The expression [][[]]
evaluates to undefined
in JavaScript. My understanding of this was that the compiler sees the second set of [...]
and interprets that to be an array subscript operator (because you can’t have two arrays next to each other).
So the compiler knows that the inner expression, []
, must be an index, and so after evaluating it, it coerces it to a number. Number([])
evaluates to 0
, and so we have [][0]
, which is undefined
.
However, [1][[]]
does not evaluate to 1
as I would expect, but rather to undefined
suggesting that in this case (or maybe also in the previous case), []
isn’t being coerced to a number. It seems that I must use the unary +
to force the type coercion:
[1][+[]] // returns 1
So if the inner []
in the expression [][[]]
is not being coerced to a number, then why does that expression evaluate to undefined
?
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Answer
The faulty assumption was that the expression that evaluates to the index is coerced to a number. It is in fact coerced to string as are all object keys (except for Symbols, which stay Symbols).
Thus, [1][[]]
turns into [1][""]
, and since the ""
property doesn’t exist on the array, we get undefined
instead of 1
.