`subClass extends baseClass` and not with `new` keyword in JavaScript [closed]

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I'm new to JavaScript and I'm trying to achieve the logic I have commented in the constructor method of the base class

export default class Building {
  constructor(sqft) {
    this._sqft = sqft;

    // when a subClass extends from this class
    // and does not implement `certainMethod()`, throw an error.
    // this should happen only when `subClass extends baseClass` and not with `new` keyword
    // object creation.
  }
}

What exactly do you mean by "this should happen only when subClass extends baseClass and not with new keyword object creation."? That the class SubBuilding extends Building {} statement itself throws the error? Or that new SubBuilding() call should throw the error, but instantiating new Building() should still be allowed? — Bergi
– Bergi, Commented Aug 3 at 10:09
@Bergi the first two lines of the comments is what i mean by this, new Building() should be allowed. subclassing with extends should throw an error when the subclass hasn't implemented the method in question. — noel-reeds
– noel-reeds, Commented Aug 3 at 10:51

Bergi · Accepted Answer · 2025-08-03 10:56:59Z

1

You can check which concrete (subclass) constructor was invoked using new.target, and exempt the base class itself from the error condition:

export default class Building {
  constructor(sqft) {
    this._sqft = sqft;

    if (new.target != Building && typeof this.certainMethod != 'function') {
      throw new Error('…');
    }
  }
}

answered Aug 3 at 10:56

Bergi

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