typeof returns the template instead of the actual string

[esthedebeste]

Bug Report

🔎 Search Terms

typeof key, typeof template, template key

⏯ Playground Link

`number_${string}` cannot be assigned to "number_three", even though that will be the value.

💻 Code

type Numbers = {
	[numberToParse: `number_${string}`]: (num: typeof numberToParse) => number;
};
const a: Numbers = {
	// Because of the `Numbers` type, typescript
	// should know that `num` is "number_three".
	// Instead, `num` is said to be a template,
	// So number_three fails to compile.
	number_three(num: "number_three") {
		return 3;
	},
};

A shorter example:

function hi(name: string): `Hi, ${typeof name}!`;
hi("World"); // Expected: "Hi, World!", got `Hi, ${string}!`

Though this could simply use generics

function hi<U extends string>(name: U): `Hi, ${U}!`;
hi("World"); // ✔️ "Hi, World!"

🙁 Actual behavior

The num variable is annotated "number_${string}"

🙂 Expected behavior

The num variable is annotated "number_three"

[MartinJohns]

[numberToParse: number_${string}]: (num: typeof numberToParse) => number;

But the typeof numberToParse is always number_${string}. It doesn't change.

[esthedebeste]

then how can I access the key as a string, aside from using typeof key?

[DanielRosenwasser]

You would need a way to encode type parameters on index signatures, or some similar feature, which is discussed a bit over at #22509. Right now, we don't have any way of saying that an index signature is generic.

[typescript-bot]

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