Remove index signature

[aspirisen]

Is your feature request related to a real problem or use-case?

Remove index signature from T. Some libraries return type with index signatures. For example in yargs

const yargsSchema = yargs(process.argv.slice(2)).options({
  MY: { type: 'string', default: 'SOME' },
})

yargsSchema.argv['MY'] // Ok, and have autocomplete for this property
yargsSchema.argv['UNKNOWN'] // This property doesn't exist, but I still can access it without errors

Describe a solution including usage in code example

There is a proposed solution on StackOverflow https://stackoverflow.com/a/51956054/3021445
Would be nice to have this here as well in utility-types

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

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

Who does this impact? Who is this for?

All TypeScript users