Function

1. 函数定义

• 函数声明 function add () {}
• 函数表达式 var add = function () {};
• 对象实例化 var add = new Function() {};

//1.函数声明与函数表达式、对象实例化的区别

add1(1,1);
add2(1,2);
add3(1,3);
function add1(i, j){
  console.log(i+j);
}
var add2 = function(i, j){
  console.log(i+j);
}
var add3 = new Function("i", "j", "console.log(i+j);");

函数声明会实现声明提前，即使函数调用在函数定义之前依然可以调用
而函数表达式和函数对象实例化不行

//2.对象实例化与函数声明与函数表达式的区别
(function(){
  var i = 10;
  function add(j) {
    console.log(i+j);
    debugger;
  }
  add(1);
})();

(function(){
  var i = 10;
  var add = new Function("j", "console.log(i+j);debugger;");
  // add 函数这时属于全局，所以无法获得 i 
  add(1);
})();

2. 构造函数

// 构造函数
function Car(type,color){
  this.type = type;
  this.color = color;
  this.status = "stop";
  this.light = "off";
}
Car.prototype.start = function(){
  this.status = "driving";
  this.light = "on";
  console.log(this.type + " is " + this.status);
}
Car.prototype.stop = function(){
  this.status = "stop";
  this.light = "off";
  console.log(this.type + " is " + this.status);
}
var audi = new Car("audi", "silver");
var benz = new Car("benz", "black");
var ferrari = new Car("ferrari", "yellow");
// 用构造函数实例化的对象可以直接获取和修改其内部的属性
// 如 audi.type = 'toyota';
// 显然这不是好的方式，它不能实现变量的私有化

// 还有其他内置的构造函数
// Boolean、String、Number、Object、Function、Array、Date、RegExp、Error
// 比如打印出对象
console.dir(new Boolean());
// 其结果如下
console.dir(new Boolean());
VM1866:2 Boolean__proto__: Booleanconstructor: Boolean()arguments: nullcaller: nulllength: 1name: "Boolean"prototype: Booleanconstructor: Boolean()toString: toString()valueOf: valueOf()__proto__: Object__defineGetter__: __defineGetter__()__defineSetter__: __defineSetter__()__lookupGetter__: __lookupGetter__()__lookupSetter__: __lookupSetter__()constructor: Object()arguments: nullassign: assign()caller: nullcreate: create()defineProperties: defineProperties()defineProperty: defineProperty()deliverChangeRecords: deliverChangeRecords()freeze: freeze()getNotifier: getNotifier()getOwnPropertyDescriptor: getOwnPropertyDescriptor()getOwnPropertyNames: getOwnPropertyNames()getOwnPropertySymbols: getOwnPropertySymbols()getPrototypeOf: getPrototypeOf()is: is()isExtensible: isExtensible()isFrozen: isFrozen()isSealed: isSealed()keys: keys()length: 1name: "Object"observe: observe()preventExtensions: preventExtensions()prototype: Objectseal: seal()setPrototypeOf: setPrototypeOf()unobserve: unobserve()__proto__: ()<function scope>hasOwnProperty: hasOwnProperty()isPrototypeOf: isPrototypeOf()propertyIsEnumerable: propertyIsEnumerable()toLocaleString: toLocaleString()toString: toString()valueOf: valueOf()get __proto__: get __proto__()set __proto__: set __proto__()[[PrimitiveValue]]: false__proto__: ()apply: apply()arguments: (...)get arguments: ThrowTypeError()set arguments: ThrowTypeError()bind: bind()call: call()caller: (...)get caller: ThrowTypeError()set caller: ThrowTypeError()constructor: Function()length: 0name: ""toString: toString()__proto__: Object<function scope><function scope>toString: toString()arguments: nullcaller: nulllength: 0name: "toString"__proto__: ()<function scope>valueOf: valueOf()arguments: nullcaller: nulllength: 0name: "valueOf"__proto__: ()<function scope>__proto__: Object__defineGetter__: __defineGetter__()__defineSetter__: __defineSetter__()__lookupGetter__: __lookupGetter__()__lookupSetter__: __lookupSetter__()constructor: Object()hasOwnProperty: hasOwnProperty()isPrototypeOf: isPrototypeOf()propertyIsEnumerable: propertyIsEnumerable()toLocaleString: toLocaleString()toString: toString()valueOf: valueOf()get __proto__: get __proto__()set __proto__: set __proto__()[[PrimitiveValue]]: false[[PrimitiveValue]]: false

3. 构造函数和普通函数的区别

• 本质上没有什么区别

• 构造函数的this会指向实例对象，并且this指定实例的属性 如：

  this.type = type;
  this.color = color;
  this.status = "stop";
  this.light = "off";

在原型对象上有一些公共方法 如：
Car.prototype.start = function(){
this.status = "driving";
this.light = "on";
console.log(this.type + " is " + this.status);
}
Car.prototype.stop = function(){
this.status = "stop";
this.light = "off";
console.log(this.type + " is " + this.status);
}

• 构造函数命名首字母为大写

4. 函数调用 (this)
   // 在每个函数中都有 this 和 arguments, this 根据调用方式不同，会绑定到不同的地方

• 构造函数调用模式
  var Audi = new Car('Audi', 'silver');
  this 绑定到实例化对象 Audi 上 (new 绑定)
• 方法调用模式
  (方法和函数的区别，方法是指绑定对象的函数，这只是一个名称上的规范)
  比如：
  Audi.start();
  // this 绑定到调用它的对象上
• 函数调用模式

// 函数调用模式
function add(i, j){
  return i+j;
}
var myNumber = {
  value: 1,
  double: function(){
    var helper = function(){
        // this 指向的是 window
      this.value = add(this.value,this.value);
    }
    helper();
  }
}
myNumber.double() 
// 通过打印对象 console.dir(myNumber); value = 1;

// 解决方法是把this保存
function add(i, j){
  return i+j;
}
var myNumber = {
  value: 1,
  double: function(){
    var that = this;
    var helper = function(){
        that.value = add(that.value,that.value);
    }
    helper();
  }
}
myNumber.double() 
// 通过打印对象 console.dir(myNumber); value = 2;

// 注意：任何在函数内部调用的子函数 this 指向全局

- apply(call) 调用模式
var a = 2;
function foo() {
    return this.a;
}
var obj = {
    a: 3,
    foo: foo
}
obj.foo(); // 3
foo.call(obj); // 3
var bar = obj.foo;
bar(); // 2



// arguments转数组
function add(i, j) {
  var args = Array.prototype.slice.apply(arguments);
  args.forEach(function(item){
    console.log(item);
  })
}
add(1,2,3);


// arguments.callee使用
console.log(
  (function(i){
    if (i==0) {
      return 1;
    }
    return i*arguments.callee(i-1);
  })(5)
);


// 递归
function factorial(i){
  if (i==0) {
    return 1;
  }
  return i*factorial(i-1);
}


// 普通递归函数跟记忆函数调用次数对比
var factorial = (function(){
  var count = 0;
  var fac = function(i){
    count++;
    if (i==0) {
      console.log('调用次数：' + count); 
      return 1;
    }
    return i*factorial(i-1);
  }
  return fac;
})();
for(var i=0;i<=10;i++){
  console.log(factorial(i)); 
}

// 记忆函数
var factorial = (function(){
  var memo = [1];
  var count = 0;
  var fac = function(i){
    count++;
    var result = memo[i];
    if(typeof result === 'number'){
      console.log('调用次数：' + count); 
      return result;
    } 
    result = i*fac(i-1);
    memo[i] = result;
    return result;
  }
  return fac;
})();
for(var i=0;i<=10;i++){
  console.log(factorial(i)); 
}


// curry函数柯里化
function add(value){
   var helper = function(next){
      value = typeof(value)==="undefined"?next:value+next;
      return helper;
   }
   helper.valueOf = function(){
     return value;
   }
   return helper
}