Traits library

Kam Kasravi kamkasravi at
Tue Feb 16 17:21:53 PST 2010

Hi Tom:

If I understand the implementation of traits, it provides a ES5 compatible way of composing 'final' properties to an existing object's prototype.
Options provide the meta-data required to define the property descriptor such as required, etc. Do traits provide an ability to bind 
a 'context' to the property in the form of a closure so that the property may be provided with additional information?
Effectively a way to curry or export additional information required by the trait when it is called in the context of the object it was added to.


From: Tom Van Cutsem <tomvc at>
To: es-discuss <es-discuss at>
Sent: Tue, February 16, 2010 2:55:50 PM
Subject: Traits library


Mark Miller and I have been working on a small traits library for Javascript. It is documented here: <>

Traits are reusable building blocks for classes, very similar to mixins, but with less gotchas. For example, traits support explicit conflict resolution upon name clashes and the order in which traits are composed is not significant.

In a nutshell:
- The library is designed for ES5, but backwards-compatible with existing ES3 implementations.
- Our library represents traits as ES5 property maps (objects mapping property names to property descriptors). The library exports:
 - a convenient trait "constructor" to generate property maps from object literals.
 - a number of "trait combinators" to compose property maps.
 - a function that can "instantiate" such property maps into objects (analogous to the ES5 Object.create function, but with awareness about trait-specific property semantics).

The interesting thing about our choice of transparently representing traits as ES5 property maps is that our library can be used as a general-purpose library for manipulating property descriptors in addition to its use as a library to compose and instantiate traits.

A small expository example that uses the library:

Mark and I were both surprised at how well Javascript accommodates a trait library with very little boilerplate. However, there is one catch to implementing traits as a library. Traits, like classes, are normally simply declared in the program text, but need not necessarily have a runtime representation. Trait composition is normally performed entirely at compile-time (in trait lingo this is called "flattening" the traits). At runtime, no trace of trait composition is left.

Because we use a library approach, traits are not declarative entities and must have a runtime representation. Thus, there is a runtime overhead associated with trait creation and composition. Moreover, because the implementation is oblivious to traits, multiple objects instantiated from the same trait "declaration" don't share structure. However, we did design the library such that, if traits are specified using object literals and property renaming depends only on string literals (which is the common case), a partial evaluator could in principle perform all trait composition statically, and replace calls to Trait.create with a specialized implementation that does support structural sharing between instances (just like an implementation that notices multiple calls to Object.create with the same property descriptor map can in principle arrange for the created objects to share structure).

Any feedback on our design is welcomed. In particular, it'd be interesting to hear how hard/easy it would be for an implementation to recognize the operations performed by our library in order to perform them statically.

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

More information about the es-discuss mailing list