How to create a typescript type to allow read-only tuples from a const

June 20, 2023

I have the following interface

interface Test {
    data: string[]
}

const DATA_LIST = {
    USER: ["some string"],
    OTHER: ["some other string", "random value", "etc"]
} as const;

const test : Test = {
    data: DATA_LIST.OTHER
}

In the above code I cannot assign DATA_LIST.OTHER to data which is my use case where I have a couple of tuples that I want to assign to data.

I cannot assign it because it only accepts string[] while I am trying to assign a readonly tuple

Is there a better way to do this?

https://www.typescriptlang.org/play?ssl=12&ssc=2&pln=1&pc=1#code/JYOwLgpgTgZghgYwgAgCoQM5mQbwFDKHIAmcYcAXMllKAOYDaAungL554ID2IWyAIgEFUggPoAZAJIBlVMgC8uAkQCq0gKIAlKgwBEGLgFsUNerqYAaZYQDyqABJad+oyi5gAFtGphaIOroWyLpQcCDERsgAbnAANgCuEIHBEGAI5mzIcBjI3LxgANwceXyQfFTofIr4RCRklALCYlKyAHR2jppsQA

>Solution :

The underlying problem is simply this:

The type 'readonly ["some other string", "random value", "etc"]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

… which means that you can’t assign a string[] to a readonly string[].

If you want to reassign test.data at some other point in time, then you could simply do data: [...DATA_LIST.OTHER], which will get rid of the readonly aspect and which allows you to assign to a simple string[] property.

If you do not want test.data to be reassigned you could change the type of the property to either readonly string[] or string[] | readonly string[].