strawman for the := operator

Axel Rauschmayer axel at
Thu Aug 9 14:10:43 PDT 2012

On Aug 9, 2012, at 18:48 , Allen Wirfs-Brock <allen at> wrote:

> There are irregularities around the edges such as when assignment auto defines a missing property but they are of minor importance if a programmer clearly understands where they are defining abstractions and where they are consuming abstractions.

Let’s, simplifyingly, call people who create abstractions “library authors” and people who use abstractions “normal developers”. It does make sense to give the latter group specialized tools that help them do their work. But I’d like to take a step back and ask the following question.

Where can the assignment operator (=) currently bite you? I’ve come up with the following list:

1. Copying properties, where you always want to override and update [[Home]] (for super references).
2. Patching non-writable configurable properties
3. Adding a property to an object (without invoking the setter), e.g. to add a unique ID to an object you put into a collection
4. Constructors should always define properties, not assign them. If you add instance methods, [[Home]] and [[MethodName]] must be set up properly.


(1) is only relevant for library authors. Normal developers can use Object.update() or similar (including an operator).

(2) is only done by library authors. They can use Object.define() et al.

(3) seems like an edge case, mostly relevant to library authors. With an object you know, assignment seems OK. With an object you haven’t created, overriding a setter with a data property seems like asking for trouble, anyway. In, private names should solve this problem.

(4) matters for normal developers, but only for instance methods. If I am not mistaken, instance methods are currently mostly used for Crockford-style privacy and will be much less common once we have private names. Furthermore, it is difficult if not impossible to add an instance method with a super-reference in But that might change later.

Did I miss anything? I agree that we should protect normal developers from getting bitten by the assignment operator. But whatever the solution, it has to be dead-simple. Perhaps there are smaller measures that can be taken so that normal programmers don’t have to understand the difference between definition and assignment. Which is subtle, because by auto-creating missing properties, assignment has taken on some of definition’s responsibilities.

Dr. Axel Rauschmayer
axel at


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the es-discuss mailing list