How to convert a tuple type to a union?

Question

How to map a tuple generic type to a union type?

type NeededUnionType<T> = T[keyof T]; // Includes all the Array properties values
  
const value: NeededUnionType<[7, string]> = 2; // This should not be allowed (2 is the tuple length)

Expected type: 7 | string

Answer 1

You can index by number instead of keyof T. keyof T will contain all keys of the tuple object which includes the length as well as any other array properties.

type NeededUnionType<T extends any[]> = T[number]; // Includes all the Array properties values

const value: NeededUnionType<[7, string]> = 2; // err value is 7 | string 

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