ts学习笔记

学习好文

interface

1. 在接口中定义方法
   定义传入参数和返回的类型

interface Counter {
    (start: number): string;
    interval: number;
    reset(): void;
}

2. interface的合并
   与类型别名不同，接口可以定义多次，会被自动合并为单个接口。

interface Point { x: number; }
interface Point { y: number; }

const point: Point = { x: 1, y: 2 };

断言

用<>或者as

let someValue: any = "this is a string";

let strLength: number = (<string>someValue).length;
let strLength: number = (someValue as string).length;

在jsx里只能使用as语法

readonly修饰符

表示属性只读，无法被赋值并且无法赋给其他变量：

let a: number[] = [1, 2, 3, 4];
let ro: ReadonlyArray<number> = a;
ro[0] = 12; // error!
ro.push(5); // error!
ro.length = 100; // error!
a = ro; // error!

可以利用断言重新赋值：

a = ro as number[];

类的readonly属性，只能在声明时或者构造函数里被初始化

class Octopus {
    readonly name: string;
    readonly numberOfLegs: number = 8;
    constructor (theName: string) {
        this.name = theName;
    }
}
let dad = new Octopus("Man with the 8 strong legs");
dad.name = "Man with the 3-piece suit"; // 错误! name 是只读的.

protected修饰符

protected成员在派生类中仍然可以访问。
构造函数也可以被标记成 protected。 这意味着这个类不能在包含它的类外被实例化，但是能被继承。比如

// Employee 能够继承 Person
class Employee extends Person {
    private department: string;

    constructor(name: string, department: string) {
        super(name);
        this.department = department;
    }

    public getElevatorPitch() {
        return `Hello, my name is ${this.name} and I work in ${this.department}.`;
    }
}

let howard = new Employee("Howard", "Sales");
let john = new Person("John"); // 错误: 'Person' 的构造函数是被保护的.

枚举

枚举类型与数字类型兼容，并且数字类型与枚举类型兼容。不同枚举类型之间是不兼容的。比如，

enum Status { Ready, Waiting };
enum Color { Red, Blue, Green };

let status = Status.Ready;
status = Color.Green;  // Error