03. Arrow function 👩‍🏫🧑‍🏫

// All of the `createGreeting` functions are equivalent
var createGreeting = function(message, name) {
  return message + ", " + name + "!";
};

// We can safely swap out function expressions with arrow functions most of the time
var createGreeting = (message, name) => {
  return message + ", " + name + "!";
};

// If the arrow function body contains only one expression, we can omit the curly braces and auto return it
var createGreeting = (message, name) => message + ", " + name + "!";

// If an arrow function only has one parameter, we can omit the parens () around the single parameter
var greet = greeting => console.log(greeting);

// We call arrow functions the same way as we call regular functions
var greeting = createGreeting("Hello", "Angie");

// Logs "Hello, Angie!";
greet(greeting);

// Depending on the environment `setTimeout` is called in, it may refer to one of two objects
// In the browser, `setTimeout` is a property of the `window` object
// In node, it belongs to a special "Timeout" object

var person = {
  name: "Hodor",
  saySomething: function() {
    console.log(this.name + " is thinking...");
    setTimeout(function() {
      console.log(this.name + "!");
    }, 100);
  }
};

person.saySomething(); // prints "Hodor is thinking..."
// prints "undefined!" 100ms later

// Arrow functions bind the `this` keyword to the object it's created inside of
// i.e. whatever `this` is where it's created
var person = {
  name: "Hodor",
  saySomething: function() {
    console.log(this.name + " is thinking...");
    setTimeout(() => console.log(this.name + "!"), 100);
  }
};

person.saySomething(); // "Prints Hodor is thinking..."
// prints `Hodor!` 100ms later

// Avoid using arrow functions for object methods
var dog = {
  name: "Lassie",
  sound: "Woof!",
  makeSound: () => console.log(this.sound),
  readTag: () => console.log("The dog's tag reads: " + this.name + ".")
};

// Prints `undefined`
dog.makeSound();

// Prints `The dog's tag reads: undefined.`
dog.readTag();

// In the makeSound and readTag methods, `this` doesn't refer to `dog`
// If this code run in node, `this` refers to `module.exports` (the object containing all the exports in this file)
// If this code was run in the browser, `this` would refer to the window