type inference issue with dynamic template literals

[playerx]

🔎 Search Terms

"template literals and type inference", "type inference with dynamic template literals"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about it

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/C4TwDgpgBAgmCWUC8UDeAoKUDaBrCIAXFAAYAWEANpQPYB0AJKgM7ABO8AdgOYC+JAXWIAKACYBDYOOKooncQFsIxVhx5ReASmQA+KADca8Uel7p0lCMCgBXZhDYBJUSvZduyKAHJRIeQvgAYyhVdy9zAHoIqABRAA9FMEsARmIAERoIZk4vawB3GjZcOihQSCgAM0LSCBIoQPEc6wAjaC4KhzYIE0CaTlZSrOBU2ARPDCxscipaRlQ7B2d+IShoJD0JrHq+5hpLOlpuYQg6fwhNTA0AGlNI6PjEywAmYgB1QtxmdF7+62Ahl6jRAoTZTCjUeisSRBAD6oR4gmIaw2lywP12+0Ox1OinOl14NzM6CisQSCiSEAAzG8Pl90dY+p4ADwAaVWcX+nFEzCg+BANAqQJ0wkufOILJuWHEgWA8D6IgkUmIcHg2BZAm06wMRlEN01KKwJN2SighyCt3QfWE0whcwWTlE-Cuq10aEu6L2JyxJzOFy0QA

💻 Code

type Api = {
  [key: `hello.${string}`]: (data: { name: string }) => void
}

let userId: string = 'dynamic string'

// Example1: Doesn't work. type for `e` can't be inferred
const test1: Api = {
  [`hello.${userId}`]: e => {
    console.log(e.name)
  },
}

// Example2: Works
const test2: Api = {
  [`hello.static_string`]: e => {
    console.log(e.name)
  },
}

// Example3: Works
const on = <K extends keyof Api>(
  key: K,
  action: (data: Api[K]) => void,
) => {
  // some logic
}

on(`hello.${userId}`, e => {
  console.log(e.name)
})

🙁 Actual behavior

Example 1 doesn't work, while Example 2 and Example 3 works properly. The type for e can't be inferred

🙂 Expected behavior

To infer type of e in the Example 1.

Additional information about the issue

No response

[Andarist]

In a sense, this could be seen as a duplicate of https://github.com/microsoft/TypeScript/issues/13948 . Like I mentioned here the computed properties like this are always widened - without considering the contextual property keys.

A somewhat related thing to this is that those assignments work because index signatures turn off common property checking:

const obj: { [key: `hello.${string}`]: string } = {} as {
  [key: string]: string;
};

This is OK but it would - IMHO - make sense for this to participate in the common property check. You can see a different issue related to this: https://github.com/microsoft/TypeScript/issues/55709

[Xriuk]

Not sure if it's related to this but I'm facing a similar problem:

type Original = `/AnyString/${number}`;
type Match = Original extends `${infer Head}/${number}${infer Tail}` ? {
    a: Head,
    b: Tail
} : never;

Here Match is never because apparently it matches /AnyString as /${number} and then fails because AnyString is not a number.