Strangest language feature

Question

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.

In JavaScript:

 '5' + 3 gives '53'

Whereas

 '5' - 3 gives 2

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;

JavaScript truth table:

''        ==   '0'           // false
0         ==   ''            // true
0         ==   '0'           // true
false     ==   'false'       // false
false     ==   '0'           // true
false     ==   undefined     // false
false     ==   null          // false
null      ==   undefined     // true
" \t\r\n" ==   0             // true

Source: Doug Crockford