CODEWITHSUNDEEP
javascripttypescriptclassoopinheritance
Exis Zhang
Exis Zhang

Reputation: 560

Javascript base class constructor run with subclass (overridden) property

I am having this problem with subclasses overriding class property in JS. I think the behavior in OOP is what I want, but I wish to achieve the same in JS.

class Base {
   constructor () {
       console.log(this.a)
   }
}

class Child extends Base {
    a = 1
}

new Child()

// undefined, but I expect (want) 1

How do I modify my code to log 1?

Example in Python, that works:

class Base:
    def __init__(self):
        print(self.a)
        
class Child(Base):
    a = 1
    
Child()

# 1 -> desirable

Upvotes: 0

Views: 110

Answers (2)

Exis Zhang
Exis Zhang

Reputation: 560

This is what I ended up doing:

class Base {
   constructor () {
       console.log(this.a)
   }
}

class Child extends Base {
    get a () { return 1 }
}

new Child()

Upvotes: 0

Thunderbolt Engineer
Thunderbolt Engineer

Reputation: 1563

undefined output is expected behavior. You haven't defined the property a in the base class so it is just natural that base class constructor outputs undefined in the console.

class Base {
   constructor () {
       console.log('Base constructor', this.a) // outputs undefined
   }
}

class Child extends Base {
    a = 1

   constructor () {
      super();
      console.log('Child constructor', this.a); // outputs 1
   }
}

new Child()

UPDATE

In Python, __init__ function is not a real constructor. Constructor is __new__.

Upvotes: 3

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