什么是TypeScript类型断言？

当TypeScript推断的类型不符合时，可以通过一定的语法人为进行类型推断即断言

断言的基本语法

1. as
2. {}
3. [ x: string ] : any;

as

看一种错误情形

let b = {}
b.name // Property 'name' does not exist on type '{}'

通过断言避免

let b = {} as { name: string}
b.name 

< T >{}

interface Person {
  name: string
}

let c = <Person>{}

c.name

在function中的应用

  class Shape {}

  class Circle extends Shape {}

  function createShape(kind: string): Shape {
    if (kind === "circle") return new Circle();
    return new Shape()
  }

  var circle = <Circle>createShape("circle"); // circle

[x: string]: any

当声明了一个固定了属性的对象时，如果对象上有不存在的情况实会报错

interface Person {
  name: string
}

let c = {
  name: 'jack',
  age: 11
}

可以通过类型断言避免

  interface PersonAssertion {
    name: string,
    [key: string]: any
  }
  
  let d: PersonAssertion = {
    name: 'jack',
    age: 11
  }

断言的缺点

当使用了断言时，是不会进行提示缺失的属性的

  interface Person {
    name: string
  }
  
  let c: Person = {} // 会提示缺失属性name而类型断言不会

双重类型断言

    
  function handler(event: Event) {
    let element = event as string; // 会报错，需要双重断言
  }

  function handler2(event: Event) {
    let element = event as any as string;
  }