How to remove index signature using mapped types

Edit: Since Typescript 4.1 there is a way of doing this directly with Key Remapping, avoiding the Pick combinator. Please see the answer by Oleg where it’s introduced.

type RemoveIndex<T> = {
  [ K in keyof T as
    string extends K
      ? never
      : number extends K
        ? never
        : symbol extends K
          ? never
          : K
  ]: T[K];
};

It is based on the fact that 'a' extends string but string doesn’t extends 'a'.

There is also a way to express that with TypeScript 2.8’s Conditional Types.

interface Foo {
  [key: string]: any;
  [key: number]: any;
  [key: symbol]: any;
  bar(): void;
}

type KnownKeys<T> = {
  [K in keyof T]:
    string extends K
      ? never 
      : number extends K 
        ? never
        : symbol extends K 
          ? never
          : K
} extends { [_ in keyof T]: infer U } ? U : never;


type FooWithOnlyBar = Pick<Foo, KnownKeys<Foo>>;

You can make a generic out of that:

// Generic !!!
type RemoveIndex<T extends Record<any,any>> = Pick<T, KnownKeys<T>>;

type FooWithOnlyBar = RemoveIndex<Foo>;

For an explanation of why exactly KnownKeys<T> works, see the following answer:

https://stackoverflow.com/a/51955852/2115619