Python's __getattr__ in Javascript [duplicate]

Question

Is there a way to simulate Python's __getattr__ method in Javascript?

I want to intercept 'gets' and 'sets' of Javascript object's properties.

In Python I can write the following:

class A:     def __getattr__(self, key):         return key  a = A() print( a.b )  # Output: b 

What about Javascript?

Answer 1

Not in standard ECMAScript-262 3rd ed.

Upcoming 5th edition (currently draft), on the other hand, introduces accessors in object initializers. For example:

var o = {   a:7,   get b() { return this.a + 1; },   set c(x) { this.a = x / 2; } }; 

Similar syntax is already supported by Javascript 1.8.1 (as a non-standard extension of course).

Note that there are no "completed" ES5 implementations at the moment (although some are in progress)

Answer 2

No. The closest is __defineGetter__ available in Firefox, which defines a function callback to invoke whenever the specified property is read:

navigator.__defineGetter__('userAgent', function(){     return 'foo' // customized user agent });  navigator.userAgent; // 'foo' 

It differs from __getattr__ in that it is called for a known property, rather than as a generic lookup for an unknown property.