1. function join<T>(first: T, second: T) {
    2.     return `${first}${second}`
    3. }
    5. join<number>(1,1)
    7. function map<T>(params: <T>[]) {
    8.     return params
    9. }
    11. map<string>(['123'])
    13. function join<T, P>(first: T, second: P) {
    14.     return `${first}${second}`
    15. }
    16. join<number, string>(1, '1') 
    17. join(1,'1') //类型推断
    19. function anotherJoin<T>(first: T, second: T): T {
    20.     return first
    21. }