怎么定义TypeScript呢？ TypeScript是一个工具 是一个编译器

1. 编译代码
   TypeScript，通过它的能力，默认使用tsc命令，可以根据.ts为后缀名的文件生成一个新的js文件

2、类型注解
TypeScript里的类型注解是一种轻量级的为函数或变量添加约束的方式。TypeScript提供了静态的代码分析，它可以分析代码结构和提供的类型注解。

function doSth(who: string): string {
  return "I want to say hello to " + who;
}

let who = 'her';

const doWhat = doSth(who);

console.log('doWhat:', doWhat)
// doWhat: I want say hello to her

这里插个题外话，如果部分语法产生error的话，要考虑tsconfig.json这样的配置文件（比如lib部分）是否支持了你需要的Javascript规范，如果不支持的话，建议使用polyfill

3、接口
允许我们在实现接口时候只要保证包含了接口要求的结构就可以

interface Girl {
  hair: string;
  face: string;
}

function describe(girl: Girl): string {
  let words = 'she has '
  let decs = Object.entries(girl)
  let len = decs.length

  for (let i = 0; i < len; i++) {
    const [where, like] = decs[i];
    words+=(like + ' ' + where)
    words+=(i === len - 1 ? '' : ' and ')
  }
  return words
}

const sentence = describe({
  hair: 'long',
  face: 'beautiful'
})

console.log('sentence:', sentence);
// sentence: she has long hair and beautiful face

4、类
支持基于类的面向对象编程
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 She {
  praise: string;
  constructor(public hair: string, public face: string) {
    // this.praise =  hair + ' hair' + " and " + face + ' face';
  }
}

interface Girl {
  hair: string;
  face: string;
}

function describe(girl: Girl): string {
  let words = 'she has '
  let decs = Object.entries(girl)
  let len = decs.length

  for (let i = 0; i < len; i++) {
    const [where, like] = decs[i];
    words+=(like + ' ' + where)
    words+=(i === len - 1 ? '' : ' and ')
  }
  return words
}

const she = new She('long', 'beautiful')

const sentence = describe(she)

console.log('sentence', sentence);

上面这段代码通过ts编译，成为了下面这段代码

var She = /** @class */ (function () {
    function She(hair, face) {
        this.hair = hair;
        this.face = face;
        // this.praise =  hair + ' hair' + " and " + face + ' face';
    }
    return She;
}());
function describe(girl) {
    var words = 'she has ';
    var decs = Object.entries(girl);
    var len = decs.length;
    for (var i = 0; i < len; i++) {
        var _a = decs[i], where = _a[0], like = _a[1];
        words += (like + ' ' + where);
        words += (i === len - 1 ? '' : ' and ');
    }
    return words;
}
var she = new She('long', 'beautiful');
var sentence = describe(she);
console.log('sentence', sentence);

看起来，只是多了一些特殊的注释，以及修改了一些局部变量名，所以ts主要的功能其实是为了规范书写，校验传入参数是否正确，从而实现更高质量的代码！