JavaScript 'this' and '=>' arrow function

gistfile1.txt

const Adder = new function() {
  this.sum = 0;
  this.add = function(numbers) {
    numbers.forEach(function(n) {
      this.sum +=n;
    });
  };
}

Adder.add([1, 2, 3]);
console.log(`sum is ${Adder.sum}`); //"sum is 0"


const Adder2 = new function() {
  this.sum = 0;
  this.add = function(numbers) {
    numbers.forEach(n => {
      this.sum +=n;
    });
  };
}

Adder2.add([1, 2, 3]);
console.log(`sum is ${Adder2.sum}`); //"sum is 6"


const Adder3 = new function() {
  this.sum = 0;
  this.add = function(numbers) {
    numbers.forEach(function(n) {
      this.sum +=n;
    }.bind(this));
  };
}

Adder3.add([1, 2, 3]);
console.log(`sum is ${Adder3.sum}`); //"sum is 6"