I am trying to add to an array, the array at the end should be > [0] (today) Name:Day1 Date:Fri June 10...
> [0] (today) Name:Day1 Date:Fri June 10...
> [1] (today) Name:Day1+2 Date:Sun June 12...
> [0] (today) Name:Day1 Date:Fri June 10...
> [1] (today) Name:Day1+2 Date:Sun June 12...
> [2] (today) Name:Day1+3Date:Mon June 13...
The link with my debug session is:
But instead, the Date: changes for each array element, and is the last date to be pushed
> [0] Day1 - Fri Jun 10 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
> [0] Day1 - Sun Jun 12 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
> [1] Day1+2 - Sun Jun 12 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
[0] Day1 - Mon Jun 13 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
[1] Day1+2 - Mon Jun 13 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
[2] Day1+3 - Mon Jun 13 2022 10:13:15 GMT-0700 (Pacific Daylight Time)
If anyone could explain this to me, I would be appreciative. I am fairly new to JS, so please be kind.
// globals
var TodaysDate;
var holidays = [];
function ShowHolidays() {
console.log(holidays);
for (let i = 0; i < holidays.length; i++) {
console.log("[" + i + "] " + holidays[i].name + " - " + holidays[i].date);
}
}
function AJSTEST() {
TodaysDate = new Date();
holidays.push({ name: "Day1", date: TodaysDate });
ShowHolidays();
debugger;
var AD = TodaysDate;
AD.setDate(AD.getDate() + 2);
debugger;
holidays.push({ name: "Day1+2", date: AD });
ShowHolidays();
AD.setDate(AD.getDate() + 1);
holidays.push({ name: "Day1+3", date: AD });
ShowHolidays();
debugger;
}
AJSTEST();What you have come across is the age old problem (which isn't really a problem, once you understand it it becomes more of a tool than a hinderance) of pass-by-reference (as opposed to pass-by-value, like you believe should be happening).
To get to the root of it, objects (basically everything you have to use the new keyword to create) are stored differently than items like ints, because they are so big. Ints are so small that when you do something like the example below, it doesnt take up a lot of storage so it passes the value of x into y and now they are separate fields with separate values
let x = 1
let y = x
x += 1
However, like I said, Date is an object. Because objects can take up a lot of space in storage, they are pass by reference. That means we just pass around the address in memory to all the variables/methods where you would like to use it.
Long story short, what you have created at the end of you method is a long list of 3 references all pointing to the same object, not 3 separate objects. So everytime you do AD.setDate(AD.getDate() + n); you are manipulating the object that AD points to, and because all the items in your array also point to the same address in memory, they reflect that same change.