Question

var DEST_VALUE = 1
var APPDAYS_AFTER = 2

}

How can i check whether the variable holds some value or not. When i do this, it does not work…

Answer 1

Of course it doesn’t do anything, because in your example DEST_VALUE resolves to true like APPDAYS_AFTER. Value that resolve to false when converted to a boolean in javascript are:

false
null
undefined
The empty string ''
The number 0
The number NaN (yep, 'Not a Number' is a number, it is a special number)

if you write

if(!DEST_VALUE){
    txtSiteId.value = fileContents.Settings.SiteID;
}

you write “if DEST_VALUE is not true do something” (in your case it does nothing). If you want to check if a variables hold a value:

if(DEST_VALUE !== undefined){
     //do something
}