Typescript mapped type - cannot add additional properties [duplicate]

Question

Suppose I have a type definition like this:

Person which must have either name or fullname property defined

type Person = {
  [k in "name" | "fullname"]: string;
}; 

Suppose I want to add one more required property age, intuitively I'd write something like this:

type Person = {
  [k in "name" | "fullname"]: string;
  age: number; // This errors
}; 

However this syntax will not work, the only way is to use intersection operator & like this:

type Person = {
  [k in "name" | "fullname"]: string;
} & { age: number }; 

Am I missing something or intersection is the only way to define this additional property on top of the mapped type?

Playground here

Answer 1

type Person = {
  [k in "name" | "fullname"]: string;
}; 

is a mapped type. It's a separate concept from an object literal type where you specify the name and type of each key.

Perhaps you confused mapped types with index signatures-

type Person = {
  age: number;
  [k: string]: string | number;
}; 

Though, I don't think that's what you want here. You either use mapped types and intersect with another type, or you explicitly write out the mapping-

type Person = {
  age: number;
  name: string;
  fullname: string;
};