On Jul 27, 2009, at 2:56 AM, Tom Van Cutsem wrote:
Could you point me to a page that explains the rationale behind
distinguishing data properties from accessor properties? At first
sight, it appears you don't need both since accessor properties can
easily subsume data properties.
ES5 is a draft standard based on de-facto standards in JavaScript
implementations. Accessor properties, commonly called "getters and
setters", originated almost ten years ago in Mozilla's SpiderMonkey JS
engine. They were reverse-engineered into other engines more recently.
JS, originally in Netscape 2 in 1995, and in ECMA-262 editions 1
through 3 (the last edition before ES5, finalized in 1999), had only
data properties, with class-wise [[Get]] and [[Put]] meta-methods in
the spec but not exposed to programmers.
My inspirations for JS back in 1995 included Scheme and Self, neither
of which subsumes anything like data properties under accessor
properties. What's more, I didn't have time in the early days to
support user-defined accessors, although native ones existed under the
hood in custom objects defined by the implementation (i.e., "the
DOM"). Thus the ES1-3 standards do not describe properties as pairs of
accessor functions, or talk about exposing such functions to
programmers and allowing custom getters and setters to be defined.
Since accessors came later, they are observable. You can tell they're
there using the ES5 meta-programming APIs. They differ from data
properties in other ways; e.g., you can't shadow an accessor property
in a prototype object by assignment. Thus ES5 can't easily or
profitably recast all properties as accessor properties. There would
still be "data properties" even if the spec formalized on top of
accessors.
/be
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