Javascript null surprise

#1

Post by gruseom » Fri, Feb 22, 2008, 8:02 PM UTC

I just encountered this while tracking down a bug in some Javascript (verified in FF and IE):

   null  false

but

   null  true

Would you expect that? I didn't. Obviously . Time to look up all uses of = in my code...

p.s. Is this too trivial to post here? Too specialized? I'm curious.

Re: Javascript null surprise

#2

Post by xirium » Fri, Feb 22, 2008, 8:14 PM UTC

This is definitely on-topic. We've had JavaScript gotchas before ( http://news.ycombinator.com/item?id=115914 ).

Re: Javascript null surprise

#3

Post by ratsbane » Fri, Feb 22, 2008, 8:35 PM UTC

Interesting and not what I would have expected. I just confirmed it in Opera. Also, nullYou can use the following javascript link to confirm this:

(Note - I can't ever remember how to embed links properly in YC... is there a reference somewhere?)

javascript:alert('null 0: '+(null>0)+'\nnull null: '+(null>null)+'\nnull<=null: ' + (null<=null));

Re: Javascript null surprise

#4

Post by randrews » Fri, Feb 22, 2008, 9:14 PM UTC

JSLint ( http://www.jslint.com ) won't catch this, either. It will complain about using == or !=, but it doesn't say anything about other comparisons.

Re: Javascript null surprise

#5

Post by dreish » Fri, Feb 22, 2008, 9:36 PM UTC

ratsbane wrote: post #3

Interesting and not what I would have expected. I just confirmed it in Opera. Also, null You can use the following javascript link to confirm this: (Note - I can't ever remember how to embed links properly in YC... is there a reference somewhere?) javascript:alert('null 0: '+(null>0)+'\nnull null: '+(null>null)+'\nnull<=null: ' + (null<=null));

In Safari 3 and Firefox 1.5, at least, null == null, so I would expect null Both Safari 3 and Firefox 1.5 do the decidedly unexpected null <= 0 thing.

Re: Javascript null surprise

#6

Post by rglullis » Fri, Feb 22, 2008, 10:21 PM UTC

Okay... I have another one.

In Python:

  >>> -1 % 3
  2

In Javascript:

  >>> -1 % 3
  -1

Re: Javascript null surprise

#7

Post by newt0311 » Fri, Feb 22, 2008, 10:34 PM UTC

rglullis wrote: post #6

Okay... I have another one. In Python: >>> -1 % 3 2 In Javascript: >>> -1 % 3 -1

That is actually an error in the python implementation. According to the language reference: "(a/b)*b + (a%b) = a" where the (a/b) step is rounded towards 0. This implies the if a is negative, (a%b) should also be negative. Note that this modulus standard is also used for the C, C++, and JAVA standard. Here, its python's mess up.

Re: Javascript null surprise

#8

Post by slurpme » Fri, Feb 22, 2008, 10:46 PM UTC

How about this surprise which caught me out a couple of days ago, try:

function hello () {

    return
    {
       value : 10
    };

}

window.alert (hello ().value);

It burps in FF and IE...

Re: Javascript null surprise

#9

Post by idea » Fri, Feb 22, 2008, 10:54 PM UTC

[Rhino book] > When null is used in a Boolean context, it converts to false. When used in a numeric context, it converts to 0. And when used in a string context, it converts to "null".

Re: Javascript null surprise

#10

Post by randrews » Fri, Feb 22, 2008, 11:08 PM UTC

idea wrote: post #9

[Rhino book] > When null is used in a Boolean context, it converts to false. When used in a numeric context, it converts to 0. And when used in a string context, it converts to "null".

So I guess the confusing bit is that null==0 is Boolean, rather than numeric, context.