Enumerability

Allen Wirfs-Brock allen at wirfs-brock.com
Sat Jan 11 10:52:01 PST 2014


On Jan 11, 2014, at 9:57 AM, Andrea Giammarchi wrote:

> Thanks Allen, makes sense ... but still no example where enumerability of properties and method defined in a "class" prototype is useful for ... something I cannot imagine right now.
> 
> Anything?

Examples that were mentioned in the discussion that lead to the change include abstraction meta methods such as extend or mixin that exist in the wild.  These were created in an world where everything that was user defined was enumerable (at least by default).

This might be particularly problematic if concise methods in object literals are non-enumerable:

$.extend( obj, {
    foo () {},
    bar () {},
    baz: function () {}
});

it would be surprising if after this call obj only acquired a baz property.  (assuming that extend is implemented using for-in or Object.keys).

It might be less surprising if we had

class Extension {
    foo () {}
    bar () {}
};

$.extend(obj, Extension.prototype);

but consider if that code had originally been:

function Extension() {}
Extension.prototype.foo = function () {};
Extension.prototype.bar = function () {};

$.extend(obj, Extension.prototype);

and somebody "cleaned up" the code by via an ES6 class declaration.  Surprise, things break...

Allen




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