type 类型别名

类型别名用于给已有类型起一个更易复用的名字，适合联合类型、对象类型、元组类型和函数类型。

type Status = 'idle' | 'loading' | 'success' | 'error'

let s1: Status = 'idle'
let s2: Status = 'success'

type Person = {
  name: string
  age?: number
  say(): void
  greet?(name: string): void
}

const person: Person = {
  name: 'foo',
  say() {},
}