TS 中 never,void,unknown类型详解

 function foo(): never {//永远不会返回结果 // 死循环 while(true) {}
 }
function bar(): never {throw new Error()
 } 

function foo(x: string | number): boolean {if (typeof x === 'string') {return true;} else if (typeof x === 'number') {return false;}// 如果不是一个 never 类型，这会报错：// - 不是所有条件都有返回值 （严格模式下）// - 或者检查到无法访问的代码// 但是由于 TypeScript 理解 `fail` 函数返回为 `never` 类型// 它可以让你调用它，因为你可能会在运行时用它来做安全或者详细的检查。return fail('Unexhaustive');
}
function fail(message: string): never {throw new Error(message);
} 

// 没有返回值的函数，其返回值类型为 void
function alertName(): void { alert('My name is Tom'); } 

let unusable: void = undefined; 

// 如果什么都不写，此时，add 函数的返回值类型为： void
const add = () => { //
}
// 如果return之后什么都不写，此时，add 函数的返回值类型为： void
const add = () => {  return 
}
const add = (): void => {// 此处，返回的 undefined 是 JS 中的一个值return undefined
}
// 这种写法是明确指定函数返回值类型为 void，与上面不指定返回值类型相同
const add = (): void => {// 
} 

function foo() {return 'abc'
}
function bar() {return 123
}
// unknown类型只能赋值给any和unknown类型
// any类型可以赋值给任意类型
let flag = true
let result: unknown // 最好不要使用any
if (flag) {result = foo()
} else {result = bar()
}
let message: string = result //报错
let num: number = result //报错
let num1: unknown = result
let num2: any = result
console.log(result) 

let value: unknown = 123;
console.log(++value )//error:'value' is of type 'unknown'.
console.log(++(value as number) ) 

let value: unknown;
value = 123;
value = "abc";
value = false;

let value1: unknown = 123;
let value2: number;
value2 = value1;

let value1: unknown = 123;
let value2: number;
value2 = value1 as number;

let value1: unknown = 123;
let value2: number;
if (typeof value1 === 'number') {
    value2 = value1;
}

let value1: unknown = 123;
value1++;

let value1: unknown = 123;
(value1 as number)++;

let value1: unknown = 123;
if (typeof value1 === 'number') {
    value1++;
}

let value1: unknown = 123;
let value2: unknown = 123;
console.log(value1 === value2);
console.log(value1 !== value2);

let value1: unknown = 123;
let value2: unknown = 123;
console.log(value1 >= value2);

type MyType = number & unknown;

type MyType = unknown & string;

type MyType1 = unknown | any;
type MyType2 = unknown | number;
type MyType3 = unknown | string | boolean;

type MyType = never extends unknown ? true : false;

type MyType = keyof unknown;

class="language-typescript">class Person {
    name: string = 'BNTang';
    say(): void {
        console.log(`name = ${this.name}`);
    }
}
let p: unknown = new Person();
p.say();
console.log(p.name);

type MyType<T> = {
    [P in keyof T]: any
}
type res = MyType<unknown>