The following figure defines a type Data that can contain a single field pointing to arbitrary Data:

https://stackoverflow.com/questions/58090665/typescript-what-is-the-type-of-the-object-name-string-string

    class aa{
      constructor(headers?: string | { [name: string]: string | string[]; }){
        
      }
    }

Meaning:
The headers type can be one of two types:

string An object with a field pointing to a string or an array of strings. The name of the field doesn’t matter, there’s no restriction, as long as the field is a string.

More of Jerry’s original articles can be found at :