1_Before.js

class ParentClass {
  constructor(controller, item_data, data) {
    Object.assign(this, {
      selectedClass: this.selectedClass || '' // should be set in subclasses
    });
    
    this.initializeItems();
  }
  
  initializeItems() {
    // Does stuff with this.initializeItems
  }
}

class ChildClass {
  constructor(controller, item_data, data) {
    if (!this.selectedClass) {
      this.selectedClass = 'SubclassValue';
    }
    super(controller, item_data, data)
  }
}

1_New.js

class ParentClass {
  constructor(controller, item_data, data, opts) {
    Object.assign(this, {
      selectedClass: '' // should be set in subclasses
    }, opts);
    
    this.initializeItems();
  }
  
  initializeItems() {
    // Does stuff with this.initializeItems
  }
}

class ChildClass {
  constructor(controller, item_data, data) {
    super(controller, item_data, data, {
      selectedClass: 'SubclassValue'
    })
  }
}

2_Before.js

class TypeaheadBase {
  constructor(name, placeholderText) {
    var node = this.getInputNode();
    node.placeholder = placeholderText;
    node.name = name;
  }

  getInputNode() {
    if (!this.inputNode) {
      this.inputNode = document.getElementById('typeahead-node');
    }
    return this.inputNode;
  }
}

class MyFooTypeahead extends TypeaheadBase{
  constructor(name) {
    var inputNode = this.getInputNode(); // nope!
    
    var placeholderText = 
      inputNode.className.indexOf('Blue')
      ? 'I Am Blue'
      : 'I Am Not Blue';
    
    super('Foo', placeholderText);
  }
}

2_New.js

class TypeaheadBase {
  constructor(name, placeholderText) {
    var node = this.getInputNode();
    node.placeholder = placeholderText;
    node.name = name;
  }

  getInputNode() {
    if (!this.inputNode) {
      this.inputNode = document.getElementById('typeahead-node');
    }
    return this.inputNode;
  }
}

class MyFooTypeahead extends TypeaheadBase {
  constructor(name) {
    var inputNode = this.getInputNode();
    
    var placeholderText = 
      inputNode.className.indexOf('Blue')
      ? 'I Am Blue'
      : 'I Am Not Blue';
    
    TypeaheadBase.call(this, 'Foo', placeholderText);
  }
}

3_Before.js

class BaseInputValidator {
  constructor(wrapper, field, position) {
    this.setupListeners();
  }
  
  setupListeners() {
    // default listeners setup
  }
}

class ChildInputValidator extends BaseInputValidator {
  constructor(wrapper, field, position, type) {
    this.domElements = document.querySelectorAll( /* ... */ );
    
    // Need to call this at the end since setupListeners needs
    // this.domElements to be initialized
    super(wrapper, field, position);
  }
  
  setupListeners() {
    this.domElements.forEach(function(el) {
      // do stuff
    });
  }
}

3_New.js

class BaseInputValidator {
  constructor(wrapper, field, position) {
    this.setupListeners();
  }
  
  setupListeners() {
    // default listeners setup
  }
}

class ChildInputValidator extends BaseInputValidator {
  constructor(wrapper, field, position, type) {
    super(wrapper, field, position);
  }
  
  setupListeners() {
    document.querySelectorAll( /* ... */ ).forEach(function(el) {
      // do stuff
    });
  }
}