class BaseClass {
    constructor(cfg) {

    // Transfer all properties to class object
    Object.assign(this, cfg);

    if(this.el instanceof HTMLElement === false) {
        return;
    }
    this.setup();
}

setup() {
    this.isSetup = true;
}

class MyClass extends BaseClass {
   setup() {
    // Calling super
    super.setup();

    this.show(); **// this.show is not a function**
   }
   show = () => {
     console.log('called');
   }
}