函数

求每个班级的男女比率

//1班学生
var students1 = [{
    name:'魏文庆'，
    gender:1
},{
    name:'蔡剑飞'，
    gender:1
},{
    name:'张贤'，
    gender:0
}
//...
];

//2班同学
var students2 = [{
    name:'陆忠芳'，
    gender:0
},{
    name:'严跃杰'，
    gender:1
},{
    name:'胡雪亮'，
    gender:1
}
//...
];

//求1班的男女比率
var maleCount1 = 0,
    femaleCount1 = 0,
    length1 = students1.length,
    students1,
    ratiol;//比率
//遍历1班的学生
for(var i = 0l i < length1; i++){
    student1 = students1[i];
    if (student1.gender === 1){
    //如果是男生，男生数量加1
        maleCount1++;
    }else{
    //否则女生数量加1
        femaleCount1++;
    }
}
ratio1 = maleCount1/femaleCount1;

//求2班的男女比率
var maleCount2 = 0,
    femaleCount2 = 0,
    length2 = students2.length,
    students2,
    ratiol;//比率
//遍历2班的学生
for(var i = 0l i < length2; i++){
    student2 = students2[i];
    if (student1.gender === 1){
    //如果是男生，男生数量加1
        maleCount2++;
    }else{
    //否则女生数量加1
        femaleCount2++;
    }
}
ratio2 = maleCount2/femaleCount2;

function ratio(students){
    var maleCount = 0,
         femaleCount = 0,
         length = students.length,
         student,
         ratio;
    for (var i = 0;i < length; i++){
        student = students[i];
        if(student.gender === 1){
            maleCount++;
        }else{
            femaleCount++;
        }
   }
   ratio = maleCount / femaleCount;
   return ratio;
}

var ratio1 = ratio(students1);
var ratio2 = ratio(students2);
var ratio3 = ratio(students3);
...

函数语法

function add(number0,number1){
    var sum = number0 + number1;
    return sum;
}
var x = add(2,3);

function 函数名(形参列表){
    执行代码
}
函数名([实参列表])

函数定义

//1.函数声明
function add(number0,number1){
    var sum = number0 + number1;
   return sum;
}

//2.函数表达式
var add = function(number0,number1){
    var sum = number0 + number1;
    return sum;
}
var x = add(4,6);//10

函数调用

function add(number0,number1){//number0=2,number1=3把实参列表复制给了形参列表
    var sum = number0 + number1;//sum = 5
    return sum;
}
var x = add(2,3);//x=5

函数参数

//实参数量少于形参时
function add(number0,number1){ //number0 = 2,number1 = undefined
    var sum = number0 + number1; //
    return sum;
}
var x = add(2);//NaN

//实参数量多于形参
function add(number0,number1){ //number0 = 2,number1 = 3
    var sum = number0 + number1; //
    return sum;
}
var x = add(2,3,4);//5

调用函数时，有个隐藏的变量

//参数不定的代码求和
add(2,3);//5
add(2,3,4);//9
add(2,3,4,5);//14

function add(){
    var length = arguments.length,
         sum = 0,
         parameter;//参数
    for(var i = 0;i < length;i++){
        parameter = arguments[i];
        sum = sum + parameter;
    }
    return sum;
}

//参数为原始类型：值传递
function increment(number){
    number = number +１；
    return number;
}
var a = 1;
var x = increment(a);//2
a;//1

//参数为对象类型：引用传递，在函数中修改对象会影响到原对象
function increment(person){
    person.age = person.age + 1;
    return person;
}

var jerry = {name:'jerry',age:1};
var x = increment(jerry);//{name:'jerry',age:2}
jerry;//{name:'jerry',age:2}

作用域

var zhoujielun = {
    name:"周杰伦",
    gender:1
};
//函数创建了一个作用域，变量的作用域其实就是限制了变量起作用的范围
function class1(){
    var zhoujielun = {
        name:"周杰伦",
        gender:0
    };
    zhoujielun.name = "周杰";
    zhoujielun.gender = 1;
}
class1();
zhoujielun;//{name: "周杰伦", gender: 1}

var zhoujielun = {
    name:"周杰伦",
    gender:1
};
function class1(){
    zhoujielun.name = "周杰";
    zhoujielun.gender = 1;
}
class1();
zhoujielun;//{name: "周杰", gender: 1}

作为对象属性

var point = {
    x:1;
    y:1;
    move:function(stepX,stepY){
        //point.x += stepX;
        //point.y += stepY;
        this.x += stepX;
        this.y += stepY; 
        //this表示调用这个函数的对象  
    }
};
point.move(2,1);//(3,2)

构造函数

function Point(x,y){                               //创建一个空对象{}
    this.x = x;                                        //{x:1}
    this.y = y;                                        //{x:1,y:1}
    this.move = function(stepX,stepY){  //{x:1,y:1,move:function(stepX,stepY){}}
        this.x += stepX;
        this.y += stepY;
    }
}
//构造函数的返回结果相当于return了this
var point = new Point(1,1);//{x:1,y:1,move:function(stepX,stepY){}}
var point2 = new Point(2,2);//{x:2,y:2,move:function(stepX,stepY){}}
var point3 = new Point(3,3);//{x:3,y:3,move:function(stepX,stepY){}}
//只需要创建一个move方法公用

原型

function Point(x,y){
    this.x = x;
    this.y = y;
}
Point.prototype.move = function(stepX,stepY){
    this.x += stepX;
    this.y += stepY;
};
var point = new Point(1,1); //{x:1,y:1}
point.move(2,1);//{x:3,y:2} 构造出来的这个对象肯定是有个隐藏的属性，指向它公共的地方，指向它构造函数的原型属性