Add a Readonly factory type/wrapper that supports Maps/Sets

[qc00]

Search Terms

"readonly" type set

Suggestion

Add something like below to the standard library

type SmartRO<T> =
    T extends ReadonlyArray<any> | ReadonlyMap<any, any> | ReadonlySet<any> ? T :
    T extends Array<infer U> ? ReadonlyArray<U> :
    T extends Map<infer K, infer V> ? ReadonlyMap<K, V> :
    T extends Set<infer U> ? ReadonlySet<U> :
    Readonly<T>;

Use Cases

Often people want to express in the type system that certain value is not to be modified by code accessing them. However, the Readonly<T> wrapper type does not work on Map and Set.

As a result, it's difficult to write a generic type that takes a mutable type argument and use a read-only version in the type body.

Examples

interface Validator<T> {
    isValid(x: SmartRO<T>);
}

Checklist

My suggestion meets these guidelines:

• [✔] This wouldn't be a breaking change in existing TypeScript / JavaScript code
• [✔] This wouldn't change the runtime behavior of existing JavaScript code
• [✔] This could be implemented without emitting different JS based on the types of the expressions
• [✔] This isn't a runtime feature (e.g. new expression-level syntax)

[mhegazy]

This was one of the proposals we discussed when adding conditional type support. The general consensuses was that we need a new readonly type operator (tracked by #10725). the type duplication for every entity causes other problems in inference, overload resolution, and in type comparisons.

I am going to say this is a duplicate of #10725, since #10725 fulfills the underlying scenario.

[mhegazy]

I should also add that you can always add this type in your project and use it as a wrapper for Readonly, but we do not have plans to add it to the standard library at the time being.