1.如下代码输出多少？如果想输出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]() )

输出为10
想输出3，可做如下改造

//方法一
var fnArr = [];
for (var i = 0; i < 10; i ++) {
  (function(i){
    fnArr[i] =  function(){
      return i
    };
  })(i)
}
console.log( fnArr[3]() )
//方法二
var fnArr = [];
for (var i = 0; i < 10; i ++) {
  fnArr[i] =  (function(j){
    return function(){
      return j
    }
  })(i)
}
console.log( fnArr[3]() )
//方法三
var fnArr = []
for (let i = 0; i < 10; i ++) {
  fnArr[i] =  function(){
    return i
  } 
}
console.log( fnArr[3]() )

2.封装一个 Car 对象

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

实现

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

3.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)
}

输出5个delayer:5
改造

//方法一
for(var i=0; i<5; i++){
  (function(i){
    setTimeout(function(){
      console.log('delayer:' + i )
    }, 0)
  })(i)
}
//方法二
for(var i=0; i<5; i++){
  setTimeout((function(i){
    return function(){
      console.log('delayer:' + i )
    }
  })(i), 0)
}
//方法三
for(let i=0; i<5; i++){
  setTimeout(function(){
    console.log('delayer:' + i )
  }, 0)
}

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() ) // ?

最后两个console.log输出的也是0和1。counter和counter2是两个互不影响的函数

5.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'))

实现

var users = [
  { name: "John", age: 20, company: "Baidu" },
  { name: "Pete", age: 18, company: "Alibaba" },
  { name: "Ann", age: 19, company: "Tecent" }
]
function byField(field){
    return function(user1, user2){
        return user1[field] > user2[field]//因为field是字符串，所以不能用.来访问
    }
}
users.sort(byField('age'))
console.log(users)

6.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

实现

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