TS2

//还要注意的是，在构造函数的参数上使用public等同于创建了同名的成员变量。
class Student {
    fullName: string;
    constructor(public firstName, public middleInitial, public lastName) {  //4个成员变量
        this.fullName = firstName + " " + middleInitial + " " + lastName;
    }
}
interface p{
    firstName:string;
    middleName:string;
    lastName:string;
    fullName:string;
}
function a(s:p){
    return "aa" + s.firstName + s.middleName + s.lastName + s.fullName;
}
var s = new Student('fff','mmm','lll');
console.log(a(s));

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元组类型:
var x: [string, number]  = ['hello', 10]; 
var x = ["sss",222];

var x : string|HTMLElement;

class Dog {woof(){}}
class Cat {meow(){}}
var pet: Dog|Cat;
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联合类型;
interface  p{
    name:string;
    age:string | string[] | ( ()=>string );
}
var p1 : p = {name:"sss",age:["sdss","dee"]};
console.log(p1.name + p1.age);
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联合类型|泛型
function cc<T>(x:T,y:T):T{  //申明时不指定参数类型
    return Math.random() > 0.5 ? x : y;
}
console.log( cc<string>('1','3') );  //调用的时候确定参数类型
console.log( cc<string|numnber>(1,'3') );

function ss<T,M>(x:T,y:M):T|M {
    return Math.random() > 0.5 ? x : y;
}
console.log( ss<string,number>("www",22) );
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模版字符串:
var name = "TypeScript";
var greeting  = `Hello, ${name}! Your name has ${name.length} characters`;
当编译目标为ES6之前的版本时，这个字符串被分解为：
var name = "TypeScript!";
var greeting = "Hello, " + name + "! Your name has " + name.length + " characters";
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类型别名:
type PrimitiveArray = Array<string|number|boolean>;
type MyNumber = number;
type NgScope = ng.IScope;
type Callback = () => void;
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const enum（完全嵌入的枚举）
const enum Suit { Clubs, Diamonds, Hearts, Spades }
var d = Suit.Diamonds;

Compiles to exactly:  var d = 1;
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解构声明会引入一个或多个命名变量, 并且初始化它们的值为对象的属性或者数组的元素对应的值.
比如说, 下面的例子声明了变量 x, y 和 z, 并且分别将它们的值初始化为 getSomeObject().x, getSomeObject().x 和 getSomeObject().z;
var { x, y, z } = getSomeObject();

交换2个值 
var x = 1;
var y = 2;
[x, y] = [y, x];
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never类型是任何类型的子类型，也可以赋值给任何类型；然而，没有类型是never的子类型或可以赋值给never类型（除了never本身之外）。 即使 any也不可以赋值给never。
// 返回never的函数必须存在无法达到的终点
function error(message: string): never {
    throw new Error(message);
}
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类型断言好比其它语言里的类型转换，
let someValue: any = "this is a string";
let strLength: number = (<string>someValue).length;
另一个为as语法：
let someValue: any = "this is a string";
let strLength: number = (someValue as string).length;
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函数参数也使用函数作用域。
解构：
数组的解构赋值：
let input = [1, 2];
let [first, second] = input;
console.log(first); // outputs 1
console.log(second); // outputs 2

// swap variables
[first, second] = [second, first];
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function f([x,y]:[number,number]){
    console.log(x);
    console.log(y);
}
let i:[number,number] = [3,4];
f(i);
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