作用域链

• 函数在执行的过程中，先从自己内部找变量
• 如果找不到，再从创建当前函数所在的作用域（词法作用域）去找，以此往上
• 注意找的是变量的当前的状态
  函数联通它的作用域链上的找的这个变量，共同构成闭包
  一般情况下使用闭包主要是为了

1. 封装数据
2. 暂存数据

典型闭包

function car(){
  var speed = 0
  function fn(){
    speed++
    console.log(speed)
  }
  return fn
}

var speedUp = car()
speedUp()   //1
speedUp()   //2

如下代码输入出多少？如果输出3，那如何改造代码？

var fnArr = [];
for (var i = 0; i < 10; i ++) {
  fnArr[i] =  function(){
    return i
  };
}
console.log( fnArr[3]() )

var fnArr = [];
for (var i = 0; i < 10; i++) {
    (function (i) {
        fnArr[i]= function () {
            return i
        }
    })(i);
}
console.log( fnArr[3]() )//3

封装一个car对象

var Car = (function(){
   var speed = 0;
   //补充
   return {
      setSpeed: setSpeed,
      get: get,
      speedUp: speedUp,
      speedDown: speedDown
   }
})()
Car.set(30)
Car.get() //30
Car.speedUp()
Car.get() //31
Car.speedDown()
Car.get()  //30

var Car = (function(){
    var speed = 0;
    function set(speed1){
        speed = speed1
        console.log(speed)
    }
    function get(){
        console.log(speed)
        return speed;
    }
    function speedUp(){
        speed++;
        console.log(speed)
    }
    function speedDown(){
        speed--;
        console.log(speed)
    }
    return {
        set: set,
        get: get,
        speedUp: speedUp,
        speedDown: speedDown
    }
})()
Car.set(30)
Car.get() //30
Car.speedUp()
Car.get() //31
Car.speedDown()
Car.get()  //30

如下代码输出多少？如何连续输出 0,1,2,3,4？

for(var i=0; i<5; i++){
  setTimeout(function(){
    console.log('delayer:' + i )
  }, 0)
}

    (function (j) {
        setTimeout(() => {
            console.log( + j ) 
        }, 0);
    })(i)  
}
//0,1,2,3,4

如下代码输出多少？

function makeCounter() {
  var count = 0

  return function() {
    return count++
  };
}

var counter = makeCounter()
var counter2 = makeCounter();

console.log( counter() ) // 0
console.log( counter() ) // 1

console.log( counter2() ) // ?
console.log( counter2() ) // ?

function makeCounter() {
    var count = 0
  
    return function() {
      return count++
    };
  }
  
  var counter = makeCounter()
  var counter2 = makeCounter();
  
  console.log( counter() ) // 0
  console.log( counter() ) // 1
  
  console.log( counter2() ) // 0
  console.log( counter2() ) // 1

补全代码，实现数组按姓名、年纪、任意字段排序。

var users = [
  { name: "John", age: 20, company: "Baidu" },
  { name: "Pete", age: 18, company: "Alibaba" },
  { name: "Ann", age: 19, company: "Tecent" }
]

users.sort(byField('age'))
users.sort(byField('company'))

var users = [
    { name: "John", age: 20, company: "Baidu" },
    { name: "Pete", age: 18, company: "Alibaba" },
    { name: "Ann", age: 19, company: "Tecent" }
]
function byField(string){
    return function(user1,user2){
        return user1[string] >user2[string]
    }
}
users.sort(byField('age'));
users.sort(byField('name'))
users.sort(byField('company'))

写一个 sum 函数，实现如下调用方式。

console.log( sum(1)(2) ) // 3
console.log( sum(5)(-1) ) // 4

function sum(a) {
    return function (b) {
        return a + b;
    }
}
console.log( sum(1)(2) ) // 3
console.log( sum(5)(-1) ) // 4

待补充