Can't add object to Array in jQuery's getJSON data function (scope issue)

Question

I have a person object and wants to store it into a global ArrayCollection I have made. Works great in normal scope:

var s = new ArrayCollection();
s.add(new person("Knud", "Mikkelsen", 35));

The problem is when I want to add people inside my jQuery function "mainFunction".

I can't seem to get it right. I know it's something to do with scope and I have to wrap something in functions like in my ArrayCollection. Please help me - thanks a lot.

function ArrayCollection() {
 var myArray = new Array;
 return {
  empty: function () {
   myArray.splice(0, myArray.length);
  },
  add: function (myElement) {
   myArray.push(myElement);
  },
  getAll: function () {
   return myArray;
  }
 }
}

function person(firstName, lastName, age) {
 this.firstName = firstName;
 this.lastName = lastName;
 this.age = parseInt(parseFloat(age));
}

function mainFunction() {
 //....
 var s = new ArrayCollection();
 s.add(new person("Knud", "Mikkelsen", 35));

 $.getJSON(url, function (data) {
  for (var x = 0; x < data.length; x++) {

   var myPerson = new person(data[x].FirstName.toString(), data[x].LastName.toString(), data[x].Age.toString());
   s.add(myPerson);
  }
 });

 alert(drawArray(s.getAll()));
}

function drawArray(myArray) {
 var v = "";
 for (var i = 0; i < myArray.length; i++) {
  v += myArray[i].firstName + " " + myArray[i].lastName + " (" + myArray[i].age + ")\n";
 }
 return v;
}

T.J. Crowder · Accepted Answer · 2010-05-06 08:40:58Z

I'm not seeing a scope issue there. Your success function for getJSON is defined within mainFunction and so has access to s. mainFunction is defined as a peer of person and so has access to it.

I am seeing a synchronous/asynchronous issue. Here's the order of execution of mainFunction:

Create a new ArrayCollection.
Add a person to it named Knud Mikkelsen
Initiate a request for data via $.getJSON.
Alert showing the one person in the ArrayCollection (Knud Mikkelsen)
Return
At some point in the future, the getJSON call completes and adds the person objects to the ArrayCollection

This is because by default, getJSON is asynchronous — it completes at some point in the future, not synchronously with the code where it was initiated. This is a good thing because synchronous Ajax calls lock up the browser UI completely and make for a poor user experience.

Your options for actually seeing that person returned by the Ajax call are to move the alert to within the getJSON success handler, and/or to pass a callback into mainFunction that gets triggered by the getJSON completion function, and/or pass the ArrayCollection to add to into mainFunction and just check it for new person objects later as part of your program flow.

Here are a couple of options for you:

Just move the alert inside the getJSON completion function:

function mainFunction() {
    //....
    var s = new ArrayCollection();
    s.add(new person("Knud", "Mikkelsen", 35));

    $.getJSON(url, function (data) {
        for (var x = 0; x < data.length; x++) {
            var myPerson = new person(data[x].FirstName.toString(), data[x].LastName.toString(), data[x].Age.toString());
            s.add(myPerson);
        }
        alert(drawArray(s.getAll())); // <== moved this into the completion function
    });
}

Or use a callback:

function mainFunction(callback) {
    //....
    var s = new ArrayCollection();
    s.add(new person("Knud", "Mikkelsen", 35));

    $.getJSON(url, function (data) {
        for (var x = 0; x < data.length; x++) {
            var myPerson = new person(data[x].FirstName.toString(), data[x].LastName.toString(), data[x].Age.toString());
            s.add(myPerson);
        }
        callback(s);                // <== Trigger callback, passing in the collection
    });
}

function mainFunctionCallback(s) {
    alert(drawArray(s.getAll()));   // <== Moved out of `mainFunction` entirely
}

// Usage:
mainFunction(mainFunctionCallback);

Works like a charm. Lol sometimes it's the simplest things you forget to think about.

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