Why Does TypeScript Mapped Type Return an Array for Tuples Instead of an Object? [duplicate]

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type T1 is Tuple, T2 is Object. but I think their implementation methods are the same.

type Tup = ['a','b']
 
 
 type Options <T>  = {
  [K in keyof T]: 1
 }
type T1 = Options<Tup>   //  type T1 = [1, 1]

type keys1 = keyof Tup
type T2 = {
  [K in keys1] :1
}
/**
 * type T2 = {
    [x: number]: 1;
    [Symbol.iterator]: 1;
    0: 1;
    1: 1;
    [Symbol.unscopables]: 1;
    length: 1;
    ...
    }
 */

I want to know how TypeScript handles these two types separately, and what is the reason for this

  • In T1, TypeScript knows that T is a tuple, so the result should be a tuple. In T2, TypeScript only has the keys to work with and doesn’t know that it is supposed to be a tuple.

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  • It has to do with whether the mapped type is homomorphic. A homomorphic mapped generic like type F<T> = {[K in keyof T]: ⋯} will map tuples to tuples, but a non-homomorphic type will not. See the answers to the linked questions for more information.

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