Weird Javascript For Loop Behaviour

Question

I've got a for loop like this:

for (var i=first; i<=last; i++)
{
    $("#markers").append("<div class='marker'>"+i+"</div>");
}

first is set to 2001 and last is 2010. This works fine. The problem is when I change it to:

for (var i=first; i<=last; i+=1)
{
     $("#markers").append("<div class='marker'>"+i+"</div>");
}

(Notice the different final declaration is different). Any variation other than i++ results in an infinite loop. It's very strange as a jsFiddle with the same parameters works happily. Any suggestions?

Curious: why would you do i+=1 instead of i++ in the first place? Seems like it's more work than it needs to be. — Jemaclus
– Jemaclus, Commented Nov 21, 2011 at 18:14
I actually needed to do i+=some_variable, I'd just simplified it down for the question. — Dormouse
– Dormouse, Commented Nov 21, 2011 at 20:17

drdwilcox · Accepted Answer · 2011-11-21 18:09:49Z

13

I would guess that first is set in a way that ambiguously could be interpreted as a string. So the first version can only be interpreted as increment, but the second is being interpreted by javascript as string concatenation.

answered Nov 21, 2011 at 18:09

drdwilcox

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3 Comments

drdwilcox Over a year ago

Does that mean this is the answer? :-)

Dormouse Over a year ago

It did however I wasn't in front of the computer after the 10 minute limit expired. Not my first rodeo. Thanks for the great answer though!

drdwilcox Over a year ago

Thanks. I knew that from your reputation. I was just enjoying my first good answer.

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