JavaScript “new Array(n)” and “Array.prototype.map” weirdness

Question

I\'ve observed this in Firefox-3.5.7/Firebug-1.5.3 and Firefox-3.6.16/Firebug-1.6.2

When I fire up Firebug:

Answer 1

From the MDC page for map:

[...] callback is invoked only for indexes of the array which have assigned value; [...]

[undefined] actually applies the setter on the index(es) so that map will iterate, whereas new Array(1) just initializes the index(es) with a default value of undefined so map skips it.

I believe this is the same for all iteration methods.