using { [key]: value } doesn't resolve type properly when using union types.

[danielFHcode]

🔎 Search Terms

"[key]", "key", "dynamic key"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about the type inference of { [key]: value } objects.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-beta#code/PQKgsAUASgpgzgewDYDd4AIAuCBckDek6x6AHjunJgE4CWAdgOYDckAvqxCMJAMYL0qZdAF50hCCXQBtAOSlZ6AIZx082QF0KsgHR7Z7SJFCRYiVBmx4IEqdPKUaDRlsd0mnNpACSgzDCUAE3QEADMsAAsYdBhSAAcYXn9A61sSe3QGNQUAH1kAT00KKndGQy4eCH4-dHzRcSJ09WVVdTzC1119dmYgA

💻 Code

/*
Resolves to:
{
    x: string;
};
*/
const x = {
    ['x' as 'x']: '...'
}

/*
Resolves to:
{
    [x: string]: string;
}
Instead of the expected:
{
    [x in 'x'|'y']: string
}
*/
const y = {
    ['x' as 'x'|'y']: '...'
};

🙁 Actual behavior

In the first example ts works as expected, resolving the key with type 'x' to have a type of 'x'.
But in the second example, ts resolves the key with type 'x'|'y' to have a type of string.

🙂 Expected behavior

The expected behaviour was for the second example to work similarly to the first one, so that the key with type 'x'|'y' resolves to type 'x'|'y'.

Additional information about the issue

This has cause errors which seem to only be fixed with the as keyword - which I prefer not to use.

[jcalz]

Duplicate of #13948, and note that [K in 'x'|'y']: string is equivalent to {x: string; y: string} and therefore not the type you actually want.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.