How to get a reversed ReadonlyArray?

Question

If I have a ReadonlyArray<T> and I want to obtain another ReadonlyArray<T> with the same items but in reverse order, what's the easiest way to achieve that?

For example:

const scoresInAscendingOrder: ReadonlyArray<number> = [1, 2, 3, 5, 9];
const scoresInDescendingOrder: ReadonlyArray<number> =  ???;

I can't use scoresInAscendingOrder.reverse() because ReadonlyArray does not support that method (and rightly so since it would modify scoresInAscendingOrder).

See [Reverse array in Javascript without mutating original array](https://stackoverflow.com/q/30610523/1715579) -- that question not about typescript but almost every answer there will also work here. — p.s.w.g, Mar 19 '19 at 13:37

Titian Cernicova-Dragomir · Accepted Answer · 2019-03-19T13:41:07.647

6

You can just use slice and reverse the result of that:

const scoresInAscendingOrder: ReadonlyArray<number> = [1, 2, 3, 5, 9];
const scoresInDescendingOrder: ReadonlyArray<number> = scoresInAscendingOrder.slice(0).reverse()

Or you can use the newer spread syntax:

const scoresInDescendingOrder: ReadonlyArray<number> = [...scoresInAscendingOrder].reverse()

edited Mar 19 '19 at 13:41

answered Mar 19 '19 at 13:38

Titian Cernicova-Dragomir

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Or spread the original array? `[...scoresInAscendingOrder].reverse()` – Gary McGill Mar 19 '19 at 13:40

How to get a reversed ReadonlyArray?

1 Answers1