# Ts 使用指南
## 6、參數類型和返回類型
```ts
// 參數類型和返回類型
function createUserId(name: string, id: number): string { // fn xx(a:參數類型): 返回類型 {}
? return name + id;
}
// 只讀屬性
interface Person {
? readonly name: string;
? age?: number;
}
// 任意屬性
interface Person {
? name: string;
? age?: number;
? [propName: string]: any;
}
// 方法類型
// 沒有返回值
type SetPoint = (x: number, y: number) => void;
interface SetPoint {
? (x: number, y: number): void;
}
// 有返回值
type SetPoint1 = (x: number, y: number) => string;
interface SetPoint1 {
? (x: number, y: number): string;
}
// 泛型語法
function identity <T, U>(value: T, message: U) : T {
? console.log(message);
? return value;
}
console.log(identity<Number, string>(68, "Semlinker"));
```
## 5、聯合類型
? * 聯合類型
```ts
// 聯合類型通常與 null 或 undefined 一起使用：
const sayHello = (name: string | undefined) => {
? /* ... */
};
// 用來約束取值只能是某幾個值中的一個
let num: 1 | 2 = 1;
type EventNames = 'click' | 'scroll' | 'mousemove';
// 類型聯合
interface Motorcycle {
? vType: "motorcycle"; // discriminant
? make: number; // year
}
interface Car {
? vType: "car"; // discriminant
? transmission: CarTransmission
}
interface Truck {
? vType: "truck"; // discriminant
? capacity: number; // in tons
}
type Vehicle = Motorcycle | Car | Truck;
```
## 4、類型守衛
? * 1、自定義類型保護的類型謂詞
```ts
function isNumber(x: any): x is number {
? return typeof x === "number";
}
function isString(x: any): x is string {
? return typeof x === "string";
}
```
## 3、確定賦值斷言
```ts
let x: number;
initialize();
// Variable 'x' is used before being assigned.(2454)
console.log(2 * x); // Error
function initialize() {
? x = 10;
}
```
```ts
let x!: number;
initialize();
console.log(2 * x); // Ok
function initialize() {
? x = 10;
}
```
## 2、非空斷言
? * 1、忽略 undefined 和 null 類型
```ts
function myFunc(maybeString: string | undefined | null) {
? const ignoreUndefinedAndNull: string = maybeString!; // Ok
}
```
? * 2、調用函數時忽略 undefined 類型
```ts
type NumGenerator = () => number;
function myFunc(numGenerator: NumGenerator | undefined) {
? const num2 = numGenerator!(); //OK
}
```
## 1、類型斷言
? * 1.“尖括號” 語法
```ts
let someValue: any = "this is a string";
let strLength: number = (<string>someValue).length;
```
? * 2.as 語法
```ts
let someValue: any = "this is a string";
let strLength: number = (someValue as string).length;
```
? * 3、變量類型
```ts
const many: any = $100; // 不確定類型用any
const arr: Array<number> = [1, 2];
const arr: number[] = [1, 2];
const arr: any[] = [1, false, 'yellow'];
const arr: object = {name:'張三', sex: '男'};
```
-end-
