8.泛型（generics）

//泛型
function echo<T>(arg: T): T {
    return arg
}
const result = echo(true)function swap<T, U>(tuple: [T, U]): [U, T] {
    return [tuple[1], tuple[0]]
}
const result2 = swap(['string', 123])//约束泛型
interface IWithLength {
    length: number
}
function echoWithLength<T extends IWithLength>(arg: T): T {
    //console.log(arg.length);return arg
}
const str = echoWithLength('str')
const obj = echoWithLength({
     length: 10, width: 10 })
const Arr2 = echoWithLength([1, 2, 3])//泛型 类和接口
class Queue<T> {
    private data = [];push(item: T) {
    return this.data.push(item)}pop(): T {
    return this.data.shift()}
}
const queue = new Queue<number>()
queue.push(1)
console.log(queue.pop().toFixed());const queue2 = new Queue<string>()
queue2.push('str')
console.log(queue2.pop().length)//接口
interface KeyPair<T, U> {
    key: T;value: U;
}
let kp1: KeyPair<number, string> = {
     key: 123, value: 'str' }
let kp2: KeyPair<string, number> = {
     key: 'test', value: 123 }let arr: number[] = [1, 2, 3]
let arrtwo: Array<number> = [1, 2, 3]  //定义数组interface IPlus<T> {
    (a: T, b: T): T
}
function plus(a: number, b: number): number {
    return a + b
}function connect(a: string, b: string): string {
    return a + b
}
const a: IPlus<number> = plus
const b: IPlus<string> = connect