🔎 Search Terms

interface method index accessor
method index type 'string'

🕗 Version & Regression Information

The bug occurs at the latest stable version published on npm. ts-loader 9.5.1 is also used in the project where this problem occurs.
The earlier versions have not been tested, but they are very likely to have the same behavior.

⏯ Playground Link

No response

💻 Code

export interface RawHeaders extends Record<Exclude <string, "get">, string[]> {
	get(key: string): string[];
}

🙁 Actual behavior

The Record<Exclude<string, "somestring">, string[]> type is being treated as a Record<string, string> type, which causes following error message at the line where I declared the get method:

Property 'get' of type '(key: string) => string[]' is not assignable to 'string' index type 'string[]'.

It also shows the same error message at the same line when using an index accessor without extending Record:

export interface RawHeaders {
	[k: Exclude<string, "get">]: string;
	get(key: string): string[];
}

🙂 Expected behavior

It should not show any errors since the key get is excluded when extending the Record interface.

Additional information about the issue

I have not found any valid solutions that would allow a Record<string, nonAnyType> interface to declare additional methods without making the value of the index accessor any, which I believe most developers would not like since it makes the code unchecked, and so is hard to place documentations. This could also confuse new developers when making a library, since it loses the advantages of using TypeScript over plain JS.

Edit:

The solution of using an intersection type works, but I don't think this design to very logical, as in reality not all keys are allocated by a Record like this. Furthermore built-in Object functions accessible, and can coexist with the interface:

interface K {
	[k: string]: string;
}
let d : K = Object.create(null);
d.toString();

whereas custom method declarations can only be a type, which makes it un-extendable by another interface, and un-implementable by a class.