JavaScript (node.js) variable not accessible on inner function call. now.js

I am using now.js and Mongoose i开发者_如何学Pythonn a node project and am having trouble accessing the this.now object inside of a mongoose function. E.g.

everyone.now.joinDoc = function (project_id){  
  this.now.talk(); //this will work
  Project.findOne({'_id':project_id}, function(err, project){
    if(project){
      this.now.talk(); // this will not work "TypeError: Cannot call method 'bark' of undefined"
    };
  });
};

Change the code to this:

everyone.now.joinDoc = function (project_id){  
  this.now.talk();  // this will work
  var that = this;  // save 'this' to something else so it will be available when 'this' has been changed
  Project.findOne({'_id':project_id}, function(err, project){
    if(project){
      that.now.talk();  // use local variable 'that' which hasn't been changed
    };
  });
};

Inside your inner function, the this is probably being set to something else. So, to preserve the value you want to access, you assign it to a different local variable that will be available in the inner function.

everyone.now.joinDoc = function (project_id){  
  this.now.talk();  // this will work
  Project.findOne({'_id':project_id}, (function(tunnel, err, project){
    if(project){
      this.now.talk(); 
    };
  }).bind(this, "tunnel")); // overwrite `this` in callback to refer to correct `this`
};

Use Function.prototype.bind to set the value of this to the value you want