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

Please see the answer to the question this duplicates for what's going on and how to approach this. Let me know if there's any outstanding issue. Good luck! — jcalz
– jcalz, Commented Oct 26, 2021 at 18:31
Thank you a lot for referencing the other answer it helped a lot! — Cristian E.
– Cristian E., Commented Oct 26, 2021 at 20:00

Chase · Accepted Answer · 2021-10-26 14:27:26Z

10

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;
};

answered Oct 26, 2021 at 14:27

Chase

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1 Comment

Cristian E. Over a year ago

Thank you for clean explanation. The referenced SO answer also helps!

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Typescript mapped type - cannot add additional properties [duplicate]

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

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