function foo(){var something = 'cool';var another = [1, 2, 3];function doSomething() {console.log( something );}function doAnother() {console.log( another.join( " ! " ) );}
}
// 是一个不明显的闭包,doSomething()和doAnother()保持了foo的内部作用域

接下来考虑以下的代码:

function CoolModule() {var something = "cool";var another = [1, 2, 3];function doSomething() {console.log( something );}function doAnother() {console.log( another.join( " ! " ) );}return {doSomething: doSomething,doAnother: doAnother};
}var foo = CoolModule();foo.doSomething(); // "cool"
foo.doAnthor(); // 1!2!3
// 上述通过 return 将doSomething和doAnother暴露出来,在js中被称为模块.

从上面可以抽象出模块模式所必须的两个必要条件:
1.必须有外部的封闭函数,该函数必须至少被调用一次(每次调用都会创建一个新的模块实例)
2.封闭函数必须返回至少一个内部函数,这样内部函数才能在私有作用域中形成闭包,并且可以访问或者修改私有的状态.

下面看一个单例模式:

var foo = (function CoolModule() {var something = 'cool';var another = [1, 2, 3,];function doSomething() {console.log( something );}function doAnother() {console.log( another.join( " ! " ) );}return {doSomething: doSomething,doAnother: doAnother}
})();
foo.doSomething(); // cool
foo.doAnother(); // 1!2!3// 注:上述的CoolModule在执行一次后便释放掉了.

接受参数的模块:

functioin CoolModule(id) {function identify() {console.log( id );}return {identify: identify}
}var foo1 = CoolModule( "foo 1" );
var foo2 = CoolModule( "foo 2" );
foo1.identify(); // "foo 1"
foo2.identify(); // "foo 2"// 没啥好说的...

现代的模块机制

var MyModules = (function Manager() {var modules = {};function define(name, deps, impl) {for (var i=0; i < deps.length; i++) {deps[i] = modules[deps[i]];}modules[name] = impl.apply( impl, deps );}function get(name) {return modules[name];}return {define: define,get: get};
})();// 上述定义了一个模块依赖加载器/管理器,下面使用上述的函数来定义模块
MyModules.define( " bar" , [], function() {function hello(who){return "Let me introduce: " + who;}return {hello: hello}
});
MyModules.define( " foo ", ["bar"], function(bar) {var hungry = "hippo";function awesome() {console.log( bar.hello( hungry ).toUpperCase() );}return {awesome: awesome};
});
var bar = MyModules.get( "bar" );
var foo = MyModules.get( "foo" );console.log(bar.hello( "hippo" )
);  // Let me introduce: hippofoo.awesome(); // LET ME INTRODUCE: HIPPO

ES6中的模块

// bar.js
function hello(who) {return "Let me introduce: " + who;
}
export hello;// foo.js
import hello from "bar";var hungry = "hippo";function awesome() {console.log(hello( hungry ).toUpperCase());
}
export awesome;// baz.js
import foo from "foo";  // 书上是module foo from "foo"
import bar from "bar";console.log(bar.hellow( "rhino" )
);    // Let me introduce: rhinofoo.awesome();  // LET ME INTRODUCE: HIPPO

ES6模块和CommonJS模块的差异:
https://blog.csdn.net/piano9425/article/details/89946163

参考《你不知道的JavaScript》(上卷)P53~P56