create instance as soon as required module

gistfile1.js

//File class.js
class = module.exports = function () {

}

class.prototype.add  = function () {console.log('hello')};

//File index.js
var class = new require('./class')();

class.add();

// Error:         
//TypeError: Cannot call method 'add' of undefined
// Why have error ??

class 应该是关键词吧

不是这个原因.javascript 没有clas关键词；

如果我这么用 就没有错:

var class = require('./class');
var c = new class
c.add();