IsNegativeNumber

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题目

Sometimes when working with numeric literals, we need to rule out (or enforce) that the provided number is a positive integer.

To do that, we first need a way to tell if the number is negative.

Write a type-level function IsNegativeNumber that accepts a number N and returns:

• true if N is negative
• false if N is positive
• false if N is 0,
• never if N is number
• never if N is a union

解答

type IsUnion<T, U = T> = U extends T ? ([T] extends [U] ? false : true) : never;

type IsNegativeNumber<T extends number> = IsUnion<T> extends true
  ? never //union
  : number extends T
  ? never //number
  : `${T}` extends `-${number}`
  ? true
  : false; //negative number