Do we really need the [[HasOwnProperty]] internal method and hasOwn trap

Tom Van Cutsem at
Wed Nov 14 00:30:36 PST 2012

2012/11/13 David Bruant <bruant.d at>

>  For the particular case you've written, when going for
> or the in operator, the JS engine knows it needs to
> output a boolean, so it can "rewrite" (or contextually compile) your trap
> last line as "e===undefined" (since "undefined" is falsy and objects
> created by object literals are truthy). In that particular case, the
> allocation isn't necessary provided some simple static analysis.
> Maybe type inference can be of some help to prevent this allocation in
> more dynamic/complicated cases too. I would really love to have
> implementors POV here.

I'm very skeptical of this.

If I may summarize, you're arguing that we can get away with spurious
allocations in handler traps because JS implementations can in theory
optimize (e.g. partially evaluate trap method bodies). I think that's
wishful thinking. Not that implementors can't do this, I just think the
cost/benefit ratio is way too high.

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