一起来学习TypeScript的类型

let 变量: 类型;
let 变量: 类型 = 值;
function fn(参数: 类型, 参数: 类型): 类型{
    ...
}

//先声明类型再赋值
let a: string;
a = 'Hello';
//声明类型并赋值
let b: number = 123;
//声明变量和赋值同时进行
let c = false; 
c = true; //编译器自动判断变量c为boolean

let a: number;
a = 10;
a = 33;
//a = 'hello'; 变量a的类型是number, 不能赋值字符串

let b: string;
b = 'hello';
//b = 123; 变量b的类型是字符串, 不能赋值number

let c: boolean;
c = true;

let d: 10; // 此时d只能等于10
//let d = 11; 则会报错，限制了该变量就是字面量的值（有点类似常量）

let A: boolean | string; // 变量A既可以是布尔类型也可以是字符串类型
A = true;
A = "Hello";
A = false;
A = "World";
// 字面量的联合类型限制变量B只能是"male"或"female"
let B: "male" | "female";
B = "male";
B = "female";
// B = "trans" 错误

let a: any;
a = true;
a = "hi";
a = 123;
// 如果声明变量不指定类型，则TS解析器会自动判断变量的类型为any
let b;
b = true;
b = "hi";
b = 123;

// any 任意类型，相当于关闭了ts对类型的限制(尽量不要用)
let B: any;
B = 10;
B = 'string';
B = true;
let S: string;
S = B; // any类型可以赋值给任意类型变量，容易出现问题。
 

let a: unknown;
a = 10;
a = "string";
a = true;
let b: string;
// b = a; 赋值时编译器会报错，不能将“unknown”类型分配给“string”。

let a: unknown;
a = 10;
a = true;
a = "string";
let b: string;
/*
*  断言：
*    1. 变量 as 类型
*    2. <类型> 变量
*/
b = a as string;
b = <string> a;
/*
*  赋值前类型判断
*/
if (typeof c === 'string'){
    b = a;
}

function foo(): void {
}
console.log(typeof foo()); // => undefined
function foo1(): void{
    return undefined;
}
function foo3(){
    // 不写void默认为void
}
// 写了返回值不写返回值类型 => 自动判断返回值类型
function foo4(){
    return 1;
}
console.log(typeof foo4()); // => number

function foo(): never{
    throw new Error('raise');
}

let o: {name: string, age: number};
o = {
    name: "Edison",
    age: 25
}
// 定义可选参数
// [propName: string]: any 任意类型的属性
let a: {name: string, [propName: string]: any};
b = {
    name: 'Edison',
    age: 24,
    gender: "female"
}
// 定义函数结构
let foo: (a: number, b: number) => number;
foo = function (n1, n2) {
    return n1 + n2;
}

/*
*  元组： 长度和类型还有顺序必须与储存元素一一对应
*    [类型，类型]
*/
let a = [string, number];
a = ["hello", 123];
//以下报错
a = [123, "hello"];
a = [123];
a = ["hello"];
a = ["hello", 123, 456];

/*
*  元组： 长度和类型还有顺序必须与储存元素一一对应
*    [类型，类型]
*/
let a = [string, number];
a = ["hello", 123];
//以下报错
a = [123, "hello"];
a = [123];
a = ["hello"];
a = ["hello", 123, 456];

enum Gender {
    Male = 0,
    Female = 1
}
let person: {name: string, gender: Gender};
person = {
    name: "Edison",
    gender: Gender.Male
}

// & 同时满足
let i: {name: string} & {age: number} & {gender: Gender};
i = person;
// i = b; Error 
// 类型别名
type myType = typeof i;
let person1: myType;
let person2: myType;