DeepReadonly Object Typescript

Question

It is possible to create a DeepReadonly type like this:

type DeepReadonly = {
  readonly [P in keyof T]: DeepReadonly;
};

         


        

Answer 1

You can have a readonly array:

interface ReadonlyArray extends Array {
    readonly [n: number]: T;
}
let a = [] as ReadonlyArray;
a[0] = "moo"; // error: Index signature in type 'ReadonlyArray' only permits reading

But you can't use it with your solution:

interface A {
    B: { C: number; };
    D: ReadonlyArray<{ E: number; }>;
}

myDeepReadonlyObject.D[0] = { E: 3 }; // still fine

The type of D is DeepReadonly> and it won't allow the ReadonlyArray to kick in.

I doubt that you'll manage to make it work to objects with arrays in them, you can have either deep read only for arrays or for objects if you want a generic interface/type and not specific ones.
For example, this will work fine:

interface A {
    readonly B: { readonly C: number; };
    D: ReadonlyArray<{ E: number; }>;
}

const myDeepReadonlyObject = {
    B: { C: 1 },
    D: [{ E: 2 }],
} as A;

myDeepReadonlyObject.B = { C: 2 }; // error
myDeepReadonlyObject.B.C = 2; // error
myDeepReadonlyObject1.D[0] = { E: 3 }; // error

But it has a specific interface to it (A) instead of a generic one DeepReadonly.

Another option is to use Immutable.js which comes with a builtin definition file and it's pretty easy to use.