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...args should not be readonly within function body #53398

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Open
...args should not be readonly within function body#53398
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Experience EnhancementNoncontroversial enhancementsSuggestionAn idea for TypeScript
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Backlog

Description

@jakebailey
Jake Bailey (jakebailey)
opened on Mar 20, 2023

Bug Report

🔎 Search Terms

readonly parameter inferred tuple array args variadic

🕗 Version & Regression Information

  • This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

declare function callFn<T extends readonly any[]>(args: T, fn: (...args: T) => void): void;

declare const input: readonly string[];

callFn(input, (...args) => {
    args; // readonly string[] ?
})

declare function callFnNonGeneric(args: readonly string[], fn: (...args: readonly string[]) => void): void;

callFnNonGeneric(input, (...args) => {
    args; // readonly string[] ?
})

🙁 Actual behavior

...args is readonly.

🙂 Expected behavior

...args is not readonly.

When this function is called, the input will be a brand new array which can be modified without affecting the caller:

> const fn = (...args) => { args[0] = "oops" }
undefined
> const arr = ["some", "values"]
undefined
> fn(...arr)
undefined
> arr
[ 'some', 'values' ]

Split out of #53258 (comment)

The "fix" here is to strip readonly from variadic args (at the top level). This also means the fix in #53258 can be reverted (as the real fix can be more general).

Activity

  1. fatcerberus commented on Mar 20, 2023

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Mar 20, 2023

    Hmm, but args is typed as T in this example, which must necessarily be instantiated with a readonly array type, and args is required to be the same type as input… so how would this work?

  2. jakebailey commented on Mar 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Mar 20, 2023
    MemberAuthor

    Like, how would it be implemented? Or something else?

  3. fatcerberus commented on Mar 20, 2023

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Mar 20, 2023

    in short: how could args not be readonly if input is, since both are typed as T? It seems like you’d need to manufacture a type parameter here.

  4. jakebailey commented on Mar 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Mar 20, 2023
    MemberAuthor

    Well, the idea I (rather Wesley Wigham (@weswigham)) had was to copy Mutable<T> into lib.d.ts and say that within the function, the type of the ...args parameter is always observed as Mutable<T>, dropping readonlyness.

    So, the function is typed the same.

  5. jakebailey commented on Mar 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Mar 20, 2023
    MemberAuthor

    Sorta kinda the same as what I did for spread types in assignments, which implicitly drops certain properties (#47078).

  6. Jake Bailey (jakebailey)
    changed the title [-]Inferred ...args should not be readonly[/-] [+]...args should not be readonly within function body[/+] on Mar 21, 2023
  7. Ryan Cavanaugh (RyanCavanaugh)
    added
    SuggestionAn idea for TypeScript
    Experience EnhancementNoncontroversial enhancements
    on Mar 21, 2023
  8. Ryan Cavanaugh (RyanCavanaugh)
    added this to the Backlog milestone on Mar 21, 2023

  9. RyanCavanaugh commented on Mar 21, 2023

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Mar 21, 2023
    Member

    Jake Bailey (@jakebailey) feel free to self-assign if you want to

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