VictorMedeiros

function multiply(a, b) {  
  if (b === undefined) {
    return function(b) {
      return a * b;
    }
  }
  return a * b;
}
let double = multiply(2);  
double(3);      // => 6  

VictorMedeiros

let multiply = (a, b) => b === undefined ? b => a * b : a * b;  
let double = multiply(2);  
double(3);      // => 6  
multiply(2, 3); // => 6 

VictorMedeiros

var Message = function(text) {  
  this.text = text;
};
var helloMessage = new Message('Hello World!');  
console.log(helloMessage.text); // => 'Hello World!' 

VictorMedeiros

var Message = (text) => {  
  this.text = text;
};
// Throws "TypeError: Message is not a constructor"
var helloMessage = new Message('Hello World!');  

VictorMedeiros

var button = document.getElementById('myButton');  
button.addEventListener('click', function() {  
  console.log(this === button); // => true
  this.innerHTML = 'Clicked button';
});

VictorMedeiros

var button = document.getElementById('myButton');  
button.addEventListener('click', () => {  
  console.log(this === window); // => true
  this.innerHTML = 'Clicked button';
});

VictorMedeiros

function MyCat(name) {  
  this.catName = name;
}
MyCat.prototype.sayCatName = function() {  
  console.log(this === cat); // => true
  return this.catName;
};
var cat = new MyCat('Mew');  
cat.sayCatName(); // => 'Mew'  

VictorMedeiros

function MyCat(name) {  
  this.catName = name;
}
MyCat.prototype.sayCatName = () => {  
  console.log(this === window); // => true
  return this.catName;
};
var cat = new MyCat('Mew');  
cat.sayCatName(); // => undefined  

VictorMedeiros

var calculate = {  
  array: [1, 2, 3],
  sum() {
    console.log(this === calculate); // => true
    return this.array.reduce((result, item) => result + item);
  }
};
calculate.sum(); // => 6  

VictorMedeiros

var calculate = {  
  array: [1, 2, 3],
  sum: () => {
    console.log(this === window); // => true
    return this.array.reduce((result, item) => result + item);
  }
};
console.log(this === window); // => true  
// Throws "TypeError: Cannot read property 'reduce' of undefined"
calculate.sum();