Returning back an array of collections in mongoose

I am new to mongoose and I tried to search a lot in order to convert a mongoose collection into an array but it returns an error that toArray() is not a method.So here is the alternative way I am trying to convert into an array.I am using passport js method for authentication and for that i require all the user information as an array.I am tryin to implement the example given in

https://github.com/jaredhanson/passport-local/blob/master/examples/express3/app.js

to mongoose driven data.

Heres the snippet I am trying to use for converting a mongoose collection into an Array.

ContactProvider.prototype.findByusername=function(callback){
    var users=Post.find({});
    var user=toObject((JSON.parse(users))) ;
    callback(null,user);

};

I got the following error

undefined:1
[object Object]
 ^
SyntaxError: Unexpected token o
    at Object.parse (native)
    at ContactProvider.findByusername (/home/r121/Desktop/nilesh/cmarin-MongoDB-Node-Express-Blog-4a5e5e9/postprovider.js:60:26)
    at Object.<anonymous> (/home/r121/Desktop/nilesh/cmarin-MongoDB-Node-Express-Blog-4a5e5e9/app.js:138:37)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.runMain (module.js:492:10)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)

How should I return an array of those collections backs.Can anyone please guide me with the syntax?