1.myInterface.js

const myInterface = Symbol('myInterface');
 
export default myInterface;

2.Foo.js

import myInterface from 'myInterface';
 
class Foo {
    constructor(value) {
        this.value = value;
    }
 
    _inc () {
        this.value = this.value + 1;
    }
 
    _getValue () {
        return this.value;
    }
 
    get [myInterface]() {
        return {
            inc: this._inc,
            get value() {
               return this._getValue();
            }
        };
    }
}
 
export default A;

3.usage.js

import myInterface from 'myInterface';
import Foo from 'foo';

function doInc(obj){
  const interface = obj[myInterface];

  if (interface === undefined) {
    throw new Error('Object does not support the interface.');
  }
  interface.inc();
}

const counter = new Foo(0);

doInc(counter);