Strangest language feature

Question

What is, in your opinion, the most surprising, weird, strange or really "WTF" language feature you have encountered?

Please only one feature per answer.

Answer 1

In C, arrays can be indexed like so:

a[10]

which is very common.

However, the lesser known form (which really does work!) is:

10[a]

which means the same as the above.

Answer 2

I struggled a bit about this:

1;

In perl, modules need to return something true.

Answer 3

In Python:

>>> x=5
>>> 1<x<10
True
>>> 1<x<3
False

Not a WTF, but a useful feature.

Answer 4

In JavaScript, the following construct

return
{
    id : 1234,
    title : 'Tony the Pony'
};

returns undefined is a syntax error due to the sneaky implicit semicolon insertion on the newline after return. The following works as you would expect though:

return {
    id : 1234,
    title : 'Tony the Pony'
};

Even worse, this one works as well (in Chrome, at least):

return /*
*/{
    id : 1234,
    title : 'Tony the Pony'
};

Here's a variant of the same issue that does not yield a syntax error, just silently fails:

return
    2 + 2;

Answer 5

Algol pass by name (illustrated using C syntax):

int a[3] = { 1, 2, 3 };
int i = 1;

void f(int j)
{
    int k;
    k = j;  // k = 2
    i = 0;
    k = j;  // k = 1 (!?!)    
}

int main()
{
    f(a[i]);
}

Answer 6

My biggest most hated feature is any configuration file syntax which includes conditional logic. This sort of thing is rife in the Java world (Ant, Maven, etc. You know who you are!).

You just end up programming in a c**p language, with limited debugging and limited editor support.

If you need logic in your configuration the "Pythonic" approach of coding the configuration in a real language is much much better.