Readonly modifier doesn't work with type aliases for tuple or array

[ikokostya]

TypeScript Version: 3.5.1

Search Terms:

readonly, type alias, tuple, array

Code

type A = number[];
type T = [number, number];

type RA = readonly A;
type RT = readonly T;

Expected behavior:

There is no errors, like in the following example:

type RA = readonly number[];
type RT = readonly [number, number];

Actual behavior:

Compile errors:

test.ts:4:11 - error TS1354: 'readonly' type modifier is only permitted on array and tuple literal types.

4 type RA = readonly A;
            ~~~~~~~~

test.ts:5:11 - error TS1354: 'readonly' type modifier is only permitted on array and tuple literal types.

5 type RT = readonly T;
            ~~~~~~~~


Found 4 errors.

Playground Link: http://www.typescriptlang.org/play/index.html#code/C4TwDgpgBAglC8UB2BXAtgIwgJwNoF0BuAKFEigBUEpdVMcAaZdLbI408aAJTkWwgBDACYB7JABsQsEmR5V+QsZOkVCQA

Related Issues:

#12

[fatcerberus]

array and tuple literal types

A and T are not array or tuple literals, they're type constants. What you want is:

type RA = Readonly<A>;
type RT = Readonly<T>;

[ikokostya]

In the wiki readonly modifier is proposed as new syntax for read-only arrays.

With current restrictions need to mix readonly modifier and Readonly type:

type Position = [number, number];

function takeCoordinates(coords: readonly Readonly<Position>[]): void {}

[fatcerberus]

On the page you linked it shows that you can't use it with type aliases:

let err1: readonly Set<number>; // error!
let err2: readonly Array<boolean>; // error!

let okay: readonly boolean[]; // works fine

And in your example, you would still need to repeat yourself even if this were supported:

function takeCoordinates(coords: readonly (readonly Position)[]): void {}

[sandersn]

This is the current design. We have special syntax and semantics for these types only.
This might change in the future, but the most likely change is that readonly could be applied to any type.