Does ECMAScript 6 have a convention for abstract classes?

ES2015 does not have Java-style classes with built-in affordances for your desired design pattern. However, it has some options which may be helpful, depending on exactly what you are trying to accomplish.

If you would like a class that cannot be constructed, but whose subclasses can, then you can use new.target:

class Abstract {
  constructor() {
    if (new.target === Abstract) {
      throw new TypeError("Cannot construct Abstract instances directly");
    }
  }
}

class Derived extends Abstract {
  constructor() {
    super();
    // more Derived-specific stuff here, maybe
  }
}

const a = new Abstract(); // new.target is Abstract, so it throws
const b = new Derived(); // new.target is Derived, so no error

For more details on new.target, you may want to read this general overview of how classes in ES2015 work: http://www.2ality.com/2015/02/es6-classes-final.html

If you’re specifically looking for requiring certain methods be implemented, you can check that in the superclass constructor as well:

class Abstract {
  constructor() {
    if (this.method === undefined) {
      // or maybe test typeof this.method === "function"
      throw new TypeError("Must override method");
    }
  }
}

class Derived1 extends Abstract {}

class Derived2 extends Abstract {
  method() {}
}

const a = new Abstract(); // this.method is undefined; error
const b = new Derived1(); // this.method is undefined; error
const c = new Derived2(); // this.method is Derived2.prototype.method; no error