Get parent class name from child with ES6?

ES6 classes inherit from each other. So when instance.constructor refers to the Child, then you can use Object.getPrototypeOf(instance.constructor) to get the Parent, and then access .name:

Object.getPrototypeOf(instance.constructor).name // == "Parent"

Of course, full ES6 compliance and non-minified code are necessary for this to work. You should never rely on function names in code.

Here is something amusing:

class J {}
    
class K extends J {}

class L extends K {}

function getBaseClass(targetClass){
  if(targetClass instanceof Function){
    let baseClass = targetClass;

    while (baseClass){
      const newBaseClass = Object.getPrototypeOf(baseClass);

      if(newBaseClass && newBaseClass !== Object && newBaseClass.name){
        baseClass = newBaseClass;
      }else{
        break;
      }
    }

    return baseClass;
  }
}

console.log(getBaseClass(L)); // Will return J.

You could technically do

// instanceProto === Child.prototype
var instanceProto = Object.getPrototypeOf(instance);

// parentProto === Parent.prototype
var parentProto = Object.getPrototypeOf(instanceProto);
console.log(parentProto.constructor.name);

keep in mind that these names may all be mangled by minifiers.