var john = {
   firstName: "John"
  }
  function func() {
   alert(this.firstName + ": hi!")
  }
  john.sayHi = func
  john.sayHi() // this = john

  func()
  function func() {
    alert(this) // [object Window] or [object global] or kind of..
  }

var numbers = { 
  numberA: 5,
  numberB: 10,
  sum: function() {
   console.log(this === numbers); // => true
   function calculate() {
    // this is window or undefined in strict mode
    console.log(this === numbers); // => false
    return this.numberA + this.numberB;
   }
   return calculate();
  }
};
numbers.sum(); // => NaN or throws TypeError in strict mode 
var numbers = { 
  numberA: 5,
  numberB: 10,
  sum: function() {
   console.log(this === numbers); // => true
   function calculate() {
    console.log(this === numbers); // => true
    return this.numberA + this.numberB;
   }
   // use .call() method to modify the context
   return calculate.call(this);
  }
};
numbers.sum(); // => 15 

  function Animal(name) {
    this.name = name
    this.canWalk = true
  }
  var animal = new Animal("beastie")
  alert(animal.name)

  function Animal() {
    this.name = 'Mousie';
    this.age = '18';
    return {
      name: 'Godzilla'
    } // <-- will be returned
  }

  var animal = new Animal()
  console.log(animal.name) // Godzilla
  console.log(animal.age)//undefined

function Vehicle(type, wheelsCount) { 
 this.type = type;
 this.wheelsCount = wheelsCount;
 return this;
}
// Function invocation
var car = Vehicle('Car', 4); 
car.type;    // => 'Car' 
car.wheelsCount // => 4 
car === window // => true

function Animal(type, legs) { 
 this.type = type;
 this.legs = legs; 
 this.logInfo = function() {
  console.log(this === myCat); // => true
  console.log('The ' + this.type + ' has ' + this.legs + ' legs');
 };
}
var myCat = new Animal('Cat', 4); 
// logs "The Cat has 4 legs"
setTimeout(myCat.logInfo.bind(myCat), 1000); 
// setTimeout??
 var john = {
  firstName: "John",
  surname: "Smith"
 }
 function func(a, b) {
  alert( this[a] + ' ' + this[b] )
 }
 func.call(john, 'firstName', 'surname') // "John Smith"

 func.call(john, 'firstName', 'surname')
 func.apply(john, ['firstName', 'surname'])

  function Runner(name) { 
   console.log(this instanceof Rabbit); // => true
   this.name = name; 
  }
  function Rabbit(name, countLegs) { 
   console.log(this instanceof Rabbit); // => true
   // 间接调用，调用了父级构造器
   Runner.call(this, name);
   this.countLegs = countLegs;
  }
  var myRabbit = new Rabbit('White Rabbit', 4); 
  myRabbit; // { name: 'White Rabbit', countLegs: 4 }

var numbers = { 
 array: [3, 5, 10],
 getNumbers: function() {
  return this.array;  
 }
};
// Create a bound function
var boundGetNumbers = numbers.getNumbers.bind(numbers); 
boundGetNumbers(); // => [3, 5, 10] 
// Extract method from object
var simpleGetNumbers = numbers.getNumbers; 
simpleGetNumbers(); // => undefined or throws an error in strict mode 

  var numbers = [1, 2]; 
  (function() { 
   var get = () => {
    console.log(this === numbers); // => true
    return this;
   };
   console.log(this === numbers); // => true
   get(); // => [1, 2]
   // 箭头函数使用 .apply() 和 .call()
   get.call([0]); // => [1, 2]
   get.apply([0]); // => [1, 2]
   // Bind
   get.bind([0])(); // => [1, 2]
  }).call(numbers);

function Period (hours, minutes) { 
   this.hours = hours;
   this.minutes = minutes;
  }
  Period.prototype.format = () => { 
   console.log(this === window); // => true
   return this.hours + ' hours and ' + this.minutes + ' minutes';
  };
  var walkPeriod = new Period(2, 30); 
  walkPeriod.format(); // => 'undefined hours and undefined minutes'