Typescript: Cannot destructure a Readonly Tuple

[SteveALee]

TypeScript Version: 2.4.0

Code

// This is OK
type Tuple = [string, number]
const t : Tuple = ['string', 2]
const [s,n] = t

// This errors 
// [ts] Type 'Readonly<[string, number]>' is not an array type.
// const rs: string
type ReadonlyTuple = Readonly<[string, number]>
const rt : ReadonlyTuple = ['string', 2]
const [rs,rn] = rt

Expected behavior:
No errors

Actual behavior:
Error as above

[jcalz]

This is probably a limitation of the Readonly<> that causes it to lose the "array-ness" of the tuple type. You can work around it as follows:

type ReadonlyTuple = Readonly<[string, number]> & [string, number];
const rt: ReadonlyTuple = ['string', 2]
rt[0] = 'zero'; // error as expected, still read-only
const [rs,rn] = rt // no error, accepts as array type

[olegdunkan]

ReadonlyTuple is mapped type, destructing requires array like type in your case

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.