WTFJS?

by mcantelon • 16 years ago

237 70 16 years ago

mrcharles4• 16 years ago

NaN === NaN being false is extremely handy. If you are trying to track down a rogue NaN in some code, equating a variable against itself finds it immediately. This is also the same in most other languages (I use it all the time in C++).

kilian• 16 years ago

I didn't know that! very useful, thank you.

mrcharles4• 16 years ago

No problem. At least once per game I end up having to track rogue NaNs. Learned this trick a while back and it has saved my bacon many a time.

rflrob• 16 years ago

Is there an advantage to doing that as opposed to using isNaN()?

mnemonicsloth• 16 years ago

IIRC, it's a speed hack in the IEEE floating point standard.

NaN can happen for a lot of different reasons. You might have divided by zero because your code is broken, or you might have divided by epsilon because your algorithm isn't stable on this problem. [1]

This test checks to make sure none of {f, g, x, y, intermediate_values} were ever NaN, wihtout having to test each one every step of the way.

    if (  f(x) == g(y)  )

Which is a big deal if it's the guard on an inner loop in some numerical code.

[1] I'm fuzzy on the details. Corrections solicited.

natep• 16 years ago

How do you know isNaN() doesn't use this fact? In Java, the body of isNaN(n) is simply 'return n!=n;'

grayrest• 16 years ago

isNaN() will return true for anything that can't be coerced into a number while !(x == x) will only be true for the NaN value itself.

jrockway3• 16 years ago

I am no fan of Javascript, but most of these WTFs make sense to me. The 111111111100 one is unexpected, although there is probably a good explanation. (Like "alert takes a string, you passed a number. nasal demons.")

mrcharles4• 16 years ago

Wouldn't that have more to do with the fact that javascript uses floats for everything? It's probably exceeding precision at a guess.

malkia• 16 years ago

Is it? I thought Javascript has integer type.

Say for example Lua (by default) is configured to use only double. It can be configured to use single-float, or even integer - but only one numeric type.

tedunangst2• 16 years ago

Indeed. I would have expected 415003079.

olliej• 16 years ago

A reasonable number of these "wtf"s are just more examples of people failing to understand floating point. sigh

kevingadd3• 16 years ago

The question is who failed to understand floating point: The authors of the JS specification, the authors of the JS runtime, or the people writing the javascript that demonstrates the 'wtf'? Is the answer "all three"?

jluxenberg• 16 years ago

The real WTF is, why does a person writing some Javascript code need to understand floating point? What high-performance calculations are you doing in you Javascript wouldn't be easier, more accurate, and not that much slower using fixed point math?

henrikschroder• 16 years ago

Here's a "wtf" from the site:

0.1 + 0.2 === 0.3 // false

I'd say that whoever posted that needs a lesson in how floating point works.

olavk• 16 years ago

The people writing the JavaScript that demonstrates the wtf.

I can see that some of the design decisions in IEEE-754, like NaN not beeing equal to NaN might seem unintuitive, but it has nothing to do with JavaScript - the standard is implemented by numerous languages and hardware.

That large numbers are rounded to a limited precision should not even be wtf if you think about it for a moment. Otherwise a single use of PI would fill up all the computer memory.

Tichy2• 16 years ago

"(x=[].reverse)() === window // true"

Didn't understand this one at first, but I guess it is easy: first x becomes the reverse function of arrays, then it is called with this === window. So it amounts to window.reverse(). Just looked it up, and reverse() works in place, so window.reverse() === window - although it is potentially different from before.

andreyf5• 16 years ago

No, it amonts to Array.prototype.reverse.call(window) - window doesn't have a reverse method AFAIK, so window.reverse() will raise a TypeError (since you can't call something "undefined").

Tichy2• 16 years ago

are you sure? i thought in js functions are not really tied to "their" object, instead they receive an implicit this argument. wouldn't x belong to window here? that is, window doesn't have reverse, but now it has x

est• 16 years ago

I smell a potential DOM injection XSS here.

jauco• 16 years ago

How? The only reaon this works is that 'this' in a function that is not called on an object will default to the global object (window for browser js).

So just saying 'this' in an arbitrary function has the same effect.

meastham2• 16 years ago

NaN === NaN is guaranteed to be false by IEEE 754. Hardly a wtf.

Actually most of these posts seem to be the author misunderstanding floating point numbers. Perhaps he should learn what he's doing before he blames the language.

kevingadd3• 16 years ago

This suggests that you don't actually know what the === operator is. To summarize:

== - 'values are equal' - in this case, are these two numbers equal? The IEEE spec would say no. I agree!

=== - 'both sides are exactly the same' - or, in more technical terms, are both the values and types of each argument exactly identical. In this case, the answer is yes, absolutely. === is not a numerical comparison and should not be.

meastham2• 16 years ago

=== means value and type are equal. In this case, the types are equal, but the values are not. I'd argue that NaN === NaN being true would imply a broken implementation of IEEE 754, and the designers of JS seem to agree with me.

knome• 16 years ago

> In this case, the answer is yes, absolutely.

No, it isn't, absolutely.

`===` means that javascript does a normal equality test, whether it be string comparison or numerical comparison, but without any type conversion. It means that while `"3" == 3` is `true` when you execute `"3" === 3"` it is false.

Given two `NaN`s, it would see they're both floats, do a floating point comparison, and return `false`. As it should.

janzer• 16 years ago

"== - 'values are equal' - in this case, are these two numbers equal? The IEEE spec would say no." "=== - ...are both the values and types..."

If the values are not equal, how can both the values and types be?

Looking at the ECMAScript 5 standard even if you take out the explicit handling of NaN in the === (Strict Equality) definition it should still evaluate to false since the next subrule compares the numeric value.

4.If Type(x) is Number, then a.If x is NaN, return false. b.If y is NaN, return false. c.If x is the same Number value as y, return true.

derefr• 16 years ago

Yup; Number.MIN_VALUE has the minimum possible mantissa and exponent, for example. If you want the most negative possible number, you don't need a separate constant, you can just use -(Number.MAX_VALUE), as 754 reals are signed-magnitude.

brianleroux• 16 years ago

Eh, msg me on twitter (same username) if you have better wtfs or want to contribute. The idea isn't to prove how awesome knowledgeable you are but rather to have a good laugh and MAYBE point out some of the less intuitive aspects of the language we all love to hate on.

icefox• 16 years ago

Just pick up JavsaScript: The Good Parts by Douglas Crockford. He has a chapter called "The bad parts".

jetako2• 16 years ago

isNaN(null) == false; is the only wtf I can never remember to avoid, because I can't rationalize it no matter how hard I try.

Edit: I get it now. null coverts to 0

alunny• 16 years ago

it's because of type conversion - null has the same value as 0 (as in `(+ null)`), so it is a number

jetako2• 16 years ago

Yea, what a weird mental block, not realizing null is equivalent to 0, even in math statements. I guess it's because null is not the same as 0 when converted to a String.

techiferous2• 16 years ago

This is awesome.

I feel compelled to mention my WTF JavaScript library, even though it is something completely different: http://github.com/techiferous/wtf-js/blob/master/javascripts...

It's a library for dealing with World Time Format conversions: http://worldtimeformat.com/

</shameless plug>

tedunangst2• 16 years ago

Floating point is hard, let's go shopping.

waldrews• 16 years ago

In what language is 0.1 + 0.2 == 0.3? (I guess C# with decimal literals, but you'd have to specify that with a suffix)

twopoint718• 16 years ago

I was curious and fired up sbcl:

    * (= (+ 0.1 0.2) 0.3)
    T
    * (type-of 0.1)
    SINGLE-FLOAT
    * (= 0.3d0 (+ 0.1d0 0.2d0))
    NIL
    * (type-of 0.1d0)
    DOUBLE-FLOAT

techiferous2• 16 years ago

In Ruby:

0.1 + 0.2 == 0.3 => false

According to "The Ruby Programming Language", all languages that use IEEE-754 floating-point numbers have this problem.

In Ruby you can get around this by using BigDecimal.

>> require 'bigdecimal'

=> true

>> require 'bigdecimal/util'

=> true

>> 0.1.to_d + 0.2.to_d == 0.3.to_d

=> true

martian• 16 years ago

In Python

  >>> decimal.Decimal(".2") + decimal.Decimal(".1") == decimal.Decimal(".3")
  True
  >>> .2 + .1 == .3
  False

Locke1689• 16 years ago

  Prelude> (0.1 + 0.2) == 0.3
  False
  Prelude Ratio> 1 % 10 + 2 % 10 == 3 % 10
  True

jrockway3• 16 years ago

Or:

    ghci> 0.1 + 0.2 == (0.3 :: Ratio Int)
    True

Same type, less syntax.

regularfry• 16 years ago

That's cheating. Haskell isn't a language, it's The Word of Simon.

The question of which Simon is left as an exercise to the reader.

pixelcort• 16 years ago

`typeof null` is 'object', not Object. (It's a string that contains the word object.)

invisible2• 16 years ago

If the programming can't handle a null object, check that the object is of the required type (by setting a key) or check that the object is not null. It's dumb to expect null to not be an object when EVERYTHING is an object in JS (except undefined, which is special). Furthermore, the programmer would be setting a value to null if they are checking for null (are there any other instances where null occurs without explicitly setting?).

statictype2• 16 years ago

>(function(){ if(false) var window; return window }()) === window

>JavaScript interpreter checks all variable declarations in scope before execution.

This is another way of saying that all variable declarations are scoped at the function level, not the individual block level. This is why generating closures in a loop is broken in javascript (Although in just about every other language as well, closures in a loop give surprising results at first - including, I think, arc?)

To get a new scope, you have to use the anonymous function trick:

    (function(){
       var x;//new scope
       ...
      })()

andreyf5• 16 years ago

This is not a WTF if you just know where a language draws scope boundaries - for most languages, it isn't just along the {}'s.

statictype2• 16 years ago

It's a WTF in the sense that every other language with curly braces scope at the block level.

kwamenum86• 16 years ago

When you run reverse with a context of anything besides an array you end up getting back that context. This is why assigning [].reverse to x, then running x() returns window. It is an interesting quirk. Not sure if this is the js specified behavior though.

andreyf5• 16 years ago

Yes, this is JS specified behavior. If there is no activation object (one referenced by an identifier before the `.'), the value of `this' is set to the global object.

invisible2• 16 years ago

A bit of Computer Science knowledge would go a long way in explaining these: e.g. "Foo" + + "Bar" === "FooNaN" (attempting to do unary + on "Bar"). Tons of other examples as documented here, but some of these are a "bit" strange yes ;).

andreyf5• 16 years ago

Psht, floating point WTF's are for noobs. == is where the money is at. Straight from my V8 console:

    >>> x = new Boolean;
    false
    >>> b = (x || !x);
    false
    >>> Boolean(b == false) && Boolean(x == false) && Boolean(b) && Boolean(x)
    true

nandemo• 16 years ago

WTFHN? Why is this voted up? Just because you don't immediately understand something doesn't make it a WTF.

tszming• 16 years ago

FYI,

Some guy registered WTFPHP.com on 2010/02/12

tmountain• 16 years ago

Eh, no biggie, a few pages of gotchas. If we were talking about PHP, I could write a few volumes.

WTFJS?

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