TypeScript函数和类型断言实例详解

function demo(message:string|undefined|null) {
    const str: string = message
}
demo(undefined)

let message:string
function demo(message:string|undefined|null) {
    const str: string = message!
}
demo(undefined)

let message: any = "scc"
let mesLength:number = (<string>message).length

let message: any = "scc"
let mesLength:number = (message as string).length

let message!:string

function demo(message: string | number | boolean) {
    if (typeof message === "string") {
        message.toUpperCase()
    }
}

interface Person {
    name: string
    age:number
}
interface Dog {
    name: string
    walk:string
}
type x = Dog|Person
function all(emp: x) {
    if ("walk" in emp) {
        console.log(d.walk);
    }
}

function demo(num:number,str?:string){
}

function demo(num:number,bol:boolean = true,str?:string){ 
}

function add(a1: number, a2: number):number
function add(a1: string, a2: string):string
function add(a1: any, a2: any):any {
        return a1 + a2
}
console.log(add(20,30));
console.log(add("scc","brd"));