TypeScript学习笔记

枚举

enum Color {Red = 1, Green, Blue}
let colorName: string = Color[2];

console.log(colorName);  // 显示'Green'因为上面代码里它的值是2

编译后

var Color;
(function (Color) {
    Color[Color["Red"] = 1] = "Red";
    Color[Color["Green"] = 2] = "Green";
    Color[Color["Blue"] = 3] = "Blue";
})(Color || (Color = {}));
var colorName = Color[2];
console.log(colorName);

此时Color对象内部结构为

image.png

所以Color[2]打印的结果会为Green

ps:解释赋值原理

var a
console.log(a="abc") //打印的结果为abc
//所以
Color[Color["Red"] = 1] = "Red";
//Color["Red"] = 1赋值完成后会演变成
Color[1] = "Red";
//所以最终枚举对象的结构才会如上图所示

private 和 protected 的共同的点以及不同点

共同点:

class Person {
    protected name: string;
    constructor(name: string) { this.name = name; }
}
let van= new Person("Van")
console.log(van.name)
//这时候编译会发生错误
class Animal{
    private name: string;
    constructor(name: string) { this.name = name; }
}
let dolphin= new Animal("Dolphin")
console.log(dolphin.name)
//这时也会出现编译错误的问题
//证明:不管是protected 或是 private 描述的成员都不可以在类外进行调用

不同点:

class Person {
    protected name: string;
    constructor(name: string) { this.name = name; }
}

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");
console.log(howard.getElevatorPitch());
//此时将ts文件编译,不会有任何问题

如果将父类(超类)的name属性前置修饰符从protected 修改成 private 如:

class Person {
    private name: string;
    constructor(name: string) { this.name = name; }
}

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");
console.log(howard.getElevatorPitch());
//那么这时候编译该ts文件就会报错
//结论:如果父类(超类)的成员修饰符为protected 那么该成员可以在子类中被调用(如:子类中的getElevatorPitch方法)
//而如果父类(超类)的成员修饰符为private 那么该成员将不可以在子类中被调用