JavaScript destructuring with an array element

Question

I have met the following questions in JavaScript:

const [x1, ...[result]] = [3, 4, 5]
console.log([result])

I know x1 is 3, but why is the logging result [4] instead of [4,5]?

result is 4, not [ 4 ]. [ result ] can never be [ 4, 5 ]. It seems to behave similarly to const [first] = array;; in this case, it seems to be spreading the rest of [ 3, 4, 5 ], i.e. [ 4, 5 ] into ...[result], then destructuring [ 4, 5 ] into [ result ]; therefore result is 4. — Sebastian Simon
– Sebastian Simon, Commented Aug 11, 2021 at 14:28

jsdev2023 · Accepted Answer · 2021-08-11 14:34:54Z

5

So basically what is happening if we follow this syntax

const [a,...b] = [3,4,5]

Javascript creates an array called b and has the value [4,5]

But in your case what is happening is,

const [a,...[b]] = [3,4,5]

This is essentially assigning to only the first variable of the empty array with first value as b, which always equals 4 and not [4,5] as you expect.

So it's equivalent to the case below

const [a,...[b,c]] = [3,4,5]

the only difference is that you are not providing a variable c in your case. So b would correspond to 4 and c would correspond to 5

answered Aug 11, 2021 at 14:34

jsdev2023

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