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In Chrome Version 125.0.6422.76 (Official Build) (64-bit), I'm seeing this weird behavior with the Array constructor:

Javascript: Array(3).map() vs [...Array(3)].map() weird behavior

Notice that a's length is correctly stored as 3 internally.

I care less if Javascript is doing an internal optimization by lazily allocating the 3 (or more) elements later (say, during element assignment) as long as an array of length 3 is returned as an object for the map call in a.map().

Apparently, it isn't, since no foo's get printed as they do with b.map().

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  • you cannot iterate when created using Array constructor stackoverflow.com/a/41246860 Commented May 25, 2024 at 6:55
  • 3
    developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/… empty slots are skipped during iteration Commented May 25, 2024 at 7:02
  • 5
    .map() skips over empty element, and spread syntax replace the empty element with undefined Commented May 25, 2024 at 7:12
  • 2
    Read about sparse arrays Commented May 25, 2024 at 7:17
  • Thanks, sparse arrays and definitions of map vs join etc is the real answer. Thanks, to all those who took the time to respond. Commented May 25, 2024 at 8:07

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When you initialize an array using Array constructor, it's an empty array. Yes, it has an length, which means JS stored a block of memory for the future use cases but it is completely empty. So, there is nothing to iterate over.

In the your second case, b array, there is an array with undefined elements. So, you can iterate over it.

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