Javascript null surprise

https://news.ycombinator.com/item?id=121508

by gruseom • 18 years ago

35 20 18 years ago

news.ycombinator.com

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

   null < 0 || null == 0    -> false

but

   null <= 0    -> true

Would you expect that? I didn't. Obviously <= is implemented as !>. Time to look up all uses of <= and >= in my code...

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

xirium• 18 years ago

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

ratsbane• 18 years ago

Interesting and not what I would have expected. I just confirmed it in Opera. Also, null<=null is true.

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

dreish• 18 years ago

In Safari 3 and Firefox 1.5, at least, null == null, so I would expect null <= null.

Both Safari 3 and Firefox 1.5 do the decidedly unexpected null <= 0 thing.

randrews4• 18 years ago

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.

rglullis• 18 years ago

Okay... I have another one.

In Python:

  >>> -1 % 3
  2

In Javascript:

  >>> -1 % 3
  -1

newt03112• 18 years ago

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.

bayareaguy• 18 years ago

This implies the if a is negative, (a%b) should also be negative.

No, python is giving you the Modulus after division (i.e. a "distance" which should be positive). If you interpret it that way everything is fine:

  >>> a = -1
  >>> b = 3
  >>> (a/b)*b
  -3
  >>> (a%b)
  2
  >>> (a/b)*b + (a%b)
  -1

Perhaps you're thinking of the Remainder?

Edit: it seems there isn't much consensus about what to do when a or b are negative. Wikipedia gives a helpful chart here how different languages choose to implement this: http://en.wikipedia.org/wiki/Modulo_operation

slurpme4• 18 years ago

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...

tolmasky• 18 years ago

This happens when putting a newline between return and any value.

This is actually a result of JavaScript's lax support of semi-colons. Semi-colons are generally not enforced in JavaScript, but this leads to some ambiguous cases. Your case may not seem ambiguous, but the following code snippet will show what is actually going on:

if (i==5) return

i += 3;

As you can see, there are 2 possible meanings for this when there is no explicit semi-colon after the return. It can either mean "if i equals 5 return, else add 3 to i" or it could mean, "if i equals 5 return i plus 3". JavaScript does not try to discern which is "more correct" (since there are no return types on functions so this would be a very difficult task), but rather defaults to "floating returns" meaning "return;".

slurpme4• 18 years ago

I know, I mentioned it as the new object because it's one of the few situations I know of where you might use a newline after the return keyword.

I understand what you mean, in this case though it would be helpful if the interpreter detects the statement as being not reachable since it's after the return.

idea• 18 years ago

[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".

randrews4• 18 years ago

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

slurpme4• 18 years ago

But isn't the context a Boolean expression, so the 0 would be converted to a Boolean as well?

gruseom• 18 years ago

Do we have an explanation yet for why

   null == 0  -> false?

It can't be because operands are converted to numbers or booleans, since 0 == 0 and false == false are both true.

tlrobinson• 18 years ago

How about this one...

    js> r = RegExp("asdf", "g"); 
    /asdf/g 
    js> r.test("asdf") 
    true 
    js> r.test("asdf") 
    false 
    js> r.test("asdf") 
    true 
    js> r.test("asdf") 
    false

Surprisingly this is not a bug, though I was certain it was at first. When using the global flag the RegExp object will keep track of the last matching character position and start the next comparison at that position (after the 4th character, in this case) which results in this seemingly incorrect alternation between "asdf" matching "asdf". Removing the global flag produces correct results.

Javascript null surprise

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