Memory shouldn't make a difference.

a = Array(1e7); for ( let i=0; i<1e7; i++ ) a[i] = i; takes ~50 ms on my machine.

Array.from(..) takes ( wait for it - literally :P ) ~3300 ms.

For 1e3 elements, it's ~.2 ms vs. ~1 ms.

How is this performance-/memory-wise compared to populating it via a for loop?

Thank you, that was helpful!

The object doesn't even contain undefined items. It only has length. There's a difference between existing properties that happen to be undefined and actual missing, or empty, ones. Cf. [undefined,0] vs. [,0] ( yes, that's valid JS ). But if you ask for a non-existent property, JS will not throw a ReferenceError, but give you undefined instead, unlike when you ask for a non-existent variable.

Someone tell me if I'm wrong please: the object the array is being mapped from contains N undefined objects, so the indexes of N undefined objects are mapped to the array.

JohaWiltink, thx.

Thanks about convention of not used variables!