函數(shù)聲明定義函數(shù)特點

• 和變量的聲明會前置一樣，函數(shù)聲明同樣會前置，如果我們使用函數(shù)表達式那么規(guī)則和變量一樣
• 用函數(shù)聲明的方式,重復(fù)定義函數(shù),最后一次定義有效

函數(shù)實例化定義函數(shù)特點

• 定義的函數(shù)只能訪問本地作用域和全局作用域

變量前置

console.log(a)   //undefined
var a = 3    

==>等同于

var a
console.log(a)
a = 3

函數(shù)聲明

console.log(sum(3,5))

function sum(a,b){
  return a + b
}

==>等同于
  

function sum(a,b){
  return a + b
}

console.log(sum(3,5))

函數(shù)表達式聲明前置

fn()  //"TypeError: fn is not a function(不是一個函數(shù))

var fn = function(){
  console.log('fn...')
}

==>


var fn
fn()

fn = function(){
  console.log('fn...')
}

立即執(zhí)行函數(shù)

立即執(zhí)行函數(shù)可以形成一個作用域,和全局隔開,不會污染全局,另一方面也是防止命名沖突的影響

(function(){
  console.log('我是立即執(zhí)行函數(shù)')
})()

==>
  
var fn = function(){
  console.log('我是立即執(zhí)行函數(shù)')
}
fn()

命名沖突

當(dāng)在同一個作用域內(nèi)定義了名字相同的變量和方法的話，會根據(jù)前置順序產(chǎn)生覆蓋

var fn = 3;
function fn(){}

console.log(fn); // 3

==>

var fn
function fn(){}  //覆蓋上面的

fn = 3  //重新賦值
console.log(fn)

console.log(fn); // 3

function fn(){}
var fn = 3;

==>
function fn(){}  
var fn  //已經(jīng)聲明的值不會變

console.log(fn)  //fn(){}
fn = 3  

遞歸

簡單來說就是自己調(diào)用自己

function factorial(n){
  if(n === 1){
    return 1
  }
  return n * factorial(n-1) 
}
alert(factorial(3))

//缺點:效率低

函數(shù)調(diào)用

• 函數(shù)調(diào)用模式
  • 函數(shù)名()
• 方法調(diào)用模式

var myNumber = {
 value: 1,
 add: function(i){
  console.log(this);
  this.value += i;
  }
}
myNumber.add(1);

• 構(gòu)造函數(shù)調(diào)用模式
  • new Function(...)首字母大寫
• apply(call)調(diào)用模式
  • apply是function構(gòu)造函數(shù)原型上的構(gòu)造方法
    • 實現(xiàn)函數(shù)借用的功能,把函數(shù)借用給對象

函數(shù)調(diào)用模式的區(qū)別-this

• 函數(shù)調(diào)用模式
  • this指向window的全局對象

//例子:
function add(i,j){
  console.log(this);  //Window
  // console.log(arguments);
  var sum = i+j;
  console.log(sum);
  (function(){
    console.log(this);  //Window
  })()
  return sum;
}
add(1,2);

• 方法調(diào)用模式
  • this指向調(diào)用者

//例子:
var myNumber = {
  value: 1,
  add: function(i){
    console.log(this);  //Object{value: 1, add: ?}
    this.value += i;
  }
}
myNumber.add(1);

• 構(gòu)造函數(shù)調(diào)用模式
  • this指向被構(gòu)造的函數(shù)

//例子:
function Car(type,color){
  this.type = type;
  this.color = color;
  this.status = "stop";
  this.light = "off";
  console.log(this);  //打印結(jié)果是我們創(chuàng)建的對象
}
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 benz = new Car("benz", "black");  //Car {type: "benz", color: "black", status: "stop", light: "off"}

• apply(call)調(diào)用模式
  • this指向第一個參數(shù)

函數(shù)傳參

• 原始類型按值傳遞-call by value

var count = 1;
var addOne = function(num) {
    num += 1;
    return num;
}
var ret = addOne(count);
console.log(ret);  //2
console.log(count);  //1結(jié)果沒有受到影響

• 對象類型按共享傳遞-call by sharing

//例子一:
var count = {a:1,b:1};
var addOne = function(obj) {
    obj.a += 1;
    obj.b += 1;
    return obj;
}
var ret = addOne(count);
console.log(ret);  //{a: 2, b: 2}
console.log(count);  //{a: 2, b: 2}

//例子二:
var count = {a:1,b:1};
var addOne = function(obj) {
    obj = {a:2, b:2};
    return obj;
}
var ret = addOne(count);
console.log(ret);  //{a: 2, b: 2} 
console.log(count);  //{a: 1, b: 1}

函數(shù)傳參