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satisfies removes the readonly constraint from a tuple #64274
Description
🔎 Search Terms
satisfies tuple readonly
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about the
satisfiesoperator.
⏯ Playground Link
💻 Code
type T = "A" | "B";
const tuple = ["A", "B"] as const;
const tupleSatisfies = ["A", "B"] as const satisfies T[]; // `tupleSatisfies` is of type `["A", "B"]` but I expected `readonly ["A", "B"]`
const record = { A: "foo", B: "bar" } as const;
const recordSatisfies = { A: "foo", B: "bar" } as const satisfies Record<T, string>; // `recordSatisfies` has the same type as `record`, which is `{ readonly A: "foo"; readonly B: "bar"; }`🙁 Actual behavior
The satisfies operator removes the readonly constraint from the tuple elements.
🙂 Expected behavior
The type of the resulting value should be same as if satisfies wasn't used at all.
Additional information about the issue
I've read through some existing issues explaining how satisfies has an impact on the contextual type, but I still don't understand why it would remove the readonly constraint from array/tuple elements which are simply properties of an array object, whereas satisfies does not remove the readonly constraint from an as const non-array object.
I am aware that I can do the following:
const tupleSatisfies = ["A", "B"] as const satisfies readonly T[];But the again I also didn't have to use
const recordSatisfies = { A: "foo", B: "bar" } as const satisfies Readonly<Record<T, string>>;
Activity
jcalz commentedon Sep 15, 2026
This is working as intended, as implemented in #55229. It's not specifically about
satisfies, but aboutas const, and as such this is effectively a duplicate of #57107.Do note there is an important difference between using
readonlyon array/tuple types and using it on non-array object types. See Improvements forReadonlyArrayandreadonlytuples. It's not just that individual properties/elements are readonly. A readonly array type lacks mutating methods likepush(). So while{readonly A: "foo", readonly B: "bar"}extends{A: "foo", B: "bar"}and vice versa,readonly ["A", "B"]does not extend["A", "B"]. Thereforesatisfies T[]is a stronger constraint thansatisfies readonly T[].Per #55229,
as constwill result in mutable array types when used in a context that expects such types. As mentioned, it isn't specifically aboutsatisfies. The same thing happens withconst x: string[] = ["a"] as const. That used to fail because["a"] as constwould unconditionally result in the typereadonly ["a"]which does not satisfystring[]. Now it succeeds because the type of["a"] as constdepends on context and sincestring[]is expected, you get type["a"]instead.Before #55229,
["A", "B"] as const satisfies T[]would just fail whereas{ A: "foo", B: "bar" } as const satisfies Record<T, string>would succeed. Now they both succeed and you get what you asked for.Sector6759 commentedon Sep 15, 2026
Thanks a lot for the explanation.