Getting Undefined Variable in Javascript

Question

UPDATED CODE: i, I'm new to Javascript programming and getting an undefined variable when trying to assign a new variable from a method.

I'm using node.js and creating a redis server using the redis-client in the "client variable".

var redis = require("redis");
var client = redis.createClient();

client.on("error", function (err) {
console.log("Error " + err); });

var numberPosts;

client.get("global:nextPostId", function(err, replies) {
  numberPosts = replies;
  console.log(numberPosts);
});

console.log(numberPosts);

When I call console.log inside the call back function it returns the proper value, however when I call the console.log outside of the callback function it returns "undefined". I'm trying to assign the value that is inside the callback function to the global variable numberPosts.

Any help is much appreciated, thanks.

Matt

When var numberPosts = client.get("global:nextPostId", redis.print); is executed it results in undefined. — Matikus1trillion
– Matikus1trillion, Commented Feb 15, 2011 at 13:49
You simply can't do that like this. See my answer for more details. — user447356
– user447356, Commented Feb 15, 2011 at 14:26

score 4 · Accepted Answer · 2011-02-15 14:59:45Z

4

I believe this will work:

client.get("global:nextPostId", function (err, reply) {
    console.log("Number of posts: " + reply.toString());
})

The AJAX call is asynchronous so it doesn't have return value.. instead you have to use callback function and only there you have the value returned by the server method.

Edit: to assign the return value to global variable, first declare global variable:

var _numOfPosts = "";

Then:

client.get("global:nextPostId", function (err, reply) {
     _numOfPosts = reply.toString());
})

However, the value won't be available until the AJAX call is finished so your original code can't work. There is not direct return value to store.

You can set timer to some reasonable response time, then have the code using the global variable in there.

Edit II: in order to call the method again once it's finished, have such code:

var _nextPostCallCount = 0;
function GetNextPost() {
   //debug
   console.log("GetNextPost called already " + _nextPostCallCount  + " times");

   //sanity check:
   if (_nextPostCallCount  > 1000) {
      console.log("too many times, aborting");
      return;
   }

   //invoke method:
   client.get("global:nextPostId", function(err, replies) {
      numberPosts = parseInt(replies.toString(), 10);
      console.log("num of replies #" + (_nextPostCallCount + 1) + ": " + numberPosts);

      //stop condition here.... for example if replies are 0
      if (!isNaN(numberPosts) && numberPosts > 0)
         GetNextPost();
   });

   //add to counter:
   _nextPostCallCount++;
}
GetNextPost();

This will call the method over and over until the result is 0 or you pass some hard coded limit to prevent endless loop.

edited Feb 15, 2011 at 14:59

answered Feb 15, 2011 at 13:45

user447356

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6 Comments

Matikus1trillion Over a year ago

That works and we've been able to get that to work. But the problem we have is when we try to assign the value "reply" into a global variable.

Matikus1trillion Over a year ago

Show wizard: Thanks for your help on this one. Unfortunately it is still returning undefined when I try to access _numOfPosts outside of the callback function.

user447356 Over a year ago

@Mat try window.setTimeout(function() { alert("num: " + _numOfPosts); }, 3000); this will show the alert after 3 seconds when the global variable should have value.

Matikus1trillion Over a year ago

As I'm rereading this it looks like you are staying this may not be possible. Let me tell you what we've trying to achieve. We essentially just want to return the value of global:nextPostId into a variable, and then loop through each post id and spit out each post body into a websocket.

Matikus1trillion Over a year ago

We're using node.js and are new to handling asynchronous data.

|

jpstrikesback · Accepted Answer · 2011-02-15 14:47:40Z

0

Try this instead to see errors:

var redis = require("redis"); 

client = redis.createClient(); 

client.on("error", function (err) {
console.log("Error " + err); });

//note the error logging
var numberPosts = client.get("global:nextPostId", function (error, response) {
  if (error) {
    console.log("async: " + error);
  } else {
    console.log("programming: " + response);
  }
});

console.log("is lotsa fun: " + numberPosts);

As Shadow Wizard has pointed out you are trying to use numberPosts before there is something in it, as client.get() hasn't returned anything.

Read this to get a handle on node.js flow:

http://www.scribd.com/doc/40366684/Nodejs-Controlling-Flow

edited Feb 15, 2011 at 14:47

answered Feb 15, 2011 at 13:43

jpstrikesback

2,30814 silver badges18 bronze badges

8 Comments

Pointy Over a year ago

This is exactly the same as what was originally posted, except that you've introduced a global variable ("client"), right? Why would this make any difference?

Matikus1trillion Over a year ago

That's not where were having the problem. Note the "," in the orignal code after require("redis"). We're specifically having the problem with the var numberPosts = client.get("global:nextPostId", redis.print); That is resulting in undefined.

user447356 Over a year ago

There is no error, he's trying to use the response before it arrive.

Matikus1trillion Over a year ago

console.log"is lotsa fun: + numberPosts); still returns "undefined"

jpstrikesback Over a year ago

it's undefined for the reasons Shadow Wizard has said, you can't use it before it exists, the call outside the closure is "faster" than the client.get and so it has nothing yet...you need to wait until there is something returned from client.get before using it

|

Russell Wong · Accepted Answer · 2012-07-20 09:29:38Z

0

I was facing the the same issue when I applied the MVC framework. To solve the problem, I employed the render function.

In the posts Model

exports.get = function(id,render) {
    client.incr('post:id:'+id, function(err, reply) {
    render(reply);
    });
};

In the posts Controller

exports.get = function(req, res) {
    posts.get('001', function (data){res.render('index',{post:data});});

};

answered Jul 20, 2012 at 9:29

Russell Wong

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