holes in arrays vs. undefined and the map function

Question

I understand that map is not called on undefined indexes on arrays and I appreciate that an undefined index is different from an array index explicitly assigned the 'undefined' value (it is, isn't it?). Yet, how is one supposed to distinguish between holes in an array and undefined values?

The below code:

foo.js

var arr = [,,undefined,,,,3,,,4];
console.log("A hole the same as undefined? "+(arr[0]===undefined?"yes":"no"));
var _ignored = arr.map(function(x) {console.log("f called on ["+x+"]");});  

... produces when running:

$ node foo.js 
A hole the same as undefined? yes
f called on [undefined]
f called on [3]
f called on [4]

... similar results are to be had when replacing map with forEach.

Answer 1

The in operator tells you if an index has been actually assigned to:

a = []
a[5] = window.foo

document.write(a[4] + "<br>")
document.write(a[5] + "<br>")

document.write((4 in a) + "<br>")
document.write((5 in a) + "<br>")

Javascript arrays are actually objects with a special length property, and array indexes are just property names (in fact, they are even strings, not numbers, internally). So the above definition is equivalent to:

a = {
  5: window.foo,
  length: 6
}

Therefore, all object functionality related to keys (like in, hasOwnProperty, Object.keys) work for array indexes as well.

forEach and other iteration methods work by iterating from 0 to length-1 and checking if n-th index is actually present in the argument, they don't "know" if the argument is actually an array or just a generic object:

a = {1:'one', 5:'five', length:100};

[].forEach.call(a, function(x) {
  document.write(x)
});