下面是一些有趣的事情:

Comparing NaN with anything (even NaN) is always false, that includes ==, < and >. NaN Stands for Not a Number but if you ask for the type it actually returns a number. Array.sort can take a comparator function and is called by a quicksort-like driver (depends on implementation). Regular expression "constants" can maintain state, like the last thing they matched. Some versions of JavaScript allow you to access $0, $1, $2 members on a regex. null is unlike anything else. It is neither an object, a boolean, a number, a string, nor undefined. It's a bit like an "alternate" undefined. (Note: typeof null == "object") In the outermost context, this yields the otherwise unnameable [Global] object. Declaring a variable with var, instead of just relying on automatic declaration of the variable gives the runtime a real chance of optimizing access to that variable The with construct will destroy such optimzations Variable names can contain Unicode characters. JavaScript regular expressions are not actually regular. They are based on Perl's regexs, and it is possible to construct expressions with lookaheads that take a very, very long time to evaluate. Blocks can be labeled and used as the targets of break. Loops can be labeled and used as the target of continue. Arrays are not sparse. Setting the 1000th element of an otherwise empty array should fill it with undefined. (depends on implementation) if (new Boolean(false)) {...} will execute the {...} block Javascript's regular expression engine's are implementation specific: e.g. it is possible to write "non-portable" regular expressions.

[更新了一点，以回应良好的评论;请参阅评论]

当你想从数组中删除一个元素时，可以使用delete操作符，如下所示:

var numbers = [1,2,3,4,5];
delete numbers[3];
//numbers is now [1,2,3,undefined,5]

正如您所看到的，元素被删除了，但是数组中留下了一个洞，因为元素被替换为一个未定义的值。

因此，要解决这个问题，不要使用删除，而是使用splice array方法…是这样的:

var numbers = [1,2,3,4,5];
numbers.splice(3,1);
//numbers is now [1,2,3,5]

splice的第一个参数是数组[index]中的序数，第二个参数是要删除的元素数量。

这似乎只适用于Firefox (SpiderMonkey)。函数内部:

Arguments[-2]给出了参数的个数(与Arguments .length相同) Arguments[-3]给出了被调用的函数(与Arguments .callee相同)

当你写回调时，你有很多代码，看起来像这样:

callback: function(){
  stuff(arg1,arg2);
}

您可以使用下面的函数使它更简洁。

callback: _(stuff, arg1, arg2) 

它使用javascript的function对象中一个不太为人所知的函数apply。

它还显示了另一个可以用作函数名的字符:_。

function _(){
        var func;
        var args = new Array();
        for(var i = 0; i < arguments.length; i++){
                if( i == 0){
                        func = arguments[i];
                } else {
                        args.push(arguments[i]);
                }
        }
        return function(){
                return func.apply(func, args);
        }
}

这并不总是一个好主意，但您可以用简洁的表达式转换大多数内容。这里重要的一点是，JavaScript中并不是每个值都是对象，所以这些表达式会成功，而对非对象(如null和undefined)的成员访问将失败。特别要注意typeof null == "object"，但你不能null. tostring()，或("name" in null)。

将任何东西转换为数字:

+anything
Number(anything)

将任何东西转换为无符号四字节整数:

anything >>> 0

将任何东西转换为字符串:

'' + anything
String(anything)

将任何东西转换为布尔值:

!!anything
Boolean(anything)

同样，使用不带“new”的类型名对于String、Number和Boolean的行为不同，返回一个基本数字、字符串或布尔值，但使用“new”将返回“盒装”的对象类型，这几乎是无用的。

如果你试图沙盒javascript代码，并禁用所有可能的方法来求值字符串到javascript代码中，要注意阻塞所有明显的eval/document。Function/setTimeout/setInterval/innerHTML和其他DOM操作是不够的。

给定任何对象o, o.constructor.constructor("alert('hi')")()将弹出一个警告对话框，其中包含单词"hi"。

可以写成

var Z="constructor";
Z[Z][Z]("alert('hi')")();

有趣的东西。