typescript - 2

补充

# undefined和null都是其他数据类型的子集
let str:string = "ddd";
str = undefined // 不会报错

# 如果不指定类型，那默认是any,后续的任何类型的赋值操作都允许而不会类型检查
let b;
b = 1;
b = "fsss";

# 联合数据类型注意点：其中toString是string和number都拥有的，不然不允许
let demo:string | number;
console.log(demo.toString());

# 剩余参数, 其中剩余参数的类型一定是any，只读属性一旦赋值便不能修改
interface Istate {
    age:number,
	readonly name: string,
    [arg:string]: any
}
let obj1:Istate = {
    age: 10,
    name: '你好'
}

# 数组声明的三种表示方法
- let arr:number [] = [1, 2, 3];
- let arrType: Array<number> = [1, 2, 3];
- interface IArr {
    [index: number]:number
}
interface Istate {
    username: string,
    age: number    
}

let arrType4:IArr = []
let arrType5: Array<Istate> = [{
    username: '张三',
    age: 90
}]
let arrType6: Istate[] = [{ username: '张三', age: 90 }]

# 函数约束
- let funcType1:(name: string, age: number) => number = function (name: string, age:number):number{}
- interface funcType6 {
    (name: string, age:number):number
}
let funcType6: funcType6 = function (name: string, age:number):number{}

# 类型断言不是类型转换，只能断言联合类型里存在的类型