set(T, S)
```

I did not add an exception when the current evaluation context's realm 
is not the same as Function's -- that seems too restrictive, forbidding 
script in the main window from fixing up typeof for same-origin code 
that it loads into an iframe.

Again, comments welcome (cc'ing Allen since he is realm-master; ES1-5 
left realm implicit and singular, contrary to reality in browsers).