学习 Typescript（二）

let numberOrString: number | stringnumberOrString.toString()	// 联合类型的共有属性和方法

// union typeslet numberOrString: number | string
function getLength(input: string | number): number {  const str = input as string  if (str.length) {    return str.length  } else {    const number = input as number    return number.toString().length  }}
//type guardfunction getLength2(input: string | number): number {  if (typeof input === 'string') {    return input.length  } else {    return input.toString().length  }}

// 数字枚举，枚举成员会被赋值为从第一个值（默认为0）开始递增的数字enum Direction {    Up,    Down,    Left,    Right,}console.log(Direction.Up)   // 0console.log(Direction[0])   // Up
// 字符串枚举enum Direction {    Up = 'UP',    Down = 'DOWN',    Left = 'LEFT',    Right = 'RIGHT',}const value = 'UP'if (value === Direction.Up) {    console.log('go up!')}
// 常量枚举，可减少性能const enum Direction {...}

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])

interface IWithLength {  length: number}
function echoWithLength<T extends IWithLength>(arg: T): T {  console.log(arg.length)  return arg}
const str = echoWithLength('str')	// 要有length属性const obj = echoWithLength({ length: 10, width: 10})const arr2 = echoWithLength([1, 2, 3])

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())

interface KeyPair<T, U> {  key: T  value: U }let kp1: KeyPair<number, string> = { key: 1, value: "string"}let kp2: KeyPair<string, number> = { key: 'str', value: 2 }let arr: number[] = [1,2,3]let arrTwo: Array<number> = [1,2,3]		// Array内置泛型