Scope issues with javascript (node.js + mongo.db)

Question

I am encountering some problems with my convertKey function, and I suspect it is due to scope issues. Basically, I try to retrieve a record from my mongo database and store it in a count variable, but when I try to return it, I get "undefined". Surprisingly, console.log(nameSearch + count) works, while return nameSearch + count doesn't. Would really appreciate it if someone could help me on this!

var dbUrl = "kidstartnow",
  collections = ["students", "studentsList"];
var db = require("mongojs").connect(dbUrl, collections);

function Student(name, src) {
  this.name = name.toLowerCase();

  //this function does not work
  this.key = convertKey(this.name);

  this.src = src;
  this.pointsTotal = 0;

  //inserts student into database
  var student = {name: this.name, key: this.key, pointsTotal: this.pointsTotal,
    src: this.src
  };
  db.students.insert(student);

  //converts name to a key by stripping white space and adding a number behind and ensures keys are unique
  //concatenates names together to form namesearch, and checks if entry exists in studentsList
  function convertKey(name) {

    var nameSearch = name.replace(/\s/g, ''),
      count = 1;



    db.studentsList.find({name: nameSearch}, function(err, student) {      
      //if nameSearch does not exist in studentsList, create entry and sets count to 1
      if(err || !student.length) {
        db.studentsList.insert({name: nameSearch, count: 1});
        count = 1;
        return nameSearch + count;
      }

      //if entry does exist, increments count by 1
      else {
        db.studentsList.update({name: nameSearch}, {$inc: {count: 1}}, function(err) {
          if(err) {
            console.log("Error incrementing records");
          }

          db.studentsList.find({name: nameSearch}, function(err, student) {
            count = student[0].count;
            //this works
            console.log(nameSearch + count)
            //but this doesn't
            return nameSearch + count;
          });
        });
      }
    });
  };
}

Jim Schubert · Accepted Answer · 2013-03-18 06:21:14Z

1

You're returning from the callback to db.studentsList.find and not from your convertKey function.

If you want the value to be returned from within db.studentsList.find, you'll need to either supply a callback to convertKey or possibly use a Promise library to make convertKey a deferred/future. Otherwise, your function will return immediately while waiting for your nested async functions to complete.

A callback allows you to pass on the result you're looking for (e.g. callback(nameSearch + count))

edit

Whenever I have questions about the return values of functions, I match braces with comments:

function convertKey(name) {

    var nameSearch = name.replace(/\s/g, ''),
      count = 1;

    db.studentsList.find({name: nameSearch}, function(err, student) {      
      //if nameSearch does not exist in studentsList, create entry and sets count to 1
      if(err || !student.length) {
        db.studentsList.insert({name: nameSearch, count: 1});
        count = 1;
        return nameSearch + count;
      } else {
        db.studentsList.update({name: nameSearch}, {$inc: {count: 1}}, function(err) {
          // ...
          db.studentsList.find({name: nameSearch}, function(err, student) {
             // ...
             return nameSearch + count;
          }); // end db.studentsList.find
        }); // end db.studentsList.update
      } // end else
    }); // end db.studentsList.find

    /**
     * Notice, no return value here...
     */
  }; // end convertKey

edited Mar 18, 2013 at 6:21

answered Mar 18, 2013 at 6:15

Jim Schubert

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2 Comments

Dan Tang Over a year ago

Thank you for this! Just one quick followup: I tried adding a callback function, and called it as such, but it does not work. Do you know where I am going wrong this.key = convertKey(this.name, function(text) { return text; });

Jim Schubert Over a year ago

@DanTang I think you need to read up on Continuation-passing style and there's a pretty good article here. Basically, you'll need to contain all logic relying on the text value within that callback function. This is the same idea as what you're doing in the callback passed to db.studentsList.find.

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