Why can't I map new Array(3) into an array of new values? [duplicate]

I'd like to understand the why behind this:

var a = new Array(3);
var b = a.map(function () {
  return 'b';
});

results in

a: [ , , ]

b: [ , , ]

When I would expect b to result in ['b', 'b', 'b'].


In further investigation, I discovered that if i were to do a.push('a'), I'd have [, , , 'a'].

And after the map function, b would become [, , , 'b'].

What's going on here? Why do these allocated cells behave differently from the initialization? I was originally expecting this to act as it would if it was an array literal, [undefined, undefined, undefined].map(fn)

Answers:

Answer

Array.map() does not invoke the callback for indexes whose values aren't defined.

From MDN docs:

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes that are undefined, those which have been deleted or which have never been assigned values.

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