Thoughts on IEEE P754

Mike Cowlishaw MFC at uk.ibm.com
Fri Aug 22 08:37:54 PDT 2008


> >>  var a = [];
> >>  a[0] = "first";
> >>  a[0.0m] = "second";   // a second property?
> >>  a[0.000m] = "third";   // a third?
> >
> > [...]
> >
> > It seems to me that if trailing zeros are retained in a call to
> > ToString, then to be consistent with ES3, the three assignments above
> > would produce three properties.  And if trailing zeros are lost, you
> > would expect to only have one.  Both approaches are internally
> > consistent, and it surprises me to see either one characterized as
> > "breaking" array lookup.
> 
> Well, the intuition is that two equal numbers should index the same
> property.  This is an invariant in ES3 and it makes a lot of sense.

Seems to me the problem here is not decimal (or binary) representations 
but arrays (consider a(0.0) and a("0.0")).

In effect the array definition in ES3 says -- in a rather roundabout way 
-- that all indexes must be an integer (which fits in 32 bits if encoded 
in binary).  So, as long as there's a toInt32 in there, decimals with 
trailing zeros after the point are just fine.  In other words the 
processing of array indexes could (conceptually) be 
toString->toNumber->toInt32 (or something like that).

Mike





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