所有“隐藏的”特性都在Mozilla wiki上:http://developer.mozilla.org/en/JavaScript。

这里有JavaScript 1.5的核心参考，JavaScript 1.6的新特性，JavaScript 1.7的新特性，以及JavaScript 1.8的新特性。看看所有这些实际工作的例子，是正确的。

我提交的第一个特性与其说是一个隐藏特性，不如说是一个很少使用的属性重定义特性应用程序。因为可以重新定义对象的方法，所以可以缓存方法调用的结果，这在计算开销很大且希望延迟求值的情况下非常有用。这是记忆的最简单形式。

function Circle(r) {
    this.setR(r);
}

Circle.prototype = {
  recalcArea: function() {
        this.area=function() {
            area = this.r * this.r * Math.PI;
            this.area = function() {return area;}
            return area;
        }
    },
  setR: function (r) {
      this.r = r;
      this.invalidateR();
    },
  invalidateR: function() {
        this.recalcArea();
    }
}

重构将结果缓存到方法中的代码，你会得到:

Object.prototype.cacheResult = function(name, _get) {
  this[name] = function() {
    var result = _get.apply(this, arguments);
    this[name] = function() {
      return result;
    }
    return result;
  };
};

function Circle(r) {
    this.setR(r);
}

Circle.prototype = {
  recalcArea: function() {
        this.cacheResult('area', function() { return this.r * this.r * Math.PI; });
    },
  setR: function (r) {
      this.r = r;
      this.invalidateR();
    },
  invalidateR: function() {
        this.recalcArea();
    }
}

如果你想要一个记忆函数，你可以用那个代替。不涉及属性重定义。

Object.prototype.memoize = function(name, implementation) {
    this[name] = function() {
        var argStr = Array.toString.call(arguments);
        if (typeof(this[name].memo[argStr]) == 'undefined') {
            this[name].memo[argStr] = implementation.apply(this, arguments);
        }
        return this[name].memo[argStr];
    }
};

注意，这依赖于标准数组toString转换，通常不能正常工作。解决这个问题留给读者练习。

第二个提交的是getter和setter。我很惊讶他们还没有被提及。因为官方标准不同于事实上的标准(defineProperty vs. define[GS]字母)，而且Internet Explorer几乎不支持官方标准，所以它们通常没什么用。也许这就是他们没有被提及的原因。注意，你可以很好地结合getter和结果缓存:

Object.prototype.defineCacher = function(name, _get) {
    this.__defineGetter__(name, function() {
        var result = _get.call(this);
        this.__defineGetter__(name, function() { return result; });
        return result;
    })
};

function Circle(r) {
    this.r = r;
}

Circle.prototype = {
  invalidateR: function() {
        this.recalcArea();
    },
  recalcArea: function() {
        this.defineCacher('area', function() {return this.r * this.r * Math.PI; });
    },
  get r() { return this._r; }
  set r(r) { this._r = r; this.invalidateR(); }
}

var unit = new Circle(1);
unit.area;

有效地组合getter, setter和结果缓存有点混乱，因为你必须防止在set上自动失效或不自动失效，这就是下面的例子所做的。如果改变一个属性会使其他多个属性失效，这是一个主要的问题(假设在这些示例中有一个“diameter”属性)。

Object.prototype.defineRecalcer = function(name, _get) {
  var recalcFunc;
  this[recalcFunc='recalc'+name.toCapitalized()] = function() {
    this.defineCacher(name, _get);
  };
  this[recalcFunc]();
  this.__defineSetter__(name, function(value) {
      _set.call(this, value);
      this.__defineGetter__(name, function() {return value; });
  });
};

function Circle(r) {
    this.defineRecalcer('area',
             function() {return this.r * this.r * Math.PI;},
             function(area) {this._r = Math.sqrt(area / Math.PI);},
    );
    this.r = r;
}

Circle.prototype = {
  invalidateR: function() {
        this.recalcArea();
    },
  get r() { return this._r; }
  set r(r) { this._r = r; this.invalidateR(); }
}

访问:

http://images.google.com/images?q=disco

将以下JavaScript代码粘贴到浏览器的地址栏:

http://amix.dk/upload/awt/spin.txt http://amix.dk/upload/awt/disco.txt

享受JavaScript的迪斯科表演:-p

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。

JavaScript使用Date()的方式让我很兴奋!

function isLeapYear(year) {
    return (new Date(year, 1, 29, 0, 0).getMonth() != 2);
}

这是真正的“隐藏特性”。

编辑:删除了评论中建议的“?”条件。 是:……new Date(year, 1,29,0,0).getMonth() != 2 ?真:假…… 详情请看评论。

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]