Is bad practice to create instance of class in constructor with new keyword?

Question

I have class in Javascript and need one object in this class from other class. My question is which is better ?

create instance of class out of class definition or create in constructor ?

sample code:

const b = new B();

class A {
  // and then use b
}
// or 
class A {
 constructor() {
 const b = new B();
 }
}

Answer 1

Whichever class controls the lifetime of the other object(s) creates those objects.

In your example, if A controls the lifetime of that particular instance of B , then it is fine to create B in the constructor.

However, if another class controls the lifetime of (or owns) the object from B , then an instance of B should be passed to A in the constructor.

The test is simple: the class that is responsible for the lifetime of other objects is responsible for creating those objects.

Answer 2

If they are both classes can you just extend the class you need.

class b {}

class a extends b {}