TypeScript: What does {[key: string]: any} means as return type of a function?

Question

I'm currently learning TypeScript and Angular. While reading about Custom validators I came across the following piece of code from https://angular.io/guide/form-validation .

export function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {
  return (control: AbstractControl): {[key: string]: any} | null => {
    const forbidden = nameRe.test(control.value);
    return forbidden ? {'forbiddenName': {value: control.value}} : null;
  };
}

I don't really understand what return type of the inner function ie {[key: string]: any} means? I understand key:string part, ie Object's key is of type string but what exactly {[key: string]: any} means?

Answer 1

It means the function returns an object that you can index into with any string value; the property's result value type is any meaning it can be, well, anything. (The | null means it can also return null instead of returning an actual object.)

Object −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−v−−−−−−−−−−−−−−−−−−v
                                        {[key: string]: any}
Key of all properties is any string −−−−−^^^^^^^^^^^^^  ^^^−−−−− type of all
                                                                 properties is `any`

It's a very broad type.

More in the documentation of index signatures .

Answer 2

It returns an object, like

{
   "name": "John"
}

or

{
   "length": 5
}