javascript - this is undefined while trying to inherit prototype -

i trying add prototype dynamically reason keyword return undefined when try inherit prototype. when run same code on browser console works fine. why not working node.js?

i have included full code @ end of question.

my main issue happens here:

let postobj = function(){   this.data = post;   this.originalindex = i; };  //postobj.prototype = postproto; // have same result postobj.prototype = object.create(postproto);  //postobj.data = post; _self.posts.push(new postobj()); 

when try access this inside prototype method return undefined. example postproto has method id looks this.data._id. can see have set constructors postobj, why not being passed prototype?

please tell me whats wrong here.

full code:

const postprototype = {   id: (){     return this.data._id;   },    title: (){     return this.data.title;   } }  function fetchposts(){   this.posts = [];    this.rawposts = [     {_id: '1', title: 'in sit amet lorem velit, in dictum lorem'},     {_id: '2', title: 'vestibulum ante ipsum primis in faucibu'},     {_id: '3', title: 'integer vulputate nibh et diam sagittis in dictum mauris dapibus'}   ];    this.inheritprototype(); }  fetchposts.prototype.inheritprototype = function(){   var _self = this;    if(!_.isempty(_self.rawposts)){     _self.rawposts.foreach(function(post, i){       try{         let postobj = function(){           this.data = post;           this.originalindex = i;         };          //postobj.prototype = postproto; // have same result         postobj.prototype = object.create(postprototype);          //postobj.data = post;         _self.posts.push(new postobj());        }catch(err){         console.log(err);       }     });   }  };  fetchposts.prototype.get = function(){   return this.posts; }   var samplefetch = new fetchposts(); samplefetch.get().foreach(function(post, i){   post.id(); // return error: cannot property _id undefined }); 

first of all, because of try catch block hided error is: "referenceerror: postproto not defined". @ top of script, have defined postprototype variable used postproto of course undefined.

also, in declaration of postprototype have used object literal shorthand not properly. should use without :.

const postprototype = {   id (){     return this.data._id;   },    title (){     return this.data.title;   } } 

i have edited code here.