String template literals as object keys don't get validated correctly

[wbolduc]

🔎 Search Terms

String template literal object keys return type function

🕗 Version & Regression Information

• This is the behavior in every version I tried, and there is nothing in the FAQ about it

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/MYewdgzgLgBBCGBbADgGwKYEYYF4YAoAPALjAFdEAjdAJwEpiYAldUGgEwB4ADBFDAPoASAN7kqtAL7cANNBoBLMAHMAfLnX4RAKBh6YAbV5I06YSMLSAuowBEACXSpUIWzO2S627aEiw+pgBMuAQk4tT0jCxsXMb8ZqLhUrLySmoaBDr6MAD0OTBKbDSsUKgAnjAAbvCoCuwwANboZbr6RgBC8PW1ULQ1MKKW3DYwtpRdMOPsbh5ePuDQcCYYAMwhRKQUEQzMrCAcPAGCiVvJclCKKuo4mln6eTBFJeUF7OhgUAoAZgro9VONZqtPS2ZTwGjjZToWx2YoxWyzIA

💻 Code

const sample1 = (x:number): Record<`sample_${number}`,string> => ({
    [`sample_${x}`]: "Hello",
})

const sample2 = (x:number): Record<`sample_${number}`,string> => ({
    // incorrectly valid key
    [`Bad literal ${x}`]: "bad bad",
})

const sample3 = (x:number): Record<`sample_${number}`,string> => ({
    // correctly identified bad key
    "garbage": "record"
})

🙁 Actual behavior

Typescript does not error on incorrect string template literals

🙂 Expected behavior

Typescript should show sample2 as having a bad object key like it does in sample3

Additional information about the issue

Bonus feature request/question: Is this the only way to get this sort of return type from a function. Putting "as const" in the key or after the object doesn't seem to correctly infer the type. It stays wide {[x:string]: string}

[jcalz]

Duplicate of #13948.

(#13948 (comment))

[fatcerberus]

Note that Record<`sample_${number}`, string> doesn’t restrict the type to only those keys, it just guarantees that any properties with keys matching the pattern are typed as string.

[wbolduc]

I mean I guess strictly from the perspective of something receiving the type but TS still checks that you're putting in the correct things when you're initially creating the object

const aa: Record<number, boolean> = {
	[2]: true,
	salami: "sandwich",
};

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAhnAXDASgU1AJwCYB4wCuAtgEZqYA0MJIIANmnGAHwwC8MA3gFACQA2gCYAusiiYCaCnwhw6cIgEtkAIllhsAd0XAAFiukBfANzcgA

[fatcerberus]

Only when assigning the object literal to the type directly:

const bb = {
	[2]: true,
	salami: "sandwich",
};
const aa: Record<number, boolean> = bb;  // ok, no error

The excess property check isn't comprehensive, you shouldn't rely on it to constrain the allowed values of a type. That's why issue #12936 exists.

[typescript-bot]

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