Nodejs 在互动模式下 下划线变量名

今天在测试代码时候发现，在nodejs 互动模式下 ，如果下划线作为对象名，会有意外事情发生

> function Testfun(par1) {
... return par1;
... }
undefined
> Testfun.prototype.fun2 = function(variables) {
... return variables;
... }
[Function]
> a=new Testfun(1)
{}
> a.fun2(2)
2
> a.fun2(3)
3
//如果用正常变量名，一切正常
> _=new Testfun(1)
{}
> _.fun2(2)
2
> _.fun2(3)
TypeError: undefined is not a function
    at repl:1:3
    at REPLServer.defaultEval (repl.js:132:27)
    at bound (domain.js:291:14)
    at REPLServer.runBound [as eval] (domain.js:304:12)
    at REPLServer. (repl.js:279:12)
    at REPLServer.emit (events.js:107:17)
    at REPLServer.Interface._onLine (readline.js:214:10)
    at REPLServer.Interface._line (readline.js:553:8)
    at REPLServer.Interface._ttyWrite (readline.js:830:14)
    at ReadStream.onkeypress (readline.js:109:10)
> _
2

//如果用下划线变量名，_会被重新赋值

还要研究下为什么出现这种状况