var Multipler = function(inc){

  this.inc = inc;

}

// Not gonna work

Multipler.prototype.multiple = function(numbers){

  return numbers.map(function(number){

    return this.inc * number;

  })

} 

// => [NaN, NaN, NaN, NaN]

console.log(new Multipler(2).multiple([1,2,3,4]))

​

// cache this or using bind

Multipler.prototype.multiple = function(numbers){

  var self = this;

  return numbers.map(function(number){

    return self.inc * number;

  })//or  .bind(this)

}

//=> [2, 4, 6, 8]

console.log(new Multipler(2).multiple([1,2,3,4]))

​

// BUT with arrow function, this will be lexical

Multipler.prototype.multiple = function(numbers){

  return numbers.map((number) => number*this.inc);

};

​

console.log(new Multipler(2).multiple([1,2,3,4]));// => [ 2, 4, 6, 8 ]

​

​