Why are these two JavaScript Date objects not the same?

Question

I`m a beginner reading a tutorial about Dates in JavaScript, and it gives this example to compare the time between two events.

I don't really understand why firstDate and var secondDate wouldn't be the exact same time? does the new Date object in var secondDate only take the time once doEvent is triggered, whereas firstDate takes the time with window.onload?

Also, why would the variable firstDate not have the "var" tag, while variable secondDate does? would that be just a typo by the author, or is it significant in some way?

var firstDate;
window.onload=startTimer;

function startTimer(){
 firstDate = new Date();
 document.getElementById("date").onclick=doEvent;
}

function doEvent() {
 var secondDate = new Date();
 alert((secondDate - firstDate) / 1000);
}

Ofcourse new Date object in var secondDate only take the time once doEvent is triggered, because writing a method in succession of another method does not execute it, rather it is executed on the event it is bind to. — Furqan Hameedi
– Furqan Hameedi, Commented Mar 1, 2011 at 5:06

Matt Ball · Accepted Answer · 2011-03-01 05:09:27Z

3

Your guess is exactly correct. firstDate is the time when the window.load event fires, and secondDate is the time when the element with id date is clicked.

In re: your edit, there is a var to go with the firstDate as well - it's outside of the scope of startTimer. It is not a typo. firstDate needs to be defined outside of the scope of the startTimer and doEvent functions so that it has a meaningful value inside of the doEvent function.

It might be useful for you to read up on JavaScript scoping.

edited Mar 1, 2011 at 5:09

answered Mar 1, 2011 at 5:04

Matt Ball

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4 Comments

mjmitche Over a year ago

o.k., thanks. Can you answer the second question I just added to the OP about why firstDate has no var tag whereas the secondDate does?

mjmitche Over a year ago

thank you. why didn`t var secondDate need to be defined outside the scope of the functions?

Matt Ball Over a year ago

@mjmitche: because it does not need to be used outside of doEvent.

mjmitche Over a year ago

thanks, so if firstDate was only defined inside of startTimer, it would have worked in that function, but not in doEvent? appreciate your help.

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