Unable to declare an interface that extends `Record<string, nonAnyType>` with additional support functions. about typescript HOT 7 CLOSED

I'd consider this a duplicate of #17867. There is no "very good solution" here right now, just various workarounds.

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@jcalz It should not be hard to add a fix, since the methods inherited from the Object interface can already be addressed without any errors:

interface K {
	[k: string]: string;
}
let d : K = {};
d.toString(); // works fine, no type errors

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Exclude<> is used to remove types from a union type. string is not a union type, so you can't remove the literal type "get" from it. So Exclude<string, "get"> is the same as string. See #47178 and many more.

The type you're trying to create can't be represented in TypeScript. This would require negated types: #4196

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If Exclude is not valid, how can I declare methods within an interface without making the value of the index accessor any?

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The closest you can get is using an intersection type:
type RawHeaders = { [k: string]: string } & { get(key: string): string[] }

But be aware that this is an unsound type. It's still possible to access the index signature with a key "get" and assign a string to it.

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This is not a very good solution as the type is not extendable by another interface, and is also not implementable by a class, which is needed in my project.

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Still belongs in #17867

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