Issue in context - how to solve that?

car.js

function Car(color) {
  this.color = color
}

Car.prototype.getColor = function() {
  return this.color;
}

car = new Car('blue');

function test(callback) {
  console.log(callback());
}

car.getColor() //'blue'
test(car.getColor); //undefined

You can use .bind in the last line. test(car.getColor.bind(car));

If you opt for the revealing pattern as opposed to the prototype pattern you can avoid using this entirely:

function Car(c) {
  var color = c;

  function getColor() {
    return color;
  }
  car.getColor = getColor;

  return car;
}

var car = new Car('blue');

function test(callback) {
  console.log(callback());
}

car.getColor() //'blue'
test(car.getColor); //'blue'

This code would work :

function Car(c) {
  var color = c;

  function getColor() {
    return color;
  }

  return {
    color: color,
    getColor: getColor
  };
}

var car = new Car('blue');

function test(callback) {
  console.log(callback());
}

car.getColor() //'blue'
test(car.getColor); //'blue'

I checked this documentation

But thanks a lot for the hint!