IIFE 立即执行函数表达式-模块化

1）立即执行

2）表达式

3）括号，分号结束 | 前缀运算符 | 表达式上下文

4）只需要执行一次，内部变量无需维护，可用于初始化

(function( ) {
})( );
或
(function( ){
}( ));
或通过前缀运算符强制执行表达式，可以无分号
+function() {
}( )

或在表达式上下文，不需要前缀或括号，无需调用就执行了
var ourter = function outter (){
}( )

4）可传入参数

var obj = {} ;
(function(export){
   export.add = function () { } ;
   export release = function () { } ;
})(this.obj = { })   // this 指向window对象

5）应用

a、用于模块化

var weekDay = (function(){
    var week = ["星期一","星期二","星期三","星期四","星期五"];
    return function (num) {
        return week[num];
    }
})()
console.log(weekDay(1))

var weekDay = (function(){
    var week = ["星期一","星期二","星期三","星期四","星期五"];
    return {
　　　　 name:function (num) {
            return week[num];
    　　},
    　　index:function (name) {
        　　return week.indexOf(name);
    　　}
    }
})()
console.log(weekDay.name(1))
console.log(weekDay.index("星期二"))
//缺点：当接口多时，不利于看 

(function(exports){
    var week = ["星期一","星期二","星期三","星期四","星期五"];
    exports.name = function (num) {
        return week[num];
    }
    exports.index = function (name) {
        return week.indexOf(name);
    }
})(this.weekDay = {});
console.log(weekDay.name(1))
console.log(weekDay.index("星期二"))
//缺点：每个模块都要定义一个全局变量存储，存在多个模块刚好使用相同名字问题