Private parameters in Typescript

问题:

I am learning Angular2 and working with classes in javascript first time.

What does the private parameter and why it couldn't be simply heroService: HeroService ?

constructor(private heroService: HeroService) { }

回答1:

Looks like a parameter property (about halfway down the page). Basically, adding an access modifier (public/private/protected/readonly) to a constructor parameter will automatically assign that parameter to a field of the same name.

Specifically, from those docs:

Parameter properties are declared by prefixing a constructor parameter with an accessibility modifier or readonly, or both. Using private for a parameter property declares and initializes a private member; likewise, the same is done for public, protected, and readonly.

So the following are equivalent:

class Foo {
    private bar: string;
    constructor(bar: string) {
        this.bar = bar;
    }
}

class Foo {
    constructor(private bar: string) {}
}

回答2:

private will scope this variable for this class (function). public will allow access from outside, if you have the instance of this class. protected is important for properties inside a abstract super class. As i started with Typescript, playground on the TypeScript page (https://www.typescriptlang.org/play/index.html) helped me to understand what really happend. Keep in mind, that TypeScript is sugar for your JavaScript

回答3:

Understanding private When a member is marked private, it cannot be accessed from outside of its containing class. For example:

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

new Animal("Cat").name; // Error: 'name' is private;

TypeScript is a structural type system. When we compare two different types, regardless of where they came from, if the types of all members are compatible, then we say the types themselves are compatible.

However, when comparing types that have private and protected members, we treat these types differently. For two types to be considered compatible, if one of them has a private member, then the other must have a private member that originated in the same declaration. The same applies to protected members.

Let’s look at an example to better see how this plays out in practice:

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

class Rhino extends Animal {
    constructor() { super("Rhino"); }
}

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

let animal = new Animal("Goat");
let rhino = new Rhino();
let employee = new Employee("Bob");

animal = rhino;
animal = employee; // Error: 'Animal' and 'Employee' are not compatible

In this example, we have an Animal and a Rhino, with Rhino being a subclass of Animal. We also have a new class Employee that looks identical to Animal in terms of shape. We create some instances of these classes and then try to assign them to each other to see what will happen. Because Animal and Rhino share the private side of their shape from the same declaration of private name: string in Animal, they are compatible. However, this is not the case for Employee. When we try to assign from an Employee to Animal we get an error that these types are not compatible. Even though Employee also has a private member called name, it’s not the one we declared in Animal.

For more details check:

https://www.typescriptlang.org/docs/handbook/classes.html