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javascriptlanguage-features

Catch access to undefined property in JavaScript

The Spider-Monkey JavaScript engine implements the __noSuchMethod__ callback function for JavaScript Objects.

This function is called whenever JavaScript tries to execute an undefined method of an Object.

I would like to set a callback function to an Object that will be called whenever an undefined property in the Object is accessed or assigned to.

I haven't found a __noSuchProperty__ function implemented for JavaScript Objects and I am curios if there is any workaround that will achieve the same result.

Consider the following code:

var a = {};
a.__defineGetter__("bla", function(){alert(1);return 2;});
alert(a.bla);

It is equivalent to [alert(1);alert(2)] - even though a.bla is undefined.

I would like to achieve the same result but to unknown properties (i.e. without knowing in advance that a."bla" will be the property accessed)

Solution

  • In ECMAScript's next version it should be quite easy, using Object.observe. Read more here: http://addyosmani.com/blog/a-few-new-things-coming-to-javascript/