Nodejs: extends class and change parent variable per child classes-CodePudding

This is my classes:

export class Parent {
  protected static name: string;

  public getName() {
    return Parent.name
  }
}

export class Child1 extends Parent {
  constructor() {
    super()
    if (!Child1.name) {
      // connect to database for get names
      Child1.name = '1';
    }
  }
}

export class Child2 extends Parent {
  constructor() {
    super()
    if (!Child2.name) {
      // connect to database for get names
      Child2.name = '2';
    }
  }
}

I run this code:

let child1 = new Child1()
let child2 = new Child2()

console.log(child1.getName())
console.log(child2.getName())

And I get this result:

undefined
undefined

But I get this result:

1
2

I want to connect to database and get names, so per new class I dont want to connect to database again.

CodePudding user response：

Parent.name will always access the name property of Parent. If you want to make it conditional on which instance the function is called on you have to use this.constructor.name instead:

public getName() {
  return this.constructor.name
}

this.constructor refers to the object's constructor function / class.

class Parent {

  getName() {
    return this.constructor.db
    //     ^^^^^^^^^^^^^^^^
  }
}

class Child1 extends Parent {
  constructor() {
    super()
    if (!Child1.db) {
      // connect to database for get names
      Child1.db = '1';
    }
  }
}

class Child2 extends Parent {
  constructor() {
    super()
    if (!Child2.db) {
      // connect to database for get names
      Child2.db = '2';
    }
  }
}

let child1 = new Child1()
let child2 = new Child2()

console.log(child1.getName())
console.log(child2.getName())

CodePudding user response：

The problem is static members are bound to the class and can not be referenced via an instance.

Use it like this:

class Parent {
  protected static name: string;

  public getName() {
    return Parent.name
  }
}

class Child1 extends Parent {
  constructor() {
    super()
    if (!Parent.name) {
      Parent.name = '1';
    }
  }
}

class Child2 extends Parent {
  constructor() {
    super()
    if (!Parent.name) {
      // connect to database for get names
      Parent.name = '2';
    }
  }
}

let child1 = new Child1();
let child2 = new Child2();

console.log(child1.getName());
console.log(child2.getName());

CodePudding user response：

export class Parent {
    protected namE: string;

    public getName() {
        return this.namE
    }
    
}

export class Child1 extends Parent {
    constructor() {
        super()
        if (!this.namE) {
            // connect to database for get namEs
            this.namE = '1';
        }
    }
}

export class Child2 extends Parent {
    constructor() {
        super()
        if (!this.namE) {
            // connect to database for get namEs
            this.namE = '2';
        }
    }
}




    let child1 = new Child1()
    let child2 = new Child2()

    console.log(child1.getName())
    console.log(child2.getName())

OutPut:

1
2

Why don't you do it this way?