函数l (f, n) {n&&l (n - 1 f, f (n));} L(函数(循环){警报(循环);}， 5);

警报5、4、3、2、1

JavaScript被认为非常擅长于暴露它的所有对象，所以不管它的窗口对象本身。

因此，如果我想用JQuery/YUI div弹出窗口覆盖浏览器警报，它也接受字符串作为参数，可以简单地使用下面的代码片段。

function divPopup(str)
{
    //code to show the divPopup
}
window.alert = divPopup;

有了这个更改，所有对alert()的调用都将显示新的基于div的弹出窗口，而不是浏览器特定的警报。

方法(或函数)可以在不是它们设计使用的类型的对象上调用。这对于在自定义对象上调用本机(快速)方法非常有用。

var listNodes = document.getElementsByTagName('a');
listNodes.sort(function(a, b){ ... });

这段代码崩溃是因为listNodes不是数组

Array.prototype.sort.apply(listNodes, [function(a, b){ ... }]);

这段代码之所以有效，是因为listNodes定义了足够多的类数组属性(length，[]运算符)供sort()使用。

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]

要正确地从对象中删除一个属性，你应该删除该属性，而不是仅仅将其设置为undefined:

var obj = { prop1: 42, prop2: 43 };

obj.prop2 = undefined;

for (var key in obj) {
    ...

prop2属性仍然是迭代的一部分。如果你想完全摆脱prop2，你应该这样做:

delete obj.prop2;

在遍历属性时，prop2属性将不再出现。

真实和虚假价值的概念。你不需要做这种事

if（someVar === undefined || someVar === zero） ...

只是做的事:

if (! someVar)。

每个值都有一个对应的布尔表示。