此范围在构造函数之后的类内

Question

class User{
    constructor(username,email){
        this.username= username,
        this.email = email,
        this.score = 0
    }
    a = [this.email]  // showing undefined 
    login(){
        console.log(`you have logged in ${this.username}`);
        return this;
    }
    logout(){
        console.log(`you have logout ${this.username}`);
        return this;
    }
    incScore() {
        this.score +=1;
        console.log(`The ${this.username} have scored ${this.score}`)
        return this;
    }


}

const userOne = new User('mdvenkatesh','mdv@gmail.com');
console.log(userOne)

// but check it here 


class User{
    constructor(username,email){
        this.username= username,
        this.email = email,
        this.score = 0
    }
    a = [this]  // showing the complete user data  
    login(){
        console.log(`you have logged in ${this.username}`);
        return this;
    }
    logout(){
        console.log(`you have logout ${this.username}`);
        return this;
    }
    incScore() {
        this.score +=1;
        console.log(`The ${this.username} have scored ${this.score}`)
        return this;
    }


}

After my class i declared a array a and intilized with the this keyword o/p

 a: Array(1) 0: User {a: Array(1), username: "mdvenkatesh", email: "mdv@gmail.com", score: 0} length: 1 __proto__: Array(0) email: "mdv@gmail.com" score: 0 

username: "mdvenkatesh"

but when i tryed with user.name it is showing undefined the o/p is following

User {a: Array(1), username: "mdvenkatesh", email: "mdv@gmail.com", score: 0}
a: [undefined]
email: "mdv@gmail.com"
score: 0
username: "mdvenkatesh"
__proto__: Object

i cantunderstand why you may thing why schould even intilize a varable outside method i want to know how a is treated here i am thinking it is proprtey of the class (plese correct me if i am wrong )

2)why this.username is showing undefined when user is show the data plese help me to know

Answer 1

When you do a = [ this.username ] , username is a string and hence its passed via value.

When you do a = [ this ] , this being an object is passes via reference. So any changes made to this will be reflected in a .

---

here i am thinking it is property of the class

Your assumption is correct. But properties are initialized during compile-time and are updated in constructor. This way is used to set default values/ static values.

If you wish to have a property that gets you value of a property, use getter function.

---

Following is the illustration of above points:

• I have created a getter function called userName that returns necessary value.
• I have also moved this.scope = 0 outside constructor as this is a default value.

 class User { score = 0; constructor(username, email) { this.username = username; this.email = email; } get userName() { return this.username; } login() { console.log(`you have logged in ${this.username}`); return this; } logout() { console.log(`you have logout ${this.username}`); return this; } incScore() { this.score += 1; console.log(`The ${this.username} have scored ${this.score}`) return this; } } const user = new User('foo', 'foo@bar.com'); console.log(user.userName) user.incScore(); 

Renerences:

• Modifying a copy of a JavaScript object is causing the original object to change
• Declare a class property outside of a class method
• Classes - MDN

Answer 2

a = [this]  // you declared this to var a then use a after a = [this] use like below
    login(){
        return console.log(`you have logged in ${a.username}`);
    }
and try user.username 
User {a: Array(1), username: "mdvenkatesh", email: "mdv@gmail.com", score: 0}
user.name not find

please check example in below link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes