Oddities about JavaScript

http://www.smashingmagazine.com/2011/05/30/10-oddities-and-secrets-about-javascript/

by fogus • 15 years ago

45 30 15 years ago

unwind• 15 years ago

That NaN (not a number) doesn't compare equal to itself is not, in my opinion, a JavaScript oddity. NaN behaves like that according to the IEEE 754, and Wikipedia suggests that using the fact that x == x returns false is a way to test if x is NaN (http://en.wikipedia.org/wiki/NaN).

dexen4• 15 years ago

Nothing odd about it, it's the good old math at work.

Is (3 / 0) == (4 / 0)?

Is even (3 / 0) == (3 / 0)?

EDIT:

also, see [1] on what becomes when dividing both sides of equation by zero and assuming equality still holds. Essentially, both sides become NaNs and everything is possible -- up to and including 1 == 2. Now that'd would be odd, should it happen in your program.

[1] http://en.wikipedia.org/wiki/Invalid_proof#All_numbers_equal...

jerf2• 15 years ago

The problem with this line of logic is that == is not equality. Two things being equal means they are actually the same thing and there aren't two things there at all, just one thing expressed two different ways. == is a binary relation [1]. In the context of that binary relation, if you made NaN == NaN it wouldn't really affect anything mathematically, because you already can't equationally reason on the basis of that binary relation, so there is no equational reasoning to become corrupted by accepting a contradiction.

[1] http://en.wikipedia.org/wiki/Binary_relation

olavk4• 15 years ago

My favorite oddity in JavaScript: Arrays are actually indexed by strings. arr[17] is just syntactic sugar for arr["17"].

city412• 15 years ago

Arrays in JavaScript are merely objects with some extra bits tacked on to make them appear "array-like". Underneath they are still a collection of key/values like all other JS objects.

justincormack• 15 years ago

But the "oddity" is that unlike many languages there are only string keys. Many languages implement the array type as a map but still distinguish between a[17] and a["17"].

program2• 15 years ago

Javascript has automatic semicolon insertion. This function:

  function a() {
    return
      true;
  }

return undefined.

JeremyBanks2• 15 years ago

This Python

    def a():
        return
        False;

returns None.

qntm• 15 years ago

That's because in Python the line break is the statement terminator.

riffraff• 15 years ago

Same in ruby

    def foo
     return
     true
    end

returns "nil"

knowtheory2• 15 years ago

I don't like referring to this as "automatic semicolon insertion". The behavior you're referring to here is really about line termination/continuation.

  var a =
  42

is only parseable one way because assignment requires an argument.

Javascript just happens to make assumptions with certain kinds of ambiguities, but then, all programming languages do.

program2• 15 years ago

Actually that is the name that appears in the ECMAScript official language specification:

http://bclary.com/2004/11/07/#a-7.9.1

olavk4• 15 years ago

The statement "Null is an Object" is probably more confusing than enlightening. I would prefer to say that typeof(null) non-intuitively returns the string "object".

dexen4• 15 years ago

The ECMA standard supports your story. The `null' is the only instance of type `Null'. A Null instance does not have any properties; in particular, doesn't have the `constructor' property all actual object do.

What alert() does is an implicit conversion to string, and that is surprisingly defined to say "object" in ECMA standard. Could be matter of backward compatibility...

olavk4• 15 years ago

It actually makes some kind of sense in a dark, twisted way. If you think of it in terms of the Java type system, then the type of null is arguably Object.

knuton• 15 years ago

Please don’t vote for this article, it’s full of half-truths and misconceptions.

You can not “fake” scope. When you call `call` on a function and pass an object as an argument, you are calling the function as a method of that object. This has nothing to do with a function’s scope.

raganwald• 15 years ago

Please:

Vote for articles you feel contribute to HN, either directly or through their discussion (a vote for a post does not necessarily imply agreement, it implies "This is worth reading.")

Please:

Point out half-truths and misconceptions.

Please don't:

Ask other people to vote for or against a post or comment.

buhrmi• 15 years ago

wow this article is full of errors

stanleydrew2• 15 years ago

Section 7 is so completely wrong:

    var someVar = 'hello';
    setTimeout((function() { alert(someVar); })(someVar), 1000);
    var someVar = 'goodbye';

This won't do what he thinks it's going to do. Instead of alerting 'hello' after 1 second (after the value has been changed to 'goodbye') it alerts 'hello' immediately and does nothing after 1 second. Perhaps he meant something like:

    var someVar = 'hello';
    setTimeout(
        (function(innerVar) {
            return function() { alert(innerVar); };
        })(someVar),
        1000
    );
    var someVar = 'goodbye';

This should alert 'hello' after 1 second even though someVar has by then been overwritten.

dexen4• 15 years ago

((EDIT: was irrelevant, as pointed out by stanleydrew in the post below. Thanks!))

stanleydrew2• 15 years ago

That's the example of what doesn't work from the beginning of section 7. That will alert 'goodbye' after one second. I'm calling out his "fixed" code, which is supposed to alert 'hello' after one second.

Mitya• 15 years ago

@stanleydrew - I changed this code late last night after I realised my mistake here. Of course I had meant to return a function from the SEF, which the code in the article now does. Apologies for the oversight.

al_james• 15 years ago

This article smacks of being written by someone who came to javscript from the design world, as opposed to coming to javascript as a programmer.

voidr• 15 years ago

> 9. 0.1 + 0.2 !== 0.3

You get this in most other programing languages too.

> 8. Firefox Reads and Returns Colors in RGB, Not Hex

The title says JavaScript, this is the DOM.

> 7. Functions Can Execute Themselves

Why is this so odd? I feel the author is too biased towards other languages. Also his example totally misses to point of closures.

> 6. You Can Fake Scope

No you can't fake scope, you override it. Why is this so odd? Again the author is not looking at it correctly, this is natural if you use it correctly.

Most of this article seems to come from someone who didn't really use JavaScript a lot, hence there is a lot of bias and prejudice. All you need to do is tell yourself that classical OOP is not the only way to code, and you will see that most of those things are really good stuff and not oddities.

Oddities about JavaScript

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