Tuesday, April 13th, 2010
Traits.js: New trait composition library
Category: JavaScript
Tom Van Cutsem and Mark Miller of Google have a new traits library at traitsjs.org that is defined to work in tandem with the new object-manipulation API defined on Object in ES5.
Traits are an alternative to mixins and enable a "safe" form of multiple inheritance.
traits.js introduces traits without also introducing classes. In this library, traits are like Javascript's functions: anonymous, first-class and freely lexically nestable.
What about the roles of classes not normally supported by traits, such as instance-private state and constructors to initialize that state? These roles are already supported by Javascript's functions: you can nest traits inside functions that close over the required state, and you can use the function body as the "constructor". Here's an obligatory 'point-and-color' example:
JAVASCRIPT:
It is also interesting to read the source and see a codebase that both supports ES5 and also is backwards compatible. Hence code such as:
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var bindThis = Function.prototype.bind
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? function(fun, self) { return Function.prototype.bind.call(fun, self); }
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: function(fun, self) {
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function funcBound(var_args) {
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return fun.apply(self, arguments);
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}
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return funcBound;
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};
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var hasOwnProperty = bindThis(call, Object.prototype.hasOwnProperty);
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var slice = bindThis(call, Array.prototype.slice);
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// feature testing such that traits.js runs on both ES3 and ES5
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var forEach = Array.prototype.forEach
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? bindThis(call, Array.prototype.forEach)
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: function(arr, fun) {
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for (var i = 0, len = arr.length; i <len; i++) { fun(arr[i]); }
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};
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var freeze = Object.freeze || function(obj) { return obj; };
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var getPrototypeOf = Object.getPrototypeOf || function(obj) { return Object.prototype };
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Some of that could make a nice ES5 shim library.
(UPDATE: Kris Kowal pointed us to the shim that Narwhal uses. Thanks Kris!)
Related Content:
I’ll probably try to exercise this traits library in Narwhal for our IO system. If you’re looking for an ES5 compatibility shim, this is what we use:
http://github.com/kriskowal/narwhal-lib/blob/narwhal-lib/lib/global-es5.js
I use Joose, they have quite solid support for this sort of thing.
it’s deeply unfortunate that this system seems to totally ignore the “new” operator. A separate “create” method forces consumers to know what sort of thing a class-like object that you can make instances of is.