MockableSystemJS Wrapper Library

[dchambers]

The MockableSystemJS library specified in #1467 will provide an API that matches what SystemJS itself provides, but it will also make sense to offer a more high level API closer to what browser-modules used to provide with its sub-realms feature.

[dchambers]

Based on this comment, it may well make sense to make the API for this library compatible with either proxyquire or rewire.

[dchambers]

One of the nice things about both proxyquire and rewire is that they don't modify global state like sub-realms do — good because this has been a source of problems with sub-realms. Instead, both libraries allow a test to require a special test version of a module, where some or all of the dependencies are replaced with stubs.

For example, in proxyquire you might write:

var proxyquire =  require('proxyquire');
var component = proxyquire('./component', {dep: StubDep});

and in rewire you might write:

var rewire = require("rewire");
var component = rewire('./component');
component.__set__("dep", StubDep);

The main limitation to this approach is that only immediate dependencies of modules directly under test can be manipulated. However, this limitation may well never be a problem for us, and so I'd suggest we delay all work on this issue until we've converted the BRJS & CT libraries, and gained some implementation experience.

As for #1467, it will still need an API that allows modules to be overriden globally so we can implement #1469, but then it may need to be extended further to allow the creation of new modules with altered dependencies.