let 變量: 類型;
let 變量: 類型 = 值;
function fn(參數(shù): 類型, 參數(shù): 類型): 類型{
    ...
}

//先聲明類型再賦值
let a: string;
a = 'Hello';
//聲明類型并賦值
let b: number = 123;
//聲明變量和賦值同時進行
let c = false; 
c = true; //編譯器自動判斷變量c為boolean

let a: number;
a = 10;
a = 33;
//a = 'hello'; 變量a的類型是number, 不能賦值字符串

let b: string;
b = 'hello';
//b = 123; 變量b的類型是字符串, 不能賦值number

let c: boolean;
c = true;

let d: 10; // 此時d只能等于10
//let d = 11; 則會報錯，限制了該變量就是字面量的值（有點類似常量）

let A: boolean | string; // 變量A既可以是布爾類型也可以是字符串類型
A = true;
A = "Hello";
A = false;
A = "World";
// 字面量的聯(lián)合類型限制變量B只能是"male"或"female"
let B: "male" | "female";
B = "male";
B = "female";
// B = "trans" 錯誤

let a: any;
a = true;
a = "hi";
a = 123;
// 如果聲明變量不指定類型，則TS解析器會自動判斷變量的類型為any
let b;
b = true;
b = "hi";
b = 123;

// any 任意類型，相當于關閉了ts對類型的限制(盡量不要用)
let B: any;
B = 10;
B = 'string';
B = true;
let S: string;
S = B; // any類型可以賦值給任意類型變量，容易出現(xiàn)問題。
 

let a: unknown;
a = 10;
a = "string";
a = true;
let b: string;
// b = a; 賦值時編譯器會報錯，不能將“unknown”類型分配給“string”。

let a: unknown;
a = 10;
a = true;
a = "string";
let b: string;
/*
*  斷言：
*    1. 變量 as 類型
*    2. <類型> 變量
*/
b = a as string;
b = <string> a;
/*
*  賦值前類型判斷
*/
if (typeof c === 'string'){
    b = a;
}

function foo(): void {
}
console.log(typeof foo()); // => undefined
function foo1(): void{
    return undefined;
}
function foo3(){
    // 不寫void默認為void
}
// 寫了返回值不寫返回值類型 => 自動判斷返回值類型
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
}
// 定義可選參數(shù)
// [propName: string]: any 任意類型的屬性
let a: {name: string, [propName: string]: any};
b = {
    name: 'Edison',
    age: 24,
    gender: "female"
}
// 定義函數(shù)結(jié)構(gòu)
let foo: (a: number, b: number) => number;
foo = function (n1, n2) {
    return n1 + n2;
}

/*
*  元組： 長度和類型還有順序必須與儲存元素一一對應
*    [類型，類型]
*/
let a = [string, number];
a = ["hello", 123];
//以下報錯
a = [123, "hello"];
a = [123];
a = ["hello"];
a = ["hello", 123, 456];

/*
*  元組： 長度和類型還有順序必須與儲存元素一一對應
*    [類型，類型]
*/
let a = [string, number];
a = ["hello", 123];
//以下報錯
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
}

// & 同時滿足
let i: {name: string} & {age: number} & {gender: Gender};
i = person;
// i = b; Error 
// 類型別名
type myType = typeof i;
let person1: myType;
let person2: myType;