How do self referenced objects work in JavaScript function?

Question

I am trying to set up a javascript object with one of it's items being a function. I am doing a little bit of self reference and it is not working.

var onTimePoints = function($a, $b, $c, $lifeotr){

    $var1 = ($b - $a) / $c;
    $var2 = ($lifeotr - $a) / $var1;
    if($var2>$c)
    {
    $var2 = $c;
    }
    if($var2<0)
    {
    $var2 = 0;
    }
    $var2 = Math.round($var2);
    return $var2;
}
var lifeData ={
      min: 25000,
      max: 65000,
      otr: 56426,
      maxPoints:100,
      otp: onTimePoints(25000, 65000, 100, 56426),
      test: function(){
        alert(this.min);
      }
}

When I do conosle.log(lifeData.otp) it works.

When I replace the hard coded numbers with this.min, this.max, etc... it doesn't work.

Here is an example of what doesn't work:

var lifeData ={
  min: 25000,
  max: 65000,
  otr: 56426,
  maxPoints:100,
  otp: onTimePoints(this.min, this.max, this.maxPoints, this.otr),
  test: function(){
    alert(this.min);
  }
}

In this case console.log(lifeData.otp) returns NaN.

I am sure that I am just overlooking something. Appreciate if you know the issue.

I deleted my original comment but then I realized I was right :-) The value of this has nothing whatsoever to do with an under-construction object literal. It has to do solely with function invocation context. Basically there's no direct way to do what you're doing in JavaScript. You have to use a separate statement. — Pointy
– Pointy, Commented Oct 11, 2012 at 18:19
When I alert this.min it alerts 25000. I am assuming that this.min is the same as saying lifeData.min — Spencer Cooley
– Spencer Cooley, Commented Oct 11, 2012 at 18:19
It is, but that's inside that "test" function call. When you call lifeData.test(), it's that function call itself that ensures that this inside the function is set to refer to the "lifeData" object. However, that's not the case in the midst of the object literal that initializes the object. In that context, this has a value determined by the context of that var declaration. — Pointy
– Pointy, Commented Oct 11, 2012 at 18:21
@My_Boon No, it's not working. Try calling lifeData.otp; and you'll see that you get a NaN back. — Pointy
– Pointy, Commented Oct 11, 2012 at 18:22

Diode · Accepted Answer · 2012-10-12 01:31:27Z

5

You will get a lot of tutorials to understand this, but here I am trying to illustrate it using a few lines of code. In firebug console or Chrome console do the following

enter this and press enter

> Window // Now it is global scope
Run the following code
```
this.a = 500;
this.b = 600;
```

Now run the following code;

var MyObject = {
    a: 20,
    b: 40,
    sum: this.a + this.b,
    getSum: function(){
        return this.a + this.b;
    }
}

Now run the following
```
   MyObject.sum
```
> 1100 // because MyObect is declared in global scope and in the expression sum:this.a+this.b this is Window object. so sum: 400 + 500
Now run
```
   MyObject.getSum();
```
> 60 // because getSum is a method of MyObject. And when invoked like MyObject.getSum() inside the getSum this will be MyObject. so it returns 20 + 40

edited Oct 12, 2012 at 1:31

answered Oct 11, 2012 at 18:34

Diode

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1 Comment

Amareswar Over a year ago

a small correction. it should not be 900 it should be 1100 500+600.

Paul Fleming · Accepted Answer · 2012-10-11 18:23:52Z

1

You can't do what you're trying to do.

Consider:

var lifeData ={
  min: 25000,
  max: 65000,
  otr: 56426,
  maxPoints:100,
  otp: onTimePoints(25000, 65000, 100, 56426),
  otNewcomm: formula,
  test: function(){
    alert(this.min);
  }
}

or

var min = 25000;
var max = 65000;
var otr = 56426;
var maxPoints = 100;
var lifeData ={
  min: min,
  max: max,
  otr: otr,
  maxPoints: maxPoints,
  otp: onTimePoints(min, max, maxPoints, otr),
  otNewcomm: formula,
  test: function(){
    alert(this.min);
  }
}

or

function lifeData(min, max, otr, maxPoints)
{
    this.min = min;
    this.max = max;
    this.otr = otr;
    this.maxPoints = maxPoints;
    this.otp = onTimePoints(min, max, maxPoints, otr);
    ...
}
var lifeDataInstance = new lifeData(25000, 65000, 56426, 100);

answered Oct 11, 2012 at 18:23

Paul Fleming

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2 Comments

Pointy Over a year ago

Or possibly the "otp" property could be a function wrapper around a call to "onTimePoints".

Spencer Cooley Over a year ago

The last chunk is what I am using because i am going to be creating a new instance every time an event is fired. It is to refresh the view with the correct numbers. Seems like the best solution.

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