Question

I am trying to get my object’s method to work, but am missing something – probably very obvious!!

I found the following topic: How do you create a method for a custom object in JavaScript? but no matter what I try, my method just returns the function code as a literal string.

I also tried looking at the object literal JavaScript – Advantages of object literal but as a newcomer to JS, I couldn’t see how to pass initial parameters to the object!

So, context….

I am creating a web page that does various astronomical calculations based on date, time and location. I have a set of inputs for year, month, day, hour, min, sec, timezone, latitude and longitude. I am using this as a learning experience for JS, as well as understanding the calculations!

I am creating a global date object to hold the date elements and I wanted to create a method that would calculate the Julian Day Number.

I have created a fiddle with the code: http://jsfiddle.net/AstroDaz/gKQ5f/5/

function DATE_OBJ (iyear,imonth,iday,idirty,ijd) {
    this.Year = iyear;
    this.Month = imonth;
    this.Day = iday;
    this.Dirty = idirty;
    this.JD = DateJD;
}

function DateJD() {
    var Y = objDate.Year;
    var M = objDate.Month;
    var D = objDate.Day;

    if (M < 3) {
        --Y;
        M +=12;
    }

    if (objDate.Year > 1582) {

      var A = parseInt(Y / 100, 10);
      var B = 2 - A + parseInt(A / 4,10);
      } else {
        if (objDate.Year = 1582 && M > 10) {
          var A = parseInt(Y / 100, 10);
          var B = 2 - A + parseInt(A / 4,10);
          } else {
            if (objDate.Year = 1582 && M = 10 && D >=15) {
              var A = parseInt(Y / 100, 10);
              var B = 2 - A + parseInt(A / 4,10);
              } else var B = 0;
          }
        }

    if (Y < 0) var C = parseInt((365.25 * Y) - 0.75,10);
    else var C = parseInt(365.25 * Y,10);

    var E = parseInt(30.6001 * (M + 1),10);
    this.JD = B + C + E + objDate.Day + 1720994.5;
}


var d = new Date();    
var objDate = new DATE_OBJ (d.getFullYear(),d.getMonth(),d.getDate(),false,0);

document.write(objDate.JD);​

As you’ll see, the result is that the JD property of the object is assigned as the literal string of the method code, not the result of the calculation!

Incidentally, if I create the DateJD as a straight function and call it, it works and returns the correct result.

Thanks for any help!
Daz

Answer 1

With this.JD = DateJD you’re not assigning the return value but a callable function. So your calculations are never executed but just assigned to the member variable.

If you change the last line of DateJD from

this.JD = B + C + E + objDate.Day + 1720994.5;

to

return B + C + E + objDate.Day + 1720994.5;

and then call it via

document.write(objDate.JD());​

It will return a number: http://jsfiddle.net/gKQ5f/6/

If you execute your function it in your ‘constructor’, then there’s no existing objDate object which can be used and you will get something like

Uncaught TypeError: Cannot read property ‘Year’ of undefined

---

Btw, you might want to change these conditions:

if (objDate.Year = 1582 && M > 10) {

This assigns objDate.Year to 1582 instead of checking if it equals.
Take objDate.Year == 1582 instead.