Narrow string | number union with utility type

string-keys.type.ts

/*
  If you try to do `keyof SomeType`, and `SomeType` has an interface of `[key: string]: x`
  TypeScript will infer that to a union of `string | number` because JavaScript coerces numeric 
  indexes to strings.  
  
  Example: obj[0] -> obj["0"]
  
  This can make it annoying to try to create an interface for interacting with one of these 
  types when we know we only want to accept string values, or that's a rule we want to enforce.
  
  Below are some examples of how we can narrow the type to only be string values.

  Reference: https://stackoverflow.com/q/51808160/7903952
*/

type MetaDefinition = {
  name?: string;
  content?: string;
} & { 
  [prop: string]: string; 
}

// Approach 1 - Extract<> Utility type
type StringKeyOf<T> = Extract<keyof T, string>;
 
function getTagsBySelector(prop: StringKeyOf<MetaDefinition>): HTMLMetaElement[];
  
// Approach 2 - Create an Intersection type with `&`
function getTagsBySelector(prop: MetaDefinition & string): HTMLMetaElement[];