constructor.js

'use strict'
const someFunction = function(){
    return this.value;
}

function Constructor(){ };


Constructor.prototype.es = {
    foobar : 'heisann',
    value : 100
};

Object.defineProperty(Constructor.prototype.es, 'method', {
  get: function() {
    return someFunction.bind(this);
  },
  writeable: true,
  configurable:true
});

module.exports = Constructor;

test.js

const Constructor = require('./constructor');
const instance = new Constructor();
const sinon = require('sinon');

console.log(instance.es.method()) // => 100

// using this won't work:
// sinon.stub(instance.__proto__.es, 'method').returns(42);
// because the getter is returning a _new_ function each time
// therefore you need to attack the actual getter function:

const stub = sinon.stub(instance.__proto__.es, 'method').value(()=>42);
console.log(instance.es.method()) // => 42
stub.get(()=>()=>84);
console.log(instance.es.method()) // => 84
stub.restore();
console.log(instance.es.method()) // => 100

// the above is working on the prototype, can't we do this on the instance?
// yes, we can, but remember that the `es` object is shared, so we
// can avoid modifying it by shadowing it further down the prototype
instance.es = { method: sinon.stub().returns(256) };
console.log(instance.es.method()) // => 256
delete instance.es
console.log(instance.es.method()) // => 100