achieve modularity and separation of concerns through feature-oriented programming

Programming is an activity very prone to human error, especially if you have multiple developers changing the same code-base at the same time. As more and more features are implemented by different programmers, progress will often slow to a crawl. Programmers can easily lose overview and step on each others toes when their code is spread across the code base surrounded by the code of others.

delta.js helps you organize your JavaScript code in terms of features. As it turns out, files, modules, objects, functions (and so on), are not the right abstractions for describing a feature. But they are very good at other stuff! This library introduces the notion of a delta, which complements those other constructs.

A delta is the place to gather all the code belonging to a specific feature. Basically, for each piece of code, you instruct the delta to put that code in the file, module, object or function where it belongs. That way, the code can physically be gathered in one place, and therefore be easy to maintain.

This has other benefits as well. By having some deltas be explicitly applied after others, you give them permission to overwrite things. If two deltas that are unordered try to overwrite each others code, you will get a friendly error message. Moreover, by separating your features so explicitly, you will be able to turn them on or off with a switch, for either debugging or production.

This might all seem overly complicated at first. But once you get started, you'll soon fall in love with this approach.

At this time, delta.js is a runtime library. But in concept, deltas could be applied by a preprocessor. This will be supported by a future version.

This library depends only on the Babel ES6 polyfill. For your convenience, a delta.js version is provided with this polyfill already baked in, but you also have the option of providing it yourself.

delta.js is available as a Bower package:

bower install delta.js

Unfortunately, the Babel polyfill is not distributed through Bower. So you'll have to either use the version of delta.js with the polyfill baked in, or you'll have to get the polyfill from someplace else, like NPM.

delta.js is available as an NPM package:

npm install delta.js

The delta.js package offers a UMD API, so it supports AMD (RequireJS), CommonJS and script-tags:

requirejs(['lib/delta.js/dist/delta.js'], function (DeltaJs) {
// use DeltaJs
});

var DeltaJs = require('lib/delta.js/dist/delta.js')
// use DeltaJs

<script src="lib/delta.js/dist/delta.js"></script>
<!-- use DeltaJs -->

###Files

The dist directory offers different files for use in different circumstances. Use the following table to determine which file to use in your situation.

If you don't know which you need, you probably want delta.full.min.js, because it will work out-of-the-box. But it is generally more elegant to load the polyfill yourself, especially if you use other libraries that depend on it.

• DeltaJs
  • .Delta
  • .newProxyMethod(method, handler)
  • .newFeature(name, options) ⇒ DeltaJs#Feature
  • .newOperationType(name, DeltaSubclass, ProxySubclass)
  • .vp(name, val) ⇒ *
  • .do() ⇒ DeltaJs#Proxy

DeltaJs

This class offers every functionality you need from delta modeling. Each instance offers its own operation types and variation points and acts as a facade (as in design pattern) to the more specific subsystems of delta.js.

Using multiple DeltaJs instances allows you to use different sets of deltas and rules in the same project that work independently from each other. But you will usually need only one DeltaJs instance per application.

do(args) ⇒ DeltaJs#Proxy

{@public}{@method}

Using multiple DeltaJs instances allows you to use different sets of deltas and rules in the same project that work independently from each other. But you will usually need only one DeltaJs instance per application.

Returns: DeltaJs#Feature - - the object representing the new feature

This method allows you to tell delta.js about a new kind of delta operation. This was also done for existing operations like modify, add, remove, and so on.

This method indicates a variation point.

Returns: * - - the value of this variation point after applying the appropriate deltas

A {DeltaJs} instance has one fundamental {DeltaJs#DeltaModel} instance, which is applied to any variation points that are encountered. This method is an alias to the eponymous method on that 'root' delta model. It returns the proxy that allows new delta operations to be added more easily. It presets the 'feature' option to 'true', but this can be overwritten manually.

Returns: DeltaJs#Proxy - - the proxy to this delta, for easily adding operations

{@public}{@method}

delta.js is released under the terms of the MIT license. It permits reuse within both open and proprietary software, provided all copies of the licensed software include a copy of the MIT License terms and the copyright notice.

Here is an incomplete list of future plans for this library:

• support for changing HTML and CSS with deltas
• transpiler to resolve deltas at build-time